Re: Reducing vdpa migration downtime because of memory pin / maps
On Thu, Jul 20, 2023 at 2:49 AM Si-Wei Liu wrote: > > > > On 7/19/2023 3:40 AM, Eugenio Perez Martin wrote: > > On Mon, Jul 17, 2023 at 9:57 PM Si-Wei Liu wrote: > >> Hey, > >> > >> I am now back from the break. Sorry for the delayed response, please see > >> in line. > >> > >> On 7/9/2023 11:04 PM, Eugenio Perez Martin wrote: > >>> On Sat, Jul 8, 2023 at 11:14 AM Si-Wei Liu wrote: > > On 7/5/2023 10:46 PM, Eugenio Perez Martin wrote: > > On Thu, Jul 6, 2023 at 2:13 AM Si-Wei Liu wrote: > >> On 7/5/2023 11:03 AM, Eugenio Perez Martin wrote: > >>> On Tue, Jun 27, 2023 at 8:36 AM Si-Wei Liu > >>> wrote: > On 6/9/2023 7:32 AM, Eugenio Perez Martin wrote: > > On Fri, Jun 9, 2023 at 12:39 AM Si-Wei Liu > > wrote: > >> On 6/7/23 01:08, Eugenio Perez Martin wrote: > >>> On Wed, Jun 7, 2023 at 12:43 AM Si-Wei Liu > >>> wrote: > Sorry for reviving this old thread, I lost the best timing to > follow up > on this while I was on vacation. I have been working on this and > found > out some discrepancy, please see below. > > On 4/5/23 04:37, Eugenio Perez Martin wrote: > > Hi! > > > > As mentioned in the last upstream virtio-networking meeting, > > one of > > the factors that adds more downtime to migration is the > > handling of > > the guest memory (pin, map, etc). At this moment this handling > > is > > bound to the virtio life cycle (DRIVER_OK, RESET). In that > > sense, the > > destination device waits until all the guest memory / state is > > migrated to start pinning all the memory. > > > > The proposal is to bind it to the char device life cycle (open > > vs > > close), > Hmmm, really? If it's the life cycle for char device, the next > guest / > qemu launch on the same vhost-vdpa device node won't make it > work. > > >>> Maybe my sentence was not accurate, but I think we're on the same > >>> page here. > >>> > >>> Two qemu instances opening the same char device at the same time > >>> are > >>> not allowed, and vhost_vdpa_release clean all the maps. So the > >>> next > >>> qemu that opens the char device should see a clean device anyway. > >> I mean the pin can't be done at the time of char device open, > >> where the > >> user address space is not known/bound yet. The earliest point > >> possible > >> for pinning would be until the vhost_attach_mm() call from > >> SET_OWNER is > >> done. > > Maybe we are deviating, let me start again. > > > > Using QEMU code, what I'm proposing is to modify the lifecycle of > > the > > .listener member of struct vhost_vdpa. > > > > At this moment, the memory listener is registered at > > vhost_vdpa_dev_start(dev, started=true) call for the last vhost_dev, > > and is unregistered in both vhost_vdpa_reset_status and > > vhost_vdpa_cleanup. > > > > My original proposal was just to move the memory listener > > registration > > to the last vhost_vdpa_init, and remove the unregister from > > vhost_vdpa_reset_status. The calls to vhost_vdpa_dma_map/unmap would > > be the same, the device should not realize this change. > This can address LM downtime latency for sure, but it won't help > downtime during dynamic SVQ switch - which still needs to go through > the > full unmap/map cycle (that includes the slow part for pinning) from > passthrough to SVQ mode. Be noted not every device could work with a > separate ASID for SVQ descriptors. The fix should expect to work on > normal vDPA vendor devices without a separate descriptor ASID, with > platform IOMMU underneath or with on-chip IOMMU. > > >>> At this moment the SVQ switch is very inefficient mapping-wise, as it > >>> unmap all the GPA->HVA maps and overrides it. In particular, SVQ is > >>> allocated in low regions of the iova space, and then the guest memory > >>> is allocated in this new IOVA region incrementally. > >> Yep. The key to build this fast path for SVQ switching I think is to > >> maintain the identity mapping for the passthrough queues so that QEMU > >> can reuse the old mappings for guest memory (e.g. GIOVA identity mapped > >> to GPA) while incrementally adding new mappings for SVQ vrings. > >> > >>> We can optimize that if we place SVQ in a free GPA area instead. > >> Here's a question though: it might not be
Re: Reducing vdpa migration downtime because of memory pin / maps
On 7/19/2023 3:40 AM, Eugenio Perez Martin wrote: On Mon, Jul 17, 2023 at 9:57 PM Si-Wei Liu wrote: Hey, I am now back from the break. Sorry for the delayed response, please see in line. On 7/9/2023 11:04 PM, Eugenio Perez Martin wrote: On Sat, Jul 8, 2023 at 11:14 AM Si-Wei Liu wrote: On 7/5/2023 10:46 PM, Eugenio Perez Martin wrote: On Thu, Jul 6, 2023 at 2:13 AM Si-Wei Liu wrote: On 7/5/2023 11:03 AM, Eugenio Perez Martin wrote: On Tue, Jun 27, 2023 at 8:36 AM Si-Wei Liu wrote: On 6/9/2023 7:32 AM, Eugenio Perez Martin wrote: On Fri, Jun 9, 2023 at 12:39 AM Si-Wei Liu wrote: On 6/7/23 01:08, Eugenio Perez Martin wrote: On Wed, Jun 7, 2023 at 12:43 AM Si-Wei Liu wrote: Sorry for reviving this old thread, I lost the best timing to follow up on this while I was on vacation. I have been working on this and found out some discrepancy, please see below. On 4/5/23 04:37, Eugenio Perez Martin wrote: Hi! As mentioned in the last upstream virtio-networking meeting, one of the factors that adds more downtime to migration is the handling of the guest memory (pin, map, etc). At this moment this handling is bound to the virtio life cycle (DRIVER_OK, RESET). In that sense, the destination device waits until all the guest memory / state is migrated to start pinning all the memory. The proposal is to bind it to the char device life cycle (open vs close), Hmmm, really? If it's the life cycle for char device, the next guest / qemu launch on the same vhost-vdpa device node won't make it work. Maybe my sentence was not accurate, but I think we're on the same page here. Two qemu instances opening the same char device at the same time are not allowed, and vhost_vdpa_release clean all the maps. So the next qemu that opens the char device should see a clean device anyway. I mean the pin can't be done at the time of char device open, where the user address space is not known/bound yet. The earliest point possible for pinning would be until the vhost_attach_mm() call from SET_OWNER is done. Maybe we are deviating, let me start again. Using QEMU code, what I'm proposing is to modify the lifecycle of the .listener member of struct vhost_vdpa. At this moment, the memory listener is registered at vhost_vdpa_dev_start(dev, started=true) call for the last vhost_dev, and is unregistered in both vhost_vdpa_reset_status and vhost_vdpa_cleanup. My original proposal was just to move the memory listener registration to the last vhost_vdpa_init, and remove the unregister from vhost_vdpa_reset_status. The calls to vhost_vdpa_dma_map/unmap would be the same, the device should not realize this change. This can address LM downtime latency for sure, but it won't help downtime during dynamic SVQ switch - which still needs to go through the full unmap/map cycle (that includes the slow part for pinning) from passthrough to SVQ mode. Be noted not every device could work with a separate ASID for SVQ descriptors. The fix should expect to work on normal vDPA vendor devices without a separate descriptor ASID, with platform IOMMU underneath or with on-chip IOMMU. At this moment the SVQ switch is very inefficient mapping-wise, as it unmap all the GPA->HVA maps and overrides it. In particular, SVQ is allocated in low regions of the iova space, and then the guest memory is allocated in this new IOVA region incrementally. Yep. The key to build this fast path for SVQ switching I think is to maintain the identity mapping for the passthrough queues so that QEMU can reuse the old mappings for guest memory (e.g. GIOVA identity mapped to GPA) while incrementally adding new mappings for SVQ vrings. We can optimize that if we place SVQ in a free GPA area instead. Here's a question though: it might not be hard to find a free GPA range for the non-vIOMMU case (allocate iova from beyond the 48bit or 52bit ranges), but I'm not sure if easy to find a free GIOVA range for the vIOMMU case - particularly this has to work in the same entire 64bit IOVA address ranges that (for now) QEMU won't be able to "reserve" a specific IOVA ranges for SVQ from the vIOMMU. Do you foresee this can be done for every QEMU emulated vIOMMU (intel-iommu amd-iommu, arm smmu and virito-iommu) so that we can call it out as a generic means for SVQ switching optimization? In the case vIOMMU allocates a new block we will use the same algorithm as now: * Find a new free IOVA chunk of the same size * Map this new SVQ IOVA, that may or may not be the same as SVQ Since we must go through the translation phase to sanitize guest's available descriptors anyway, it has zero added cost. Not sure I followed, this can work but doesn't seem able to reuse the old host kernel mappings for guest memory, hence still requires remap of the entire host IOVA ranges when SVQ IOVA comes along. I think by maintaining 1:1 identity map on guest memory, we don't have to bother tearing down existing HVA->HPA mappings in kernel thus save the expensive pinning calls at large. I
Re: Reducing vdpa migration downtime because of memory pin / maps
On Mon, Jul 17, 2023 at 9:57 PM Si-Wei Liu wrote: > > Hey, > > I am now back from the break. Sorry for the delayed response, please see > in line. > > On 7/9/2023 11:04 PM, Eugenio Perez Martin wrote: > > On Sat, Jul 8, 2023 at 11:14 AM Si-Wei Liu wrote: > >> > >> > >> On 7/5/2023 10:46 PM, Eugenio Perez Martin wrote: > >>> On Thu, Jul 6, 2023 at 2:13 AM Si-Wei Liu wrote: > > On 7/5/2023 11:03 AM, Eugenio Perez Martin wrote: > > On Tue, Jun 27, 2023 at 8:36 AM Si-Wei Liu > > wrote: > >> On 6/9/2023 7:32 AM, Eugenio Perez Martin wrote: > >>> On Fri, Jun 9, 2023 at 12:39 AM Si-Wei Liu > >>> wrote: > On 6/7/23 01:08, Eugenio Perez Martin wrote: > > On Wed, Jun 7, 2023 at 12:43 AM Si-Wei Liu > > wrote: > >> Sorry for reviving this old thread, I lost the best timing to > >> follow up > >> on this while I was on vacation. I have been working on this and > >> found > >> out some discrepancy, please see below. > >> > >> On 4/5/23 04:37, Eugenio Perez Martin wrote: > >>> Hi! > >>> > >>> As mentioned in the last upstream virtio-networking meeting, one > >>> of > >>> the factors that adds more downtime to migration is the handling > >>> of > >>> the guest memory (pin, map, etc). At this moment this handling is > >>> bound to the virtio life cycle (DRIVER_OK, RESET). In that sense, > >>> the > >>> destination device waits until all the guest memory / state is > >>> migrated to start pinning all the memory. > >>> > >>> The proposal is to bind it to the char device life cycle (open vs > >>> close), > >> Hmmm, really? If it's the life cycle for char device, the next > >> guest / > >> qemu launch on the same vhost-vdpa device node won't make it work. > >> > > Maybe my sentence was not accurate, but I think we're on the same > > page here. > > > > Two qemu instances opening the same char device at the same time are > > not allowed, and vhost_vdpa_release clean all the maps. So the next > > qemu that opens the char device should see a clean device anyway. > I mean the pin can't be done at the time of char device open, where > the > user address space is not known/bound yet. The earliest point > possible > for pinning would be until the vhost_attach_mm() call from SET_OWNER > is > done. > >>> Maybe we are deviating, let me start again. > >>> > >>> Using QEMU code, what I'm proposing is to modify the lifecycle of the > >>> .listener member of struct vhost_vdpa. > >>> > >>> At this moment, the memory listener is registered at > >>> vhost_vdpa_dev_start(dev, started=true) call for the last vhost_dev, > >>> and is unregistered in both vhost_vdpa_reset_status and > >>> vhost_vdpa_cleanup. > >>> > >>> My original proposal was just to move the memory listener registration > >>> to the last vhost_vdpa_init, and remove the unregister from > >>> vhost_vdpa_reset_status. The calls to vhost_vdpa_dma_map/unmap would > >>> be the same, the device should not realize this change. > >> This can address LM downtime latency for sure, but it won't help > >> downtime during dynamic SVQ switch - which still needs to go through > >> the > >> full unmap/map cycle (that includes the slow part for pinning) from > >> passthrough to SVQ mode. Be noted not every device could work with a > >> separate ASID for SVQ descriptors. The fix should expect to work on > >> normal vDPA vendor devices without a separate descriptor ASID, with > >> platform IOMMU underneath or with on-chip IOMMU. > >> > > At this moment the SVQ switch is very inefficient mapping-wise, as it > > unmap all the GPA->HVA maps and overrides it. In particular, SVQ is > > allocated in low regions of the iova space, and then the guest memory > > is allocated in this new IOVA region incrementally. > Yep. The key to build this fast path for SVQ switching I think is to > maintain the identity mapping for the passthrough queues so that QEMU > can reuse the old mappings for guest memory (e.g. GIOVA identity mapped > to GPA) while incrementally adding new mappings for SVQ vrings. > > > We can optimize that if we place SVQ in a free GPA area instead. > Here's a question though: it might not be hard to find a free GPA range > for the non-vIOMMU case (allocate iova from beyond the 48bit or 52bit > ranges), but I'm not sure if easy to find a free GIOVA range for the > vIOMMU case - particularly this has to work in the same entire 64bit > IOVA address ranges that (for now) QEMU won't be able to "reserve" a > specific IOVA ranges for SVQ from the vIOMMU. Do
Re: Reducing vdpa migration downtime because of memory pin / maps
Hey, I am now back from the break. Sorry for the delayed response, please see in line. On 7/9/2023 11:04 PM, Eugenio Perez Martin wrote: On Sat, Jul 8, 2023 at 11:14 AM Si-Wei Liu wrote: On 7/5/2023 10:46 PM, Eugenio Perez Martin wrote: On Thu, Jul 6, 2023 at 2:13 AM Si-Wei Liu wrote: On 7/5/2023 11:03 AM, Eugenio Perez Martin wrote: On Tue, Jun 27, 2023 at 8:36 AM Si-Wei Liu wrote: On 6/9/2023 7:32 AM, Eugenio Perez Martin wrote: On Fri, Jun 9, 2023 at 12:39 AM Si-Wei Liu wrote: On 6/7/23 01:08, Eugenio Perez Martin wrote: On Wed, Jun 7, 2023 at 12:43 AM Si-Wei Liu wrote: Sorry for reviving this old thread, I lost the best timing to follow up on this while I was on vacation. I have been working on this and found out some discrepancy, please see below. On 4/5/23 04:37, Eugenio Perez Martin wrote: Hi! As mentioned in the last upstream virtio-networking meeting, one of the factors that adds more downtime to migration is the handling of the guest memory (pin, map, etc). At this moment this handling is bound to the virtio life cycle (DRIVER_OK, RESET). In that sense, the destination device waits until all the guest memory / state is migrated to start pinning all the memory. The proposal is to bind it to the char device life cycle (open vs close), Hmmm, really? If it's the life cycle for char device, the next guest / qemu launch on the same vhost-vdpa device node won't make it work. Maybe my sentence was not accurate, but I think we're on the same page here. Two qemu instances opening the same char device at the same time are not allowed, and vhost_vdpa_release clean all the maps. So the next qemu that opens the char device should see a clean device anyway. I mean the pin can't be done at the time of char device open, where the user address space is not known/bound yet. The earliest point possible for pinning would be until the vhost_attach_mm() call from SET_OWNER is done. Maybe we are deviating, let me start again. Using QEMU code, what I'm proposing is to modify the lifecycle of the .listener member of struct vhost_vdpa. At this moment, the memory listener is registered at vhost_vdpa_dev_start(dev, started=true) call for the last vhost_dev, and is unregistered in both vhost_vdpa_reset_status and vhost_vdpa_cleanup. My original proposal was just to move the memory listener registration to the last vhost_vdpa_init, and remove the unregister from vhost_vdpa_reset_status. The calls to vhost_vdpa_dma_map/unmap would be the same, the device should not realize this change. This can address LM downtime latency for sure, but it won't help downtime during dynamic SVQ switch - which still needs to go through the full unmap/map cycle (that includes the slow part for pinning) from passthrough to SVQ mode. Be noted not every device could work with a separate ASID for SVQ descriptors. The fix should expect to work on normal vDPA vendor devices without a separate descriptor ASID, with platform IOMMU underneath or with on-chip IOMMU. At this moment the SVQ switch is very inefficient mapping-wise, as it unmap all the GPA->HVA maps and overrides it. In particular, SVQ is allocated in low regions of the iova space, and then the guest memory is allocated in this new IOVA region incrementally. Yep. The key to build this fast path for SVQ switching I think is to maintain the identity mapping for the passthrough queues so that QEMU can reuse the old mappings for guest memory (e.g. GIOVA identity mapped to GPA) while incrementally adding new mappings for SVQ vrings. We can optimize that if we place SVQ in a free GPA area instead. Here's a question though: it might not be hard to find a free GPA range for the non-vIOMMU case (allocate iova from beyond the 48bit or 52bit ranges), but I'm not sure if easy to find a free GIOVA range for the vIOMMU case - particularly this has to work in the same entire 64bit IOVA address ranges that (for now) QEMU won't be able to "reserve" a specific IOVA ranges for SVQ from the vIOMMU. Do you foresee this can be done for every QEMU emulated vIOMMU (intel-iommu amd-iommu, arm smmu and virito-iommu) so that we can call it out as a generic means for SVQ switching optimization? In the case vIOMMU allocates a new block we will use the same algorithm as now: * Find a new free IOVA chunk of the same size * Map this new SVQ IOVA, that may or may not be the same as SVQ Since we must go through the translation phase to sanitize guest's available descriptors anyway, it has zero added cost. Not sure I followed, this can work but doesn't seem able to reuse the old host kernel mappings for guest memory, hence still requires remap of the entire host IOVA ranges when SVQ IOVA comes along. I think by maintaining 1:1 identity map on guest memory, we don't have to bother tearing down existing HVA->HPA mappings in kernel thus save the expensive pinning calls at large. I don't clearly see under this scheme, how the new SVQ IOVA may work with potential conflict on IOVA space
Re: Reducing vdpa migration downtime because of memory pin / maps
On Sat, Jul 8, 2023 at 11:14 AM Si-Wei Liu wrote: > > > > On 7/5/2023 10:46 PM, Eugenio Perez Martin wrote: > > On Thu, Jul 6, 2023 at 2:13 AM Si-Wei Liu wrote: > >> > >> > >> On 7/5/2023 11:03 AM, Eugenio Perez Martin wrote: > >>> On Tue, Jun 27, 2023 at 8:36 AM Si-Wei Liu wrote: > > On 6/9/2023 7:32 AM, Eugenio Perez Martin wrote: > > On Fri, Jun 9, 2023 at 12:39 AM Si-Wei Liu > > wrote: > >> On 6/7/23 01:08, Eugenio Perez Martin wrote: > >>> On Wed, Jun 7, 2023 at 12:43 AM Si-Wei Liu > >>> wrote: > Sorry for reviving this old thread, I lost the best timing to follow > up > on this while I was on vacation. I have been working on this and > found > out some discrepancy, please see below. > > On 4/5/23 04:37, Eugenio Perez Martin wrote: > > Hi! > > > > As mentioned in the last upstream virtio-networking meeting, one of > > the factors that adds more downtime to migration is the handling of > > the guest memory (pin, map, etc). At this moment this handling is > > bound to the virtio life cycle (DRIVER_OK, RESET). In that sense, > > the > > destination device waits until all the guest memory / state is > > migrated to start pinning all the memory. > > > > The proposal is to bind it to the char device life cycle (open vs > > close), > Hmmm, really? If it's the life cycle for char device, the next guest > / > qemu launch on the same vhost-vdpa device node won't make it work. > > >>> Maybe my sentence was not accurate, but I think we're on the same > >>> page here. > >>> > >>> Two qemu instances opening the same char device at the same time are > >>> not allowed, and vhost_vdpa_release clean all the maps. So the next > >>> qemu that opens the char device should see a clean device anyway. > >> I mean the pin can't be done at the time of char device open, where the > >> user address space is not known/bound yet. The earliest point possible > >> for pinning would be until the vhost_attach_mm() call from SET_OWNER is > >> done. > > Maybe we are deviating, let me start again. > > > > Using QEMU code, what I'm proposing is to modify the lifecycle of the > > .listener member of struct vhost_vdpa. > > > > At this moment, the memory listener is registered at > > vhost_vdpa_dev_start(dev, started=true) call for the last vhost_dev, > > and is unregistered in both vhost_vdpa_reset_status and > > vhost_vdpa_cleanup. > > > > My original proposal was just to move the memory listener registration > > to the last vhost_vdpa_init, and remove the unregister from > > vhost_vdpa_reset_status. The calls to vhost_vdpa_dma_map/unmap would > > be the same, the device should not realize this change. > This can address LM downtime latency for sure, but it won't help > downtime during dynamic SVQ switch - which still needs to go through the > full unmap/map cycle (that includes the slow part for pinning) from > passthrough to SVQ mode. Be noted not every device could work with a > separate ASID for SVQ descriptors. The fix should expect to work on > normal vDPA vendor devices without a separate descriptor ASID, with > platform IOMMU underneath or with on-chip IOMMU. > > >>> At this moment the SVQ switch is very inefficient mapping-wise, as it > >>> unmap all the GPA->HVA maps and overrides it. In particular, SVQ is > >>> allocated in low regions of the iova space, and then the guest memory > >>> is allocated in this new IOVA region incrementally. > >> Yep. The key to build this fast path for SVQ switching I think is to > >> maintain the identity mapping for the passthrough queues so that QEMU > >> can reuse the old mappings for guest memory (e.g. GIOVA identity mapped > >> to GPA) while incrementally adding new mappings for SVQ vrings. > >> > >>> We can optimize that if we place SVQ in a free GPA area instead. > >> Here's a question though: it might not be hard to find a free GPA range > >> for the non-vIOMMU case (allocate iova from beyond the 48bit or 52bit > >> ranges), but I'm not sure if easy to find a free GIOVA range for the > >> vIOMMU case - particularly this has to work in the same entire 64bit > >> IOVA address ranges that (for now) QEMU won't be able to "reserve" a > >> specific IOVA ranges for SVQ from the vIOMMU. Do you foresee this can be > >> done for every QEMU emulated vIOMMU (intel-iommu amd-iommu, arm smmu and > >> virito-iommu) so that we can call it out as a generic means for SVQ > >> switching optimization? > >> > > In the case vIOMMU allocates a new block we will use the same algorithm as > > now: > > * Find a new free IOVA chunk of the same size > > * Map this new SVQ IOVA, that may or may not be the same as SVQ > > > > Since we must go through
Re: Reducing vdpa migration downtime because of memory pin / maps
On 7/5/2023 10:46 PM, Eugenio Perez Martin wrote: On Thu, Jul 6, 2023 at 2:13 AM Si-Wei Liu wrote: On 7/5/2023 11:03 AM, Eugenio Perez Martin wrote: On Tue, Jun 27, 2023 at 8:36 AM Si-Wei Liu wrote: On 6/9/2023 7:32 AM, Eugenio Perez Martin wrote: On Fri, Jun 9, 2023 at 12:39 AM Si-Wei Liu wrote: On 6/7/23 01:08, Eugenio Perez Martin wrote: On Wed, Jun 7, 2023 at 12:43 AM Si-Wei Liu wrote: Sorry for reviving this old thread, I lost the best timing to follow up on this while I was on vacation. I have been working on this and found out some discrepancy, please see below. On 4/5/23 04:37, Eugenio Perez Martin wrote: Hi! As mentioned in the last upstream virtio-networking meeting, one of the factors that adds more downtime to migration is the handling of the guest memory (pin, map, etc). At this moment this handling is bound to the virtio life cycle (DRIVER_OK, RESET). In that sense, the destination device waits until all the guest memory / state is migrated to start pinning all the memory. The proposal is to bind it to the char device life cycle (open vs close), Hmmm, really? If it's the life cycle for char device, the next guest / qemu launch on the same vhost-vdpa device node won't make it work. Maybe my sentence was not accurate, but I think we're on the same page here. Two qemu instances opening the same char device at the same time are not allowed, and vhost_vdpa_release clean all the maps. So the next qemu that opens the char device should see a clean device anyway. I mean the pin can't be done at the time of char device open, where the user address space is not known/bound yet. The earliest point possible for pinning would be until the vhost_attach_mm() call from SET_OWNER is done. Maybe we are deviating, let me start again. Using QEMU code, what I'm proposing is to modify the lifecycle of the .listener member of struct vhost_vdpa. At this moment, the memory listener is registered at vhost_vdpa_dev_start(dev, started=true) call for the last vhost_dev, and is unregistered in both vhost_vdpa_reset_status and vhost_vdpa_cleanup. My original proposal was just to move the memory listener registration to the last vhost_vdpa_init, and remove the unregister from vhost_vdpa_reset_status. The calls to vhost_vdpa_dma_map/unmap would be the same, the device should not realize this change. This can address LM downtime latency for sure, but it won't help downtime during dynamic SVQ switch - which still needs to go through the full unmap/map cycle (that includes the slow part for pinning) from passthrough to SVQ mode. Be noted not every device could work with a separate ASID for SVQ descriptors. The fix should expect to work on normal vDPA vendor devices without a separate descriptor ASID, with platform IOMMU underneath or with on-chip IOMMU. At this moment the SVQ switch is very inefficient mapping-wise, as it unmap all the GPA->HVA maps and overrides it. In particular, SVQ is allocated in low regions of the iova space, and then the guest memory is allocated in this new IOVA region incrementally. Yep. The key to build this fast path for SVQ switching I think is to maintain the identity mapping for the passthrough queues so that QEMU can reuse the old mappings for guest memory (e.g. GIOVA identity mapped to GPA) while incrementally adding new mappings for SVQ vrings. We can optimize that if we place SVQ in a free GPA area instead. Here's a question though: it might not be hard to find a free GPA range for the non-vIOMMU case (allocate iova from beyond the 48bit or 52bit ranges), but I'm not sure if easy to find a free GIOVA range for the vIOMMU case - particularly this has to work in the same entire 64bit IOVA address ranges that (for now) QEMU won't be able to "reserve" a specific IOVA ranges for SVQ from the vIOMMU. Do you foresee this can be done for every QEMU emulated vIOMMU (intel-iommu amd-iommu, arm smmu and virito-iommu) so that we can call it out as a generic means for SVQ switching optimization? In the case vIOMMU allocates a new block we will use the same algorithm as now: * Find a new free IOVA chunk of the same size * Map this new SVQ IOVA, that may or may not be the same as SVQ Since we must go through the translation phase to sanitize guest's available descriptors anyway, it has zero added cost. Not sure I followed, this can work but doesn't seem able to reuse the old host kernel mappings for guest memory, hence still requires remap of the entire host IOVA ranges when SVQ IOVA comes along. I think by maintaining 1:1 identity map on guest memory, we don't have to bother tearing down existing HVA->HPA mappings in kernel thus save the expensive pinning calls at large. I don't clearly see under this scheme, how the new SVQ IOVA may work with potential conflict on IOVA space from hotplugged memory - in this case the 1:1 IOVA->GPA identity guest memory mapping can't be kept. Another option would be to move the SVQ vring to a new region, but I don't see
Re: Reducing vdpa migration downtime because of memory pin / maps
On Thu, Jul 6, 2023 at 2:13 AM Si-Wei Liu wrote: > > > > On 7/5/2023 11:03 AM, Eugenio Perez Martin wrote: > > On Tue, Jun 27, 2023 at 8:36 AM Si-Wei Liu wrote: > >> > >> > >> On 6/9/2023 7:32 AM, Eugenio Perez Martin wrote: > >>> On Fri, Jun 9, 2023 at 12:39 AM Si-Wei Liu wrote: > On 6/7/23 01:08, Eugenio Perez Martin wrote: > > On Wed, Jun 7, 2023 at 12:43 AM Si-Wei Liu > > wrote: > >> Sorry for reviving this old thread, I lost the best timing to follow up > >> on this while I was on vacation. I have been working on this and found > >> out some discrepancy, please see below. > >> > >> On 4/5/23 04:37, Eugenio Perez Martin wrote: > >>> Hi! > >>> > >>> As mentioned in the last upstream virtio-networking meeting, one of > >>> the factors that adds more downtime to migration is the handling of > >>> the guest memory (pin, map, etc). At this moment this handling is > >>> bound to the virtio life cycle (DRIVER_OK, RESET). In that sense, the > >>> destination device waits until all the guest memory / state is > >>> migrated to start pinning all the memory. > >>> > >>> The proposal is to bind it to the char device life cycle (open vs > >>> close), > >> Hmmm, really? If it's the life cycle for char device, the next guest / > >> qemu launch on the same vhost-vdpa device node won't make it work. > >> > > Maybe my sentence was not accurate, but I think we're on the same page > > here. > > > > Two qemu instances opening the same char device at the same time are > > not allowed, and vhost_vdpa_release clean all the maps. So the next > > qemu that opens the char device should see a clean device anyway. > I mean the pin can't be done at the time of char device open, where the > user address space is not known/bound yet. The earliest point possible > for pinning would be until the vhost_attach_mm() call from SET_OWNER is > done. > >>> Maybe we are deviating, let me start again. > >>> > >>> Using QEMU code, what I'm proposing is to modify the lifecycle of the > >>> .listener member of struct vhost_vdpa. > >>> > >>> At this moment, the memory listener is registered at > >>> vhost_vdpa_dev_start(dev, started=true) call for the last vhost_dev, > >>> and is unregistered in both vhost_vdpa_reset_status and > >>> vhost_vdpa_cleanup. > >>> > >>> My original proposal was just to move the memory listener registration > >>> to the last vhost_vdpa_init, and remove the unregister from > >>> vhost_vdpa_reset_status. The calls to vhost_vdpa_dma_map/unmap would > >>> be the same, the device should not realize this change. > >> This can address LM downtime latency for sure, but it won't help > >> downtime during dynamic SVQ switch - which still needs to go through the > >> full unmap/map cycle (that includes the slow part for pinning) from > >> passthrough to SVQ mode. Be noted not every device could work with a > >> separate ASID for SVQ descriptors. The fix should expect to work on > >> normal vDPA vendor devices without a separate descriptor ASID, with > >> platform IOMMU underneath or with on-chip IOMMU. > >> > > At this moment the SVQ switch is very inefficient mapping-wise, as it > > unmap all the GPA->HVA maps and overrides it. In particular, SVQ is > > allocated in low regions of the iova space, and then the guest memory > > is allocated in this new IOVA region incrementally. > Yep. The key to build this fast path for SVQ switching I think is to > maintain the identity mapping for the passthrough queues so that QEMU > can reuse the old mappings for guest memory (e.g. GIOVA identity mapped > to GPA) while incrementally adding new mappings for SVQ vrings. > > > > > We can optimize that if we place SVQ in a free GPA area instead. > Here's a question though: it might not be hard to find a free GPA range > for the non-vIOMMU case (allocate iova from beyond the 48bit or 52bit > ranges), but I'm not sure if easy to find a free GIOVA range for the > vIOMMU case - particularly this has to work in the same entire 64bit > IOVA address ranges that (for now) QEMU won't be able to "reserve" a > specific IOVA ranges for SVQ from the vIOMMU. Do you foresee this can be > done for every QEMU emulated vIOMMU (intel-iommu amd-iommu, arm smmu and > virito-iommu) so that we can call it out as a generic means for SVQ > switching optimization? > In the case vIOMMU allocates a new block we will use the same algorithm as now: * Find a new free IOVA chunk of the same size * Map this new SVQ IOVA, that may or may not be the same as SVQ Since we must go through the translation phase to sanitize guest's available descriptors anyway, it has zero added cost. Another option would be to move the SVQ vring to a new region, but I don't see any advantage on maintaining 1:1 mapping at that point. > If this QEMU/vIOMMU "hack" is not universally feasible, I would rather > build a fast path in the kernel via a new vhost IOTLB
Re: Reducing vdpa migration downtime because of memory pin / maps
On 7/5/2023 11:03 AM, Eugenio Perez Martin wrote: On Tue, Jun 27, 2023 at 8:36 AM Si-Wei Liu wrote: On 6/9/2023 7:32 AM, Eugenio Perez Martin wrote: On Fri, Jun 9, 2023 at 12:39 AM Si-Wei Liu wrote: On 6/7/23 01:08, Eugenio Perez Martin wrote: On Wed, Jun 7, 2023 at 12:43 AM Si-Wei Liu wrote: Sorry for reviving this old thread, I lost the best timing to follow up on this while I was on vacation. I have been working on this and found out some discrepancy, please see below. On 4/5/23 04:37, Eugenio Perez Martin wrote: Hi! As mentioned in the last upstream virtio-networking meeting, one of the factors that adds more downtime to migration is the handling of the guest memory (pin, map, etc). At this moment this handling is bound to the virtio life cycle (DRIVER_OK, RESET). In that sense, the destination device waits until all the guest memory / state is migrated to start pinning all the memory. The proposal is to bind it to the char device life cycle (open vs close), Hmmm, really? If it's the life cycle for char device, the next guest / qemu launch on the same vhost-vdpa device node won't make it work. Maybe my sentence was not accurate, but I think we're on the same page here. Two qemu instances opening the same char device at the same time are not allowed, and vhost_vdpa_release clean all the maps. So the next qemu that opens the char device should see a clean device anyway. I mean the pin can't be done at the time of char device open, where the user address space is not known/bound yet. The earliest point possible for pinning would be until the vhost_attach_mm() call from SET_OWNER is done. Maybe we are deviating, let me start again. Using QEMU code, what I'm proposing is to modify the lifecycle of the .listener member of struct vhost_vdpa. At this moment, the memory listener is registered at vhost_vdpa_dev_start(dev, started=true) call for the last vhost_dev, and is unregistered in both vhost_vdpa_reset_status and vhost_vdpa_cleanup. My original proposal was just to move the memory listener registration to the last vhost_vdpa_init, and remove the unregister from vhost_vdpa_reset_status. The calls to vhost_vdpa_dma_map/unmap would be the same, the device should not realize this change. This can address LM downtime latency for sure, but it won't help downtime during dynamic SVQ switch - which still needs to go through the full unmap/map cycle (that includes the slow part for pinning) from passthrough to SVQ mode. Be noted not every device could work with a separate ASID for SVQ descriptors. The fix should expect to work on normal vDPA vendor devices without a separate descriptor ASID, with platform IOMMU underneath or with on-chip IOMMU. At this moment the SVQ switch is very inefficient mapping-wise, as it unmap all the GPA->HVA maps and overrides it. In particular, SVQ is allocated in low regions of the iova space, and then the guest memory is allocated in this new IOVA region incrementally. Yep. The key to build this fast path for SVQ switching I think is to maintain the identity mapping for the passthrough queues so that QEMU can reuse the old mappings for guest memory (e.g. GIOVA identity mapped to GPA) while incrementally adding new mappings for SVQ vrings. We can optimize that if we place SVQ in a free GPA area instead. Here's a question though: it might not be hard to find a free GPA range for the non-vIOMMU case (allocate iova from beyond the 48bit or 52bit ranges), but I'm not sure if easy to find a free GIOVA range for the vIOMMU case - particularly this has to work in the same entire 64bit IOVA address ranges that (for now) QEMU won't be able to "reserve" a specific IOVA ranges for SVQ from the vIOMMU. Do you foresee this can be done for every QEMU emulated vIOMMU (intel-iommu amd-iommu, arm smmu and virito-iommu) so that we can call it out as a generic means for SVQ switching optimization? If this QEMU/vIOMMU "hack" is not universally feasible, I would rather build a fast path in the kernel via a new vhost IOTLB command, say INVALIDATE_AND_UPDATE_ALL, to atomically flush all existing (passthrough) mappings and update to use the SVQ ones in a single batch, while keeping the pages for guest memory always pinned (the kernel will make this decision). This doesn't expose pinning to userspace, and can also fix downtime issue. All of the "translations" still need to be done, to ensure the guest doesn't have access to SVQ vring. That way, qemu will not send all the unmaps & maps, only the new ones. And vhost/vdpa does not need to call unpin_user_page / pin_user_pages for all the guest memory. More optimizations include the batching of the SVQ vrings. Nods. One of the concerns was that it could delay VM initialization, and I didn't profile it but I think that may be the case. Yes, that's the concern here - we should not introduce regression to normal VM boot process/time. In case of large VM it's very easy to see the side effect if we go
Re: Reducing vdpa migration downtime because of memory pin / maps
On Tue, Jun 27, 2023 at 8:36 AM Si-Wei Liu wrote: > > > > On 6/9/2023 7:32 AM, Eugenio Perez Martin wrote: > > On Fri, Jun 9, 2023 at 12:39 AM Si-Wei Liu wrote: > >> > >> On 6/7/23 01:08, Eugenio Perez Martin wrote: > >>> On Wed, Jun 7, 2023 at 12:43 AM Si-Wei Liu wrote: > Sorry for reviving this old thread, I lost the best timing to follow up > on this while I was on vacation. I have been working on this and found > out some discrepancy, please see below. > > On 4/5/23 04:37, Eugenio Perez Martin wrote: > > Hi! > > > > As mentioned in the last upstream virtio-networking meeting, one of > > the factors that adds more downtime to migration is the handling of > > the guest memory (pin, map, etc). At this moment this handling is > > bound to the virtio life cycle (DRIVER_OK, RESET). In that sense, the > > destination device waits until all the guest memory / state is > > migrated to start pinning all the memory. > > > > The proposal is to bind it to the char device life cycle (open vs > > close), > Hmmm, really? If it's the life cycle for char device, the next guest / > qemu launch on the same vhost-vdpa device node won't make it work. > > >>> Maybe my sentence was not accurate, but I think we're on the same page > >>> here. > >>> > >>> Two qemu instances opening the same char device at the same time are > >>> not allowed, and vhost_vdpa_release clean all the maps. So the next > >>> qemu that opens the char device should see a clean device anyway. > >> I mean the pin can't be done at the time of char device open, where the > >> user address space is not known/bound yet. The earliest point possible > >> for pinning would be until the vhost_attach_mm() call from SET_OWNER is > >> done. > > Maybe we are deviating, let me start again. > > > > Using QEMU code, what I'm proposing is to modify the lifecycle of the > > .listener member of struct vhost_vdpa. > > > > At this moment, the memory listener is registered at > > vhost_vdpa_dev_start(dev, started=true) call for the last vhost_dev, > > and is unregistered in both vhost_vdpa_reset_status and > > vhost_vdpa_cleanup. > > > > My original proposal was just to move the memory listener registration > > to the last vhost_vdpa_init, and remove the unregister from > > vhost_vdpa_reset_status. The calls to vhost_vdpa_dma_map/unmap would > > be the same, the device should not realize this change. > This can address LM downtime latency for sure, but it won't help > downtime during dynamic SVQ switch - which still needs to go through the > full unmap/map cycle (that includes the slow part for pinning) from > passthrough to SVQ mode. Be noted not every device could work with a > separate ASID for SVQ descriptors. The fix should expect to work on > normal vDPA vendor devices without a separate descriptor ASID, with > platform IOMMU underneath or with on-chip IOMMU. > At this moment the SVQ switch is very inefficient mapping-wise, as it unmap all the GPA->HVA maps and overrides it. In particular, SVQ is allocated in low regions of the iova space, and then the guest memory is allocated in this new IOVA region incrementally. We can optimize that if we place SVQ in a free GPA area instead. All of the "translations" still need to be done, to ensure the guest doesn't have access to SVQ vring. That way, qemu will not send all the unmaps & maps, only the new ones. And vhost/vdpa does not need to call unpin_user_page / pin_user_pages for all the guest memory. More optimizations include the batching of the SVQ vrings. > > > > One of the concerns was that it could delay VM initialization, and I > > didn't profile it but I think that may be the case. > Yes, that's the concern here - we should not introduce regression to > normal VM boot process/time. In case of large VM it's very easy to see > the side effect if we go this way. > > > I'm not sure about > > the right fix but I think the change is easy to profile. If that is > > the case, we could: > > * use a flag (listener->address_space ?) and only register the > > listener in _init if waiting for a migration, do it in _start > > otherwise. > Just doing this alone won't help SVQ mode switch downtime, see the > reason stated above. > > > * something like io_uring, where the map can be done in parallel and > > we can fail _start if some of them fails. > This can alleviate the problem somehow, but still sub-optimal and not > scalable with larger size. I'd like zero or least pinning to be done at > the SVQ switch or migration time. > To reduce even further the pinning at SVQ time we would need to preallocate SVQ vrings before suspending the device. > > > >> Actually I think the counterpart vhost_detach_mm() only gets > >> handled in vhost_vdpa_release() at device close time is a resulting > >> artifact and amiss of today's vhost protocol - the opposite RESET_OWNER > >> call is not made mandatory hence only seen implemented in vhost-net > >>
Re: Reducing vdpa migration downtime because of memory pin / maps
On 6/9/2023 7:32 AM, Eugenio Perez Martin wrote: On Fri, Jun 9, 2023 at 12:39 AM Si-Wei Liu wrote: On 6/7/23 01:08, Eugenio Perez Martin wrote: On Wed, Jun 7, 2023 at 12:43 AM Si-Wei Liu wrote: Sorry for reviving this old thread, I lost the best timing to follow up on this while I was on vacation. I have been working on this and found out some discrepancy, please see below. On 4/5/23 04:37, Eugenio Perez Martin wrote: Hi! As mentioned in the last upstream virtio-networking meeting, one of the factors that adds more downtime to migration is the handling of the guest memory (pin, map, etc). At this moment this handling is bound to the virtio life cycle (DRIVER_OK, RESET). In that sense, the destination device waits until all the guest memory / state is migrated to start pinning all the memory. The proposal is to bind it to the char device life cycle (open vs close), Hmmm, really? If it's the life cycle for char device, the next guest / qemu launch on the same vhost-vdpa device node won't make it work. Maybe my sentence was not accurate, but I think we're on the same page here. Two qemu instances opening the same char device at the same time are not allowed, and vhost_vdpa_release clean all the maps. So the next qemu that opens the char device should see a clean device anyway. I mean the pin can't be done at the time of char device open, where the user address space is not known/bound yet. The earliest point possible for pinning would be until the vhost_attach_mm() call from SET_OWNER is done. Maybe we are deviating, let me start again. Using QEMU code, what I'm proposing is to modify the lifecycle of the .listener member of struct vhost_vdpa. At this moment, the memory listener is registered at vhost_vdpa_dev_start(dev, started=true) call for the last vhost_dev, and is unregistered in both vhost_vdpa_reset_status and vhost_vdpa_cleanup. My original proposal was just to move the memory listener registration to the last vhost_vdpa_init, and remove the unregister from vhost_vdpa_reset_status. The calls to vhost_vdpa_dma_map/unmap would be the same, the device should not realize this change. This can address LM downtime latency for sure, but it won't help downtime during dynamic SVQ switch - which still needs to go through the full unmap/map cycle (that includes the slow part for pinning) from passthrough to SVQ mode. Be noted not every device could work with a separate ASID for SVQ descriptors. The fix should expect to work on normal vDPA vendor devices without a separate descriptor ASID, with platform IOMMU underneath or with on-chip IOMMU. One of the concerns was that it could delay VM initialization, and I didn't profile it but I think that may be the case. Yes, that's the concern here - we should not introduce regression to normal VM boot process/time. In case of large VM it's very easy to see the side effect if we go this way. I'm not sure about the right fix but I think the change is easy to profile. If that is the case, we could: * use a flag (listener->address_space ?) and only register the listener in _init if waiting for a migration, do it in _start otherwise. Just doing this alone won't help SVQ mode switch downtime, see the reason stated above. * something like io_uring, where the map can be done in parallel and we can fail _start if some of them fails. This can alleviate the problem somehow, but still sub-optimal and not scalable with larger size. I'd like zero or least pinning to be done at the SVQ switch or migration time. Actually I think the counterpart vhost_detach_mm() only gets handled in vhost_vdpa_release() at device close time is a resulting artifact and amiss of today's vhost protocol - the opposite RESET_OWNER call is not made mandatory hence only seen implemented in vhost-net device today. One qemu instance could well exec(3) another new qemu instance to live upgrade itself while keeping all emulated devices and guest alive. The current vhost design simply prohibits this from happening. Ok, I was not aware of this. Thanks for explaining it! so all the guest memory can be pinned for all the guest / qemu lifecycle. I think to tie pinning to guest / qemu process life cycle makes more sense. Essentially this pinning part needs to be decoupled from the iotlb mapping abstraction layer, and can / should work as a standalone uAPI. Such that QEMU at the destination may launch and pin all guest's memory as needed without having to start the device, while awaiting any incoming migration request. Though problem is, there's no existing vhost uAPI that could properly serve as the vehicle for that. SET_OWNER / SET_MEM_TABLE / RESET_OWNER seems a remote fit.. Any objection against introducing a new but clean vhost uAPI for pinning guest pages, subject to guest's life cycle? I think that to pin or not pin memory maps should be a kernel decision, not to be driven by qemu. It's kernel decision for sure. I am with this part. I'm not
Re: Reducing vdpa migration downtime because of memory pin / maps
On Fri, Jun 9, 2023 at 12:39 AM Si-Wei Liu wrote: > > > On 6/7/23 01:08, Eugenio Perez Martin wrote: > > On Wed, Jun 7, 2023 at 12:43 AM Si-Wei Liu wrote: > >> Sorry for reviving this old thread, I lost the best timing to follow up > >> on this while I was on vacation. I have been working on this and found > >> out some discrepancy, please see below. > >> > >> On 4/5/23 04:37, Eugenio Perez Martin wrote: > >>> Hi! > >>> > >>> As mentioned in the last upstream virtio-networking meeting, one of > >>> the factors that adds more downtime to migration is the handling of > >>> the guest memory (pin, map, etc). At this moment this handling is > >>> bound to the virtio life cycle (DRIVER_OK, RESET). In that sense, the > >>> destination device waits until all the guest memory / state is > >>> migrated to start pinning all the memory. > >>> > >>> The proposal is to bind it to the char device life cycle (open vs > >>> close), > >> Hmmm, really? If it's the life cycle for char device, the next guest / > >> qemu launch on the same vhost-vdpa device node won't make it work. > >> > > Maybe my sentence was not accurate, but I think we're on the same page here. > > > > Two qemu instances opening the same char device at the same time are > > not allowed, and vhost_vdpa_release clean all the maps. So the next > > qemu that opens the char device should see a clean device anyway. > > I mean the pin can't be done at the time of char device open, where the > user address space is not known/bound yet. The earliest point possible > for pinning would be until the vhost_attach_mm() call from SET_OWNER is > done. Maybe we are deviating, let me start again. Using QEMU code, what I'm proposing is to modify the lifecycle of the .listener member of struct vhost_vdpa. At this moment, the memory listener is registered at vhost_vdpa_dev_start(dev, started=true) call for the last vhost_dev, and is unregistered in both vhost_vdpa_reset_status and vhost_vdpa_cleanup. My original proposal was just to move the memory listener registration to the last vhost_vdpa_init, and remove the unregister from vhost_vdpa_reset_status. The calls to vhost_vdpa_dma_map/unmap would be the same, the device should not realize this change. One of the concerns was that it could delay VM initialization, and I didn't profile it but I think that may be the case. I'm not sure about the right fix but I think the change is easy to profile. If that is the case, we could: * use a flag (listener->address_space ?) and only register the listener in _init if waiting for a migration, do it in _start otherwise. * something like io_uring, where the map can be done in parallel and we can fail _start if some of them fails. > Actually I think the counterpart vhost_detach_mm() only gets > handled in vhost_vdpa_release() at device close time is a resulting > artifact and amiss of today's vhost protocol - the opposite RESET_OWNER > call is not made mandatory hence only seen implemented in vhost-net > device today. One qemu instance could well exec(3) another new qemu > instance to live upgrade itself while keeping all emulated devices and > guest alive. The current vhost design simply prohibits this from happening. > Ok, I was not aware of this. Thanks for explaining it! > > > > >>>so all the guest memory can be pinned for all the guest / qemu > >>> lifecycle. > >> I think to tie pinning to guest / qemu process life cycle makes more > >> sense. Essentially this pinning part needs to be decoupled from the > >> iotlb mapping abstraction layer, and can / should work as a standalone > >> uAPI. Such that QEMU at the destination may launch and pin all guest's > >> memory as needed without having to start the device, while awaiting any > >> incoming migration request. Though problem is, there's no existing vhost > >> uAPI that could properly serve as the vehicle for that. SET_OWNER / > >> SET_MEM_TABLE / RESET_OWNER seems a remote fit.. Any objection against > >> introducing a new but clean vhost uAPI for pinning guest pages, subject > >> to guest's life cycle? > >> > > I think that to pin or not pin memory maps should be a kernel > > decision, not to be driven by qemu. > > It's kernel decision for sure. I am with this part. > > > I'm not against it if needed, but > > let me know if the current "clean at close" address your concerns. > > To better facilitate QEMU exec (live update) case, I propose we add new > vhost uAPI pair for explicit pinning request - which would live with > user mm's, or more precisely qemu instance's lifecycle. > Ok I see your problem better now, but I think it should be solved at kernel level. Does that live update need to forcefully unpin and pin the memory again, or that is just a consequence of how it works the memory listener right now? Why not extend the RESET_OWNER to the rest of devices? It seems the most natural way to me. Thanks! > > > >> Another concern is the use_va stuff, originally it tags to the device > >> level and is made static at
Re: Reducing vdpa migration downtime because of memory pin / maps
On Fri, Jun 9, 2023 at 6:39 AM Si-Wei Liu wrote: > > > On 6/7/23 01:08, Eugenio Perez Martin wrote: > > On Wed, Jun 7, 2023 at 12:43 AM Si-Wei Liu wrote: > >> Sorry for reviving this old thread, I lost the best timing to follow up > >> on this while I was on vacation. I have been working on this and found > >> out some discrepancy, please see below. > >> > >> On 4/5/23 04:37, Eugenio Perez Martin wrote: > >>> Hi! > >>> > >>> As mentioned in the last upstream virtio-networking meeting, one of > >>> the factors that adds more downtime to migration is the handling of > >>> the guest memory (pin, map, etc). At this moment this handling is > >>> bound to the virtio life cycle (DRIVER_OK, RESET). In that sense, the > >>> destination device waits until all the guest memory / state is > >>> migrated to start pinning all the memory. > >>> > >>> The proposal is to bind it to the char device life cycle (open vs > >>> close), > >> Hmmm, really? If it's the life cycle for char device, the next guest / > >> qemu launch on the same vhost-vdpa device node won't make it work. > >> > > Maybe my sentence was not accurate, but I think we're on the same page here. > > > > Two qemu instances opening the same char device at the same time are > > not allowed, and vhost_vdpa_release clean all the maps. So the next > > qemu that opens the char device should see a clean device anyway. > > I mean the pin can't be done at the time of char device open, where the > user address space is not known/bound yet. The earliest point possible > for pinning would be until the vhost_attach_mm() call from SET_OWNER is > done. Actually I think the counterpart vhost_detach_mm() only gets > handled in vhost_vdpa_release() at device close time is a resulting > artifact and amiss of today's vhost protocol - the opposite RESET_OWNER > call is not made mandatory hence only seen implemented in vhost-net > device today. One qemu instance could well exec(3) another new qemu > instance to live upgrade itself while keeping all emulated devices and > guest alive. The current vhost design simply prohibits this from happening. I'm not sure I fully understand the issue you mention here. What is missed and can iommufd help anyhow? > > > > > >>>so all the guest memory can be pinned for all the guest / qemu > >>> lifecycle. > >> I think to tie pinning to guest / qemu process life cycle makes more > >> sense. Essentially this pinning part needs to be decoupled from the > >> iotlb mapping abstraction layer, and can / should work as a standalone > >> uAPI. Such that QEMU at the destination may launch and pin all guest's > >> memory as needed without having to start the device, while awaiting any > >> incoming migration request. Though problem is, there's no existing vhost > >> uAPI that could properly serve as the vehicle for that. SET_OWNER / > >> SET_MEM_TABLE / RESET_OWNER seems a remote fit.. Any objection against > >> introducing a new but clean vhost uAPI for pinning guest pages, subject > >> to guest's life cycle? > >> > > I think that to pin or not pin memory maps should be a kernel > > decision, not to be driven by qemu. > > It's kernel decision for sure. I am with this part. > > > I'm not against it if needed, but > > let me know if the current "clean at close" address your concerns. > > To better facilitate QEMU exec (live update) case, I propose we add new > vhost uAPI pair for explicit pinning request - which would live with > user mm's, or more precisely qemu instance's lifecycle. Is this something similar to iommufd? Btw, I'm not sure whether or not it's good to expose pinning to the userspace. For example, vhost allows virtual mapping instead of dma mapping which doesn't require pinning at all. Thanks > > > > >> Another concern is the use_va stuff, originally it tags to the device > >> level and is made static at the time of device instantiation, which is > >> fine. But others to come just find a new home at per-group level or > >> per-vq level struct. Hard to tell whether or not pinning is actually > >> needed for the latter use_va friends, as they are essentially tied to > >> the virtio life cycle or feature negotiation. While guest / Qemu starts > >> way earlier than that. Perhaps just ignore those sub-device level use_va > >> usages? Presumably !use_va at the device level is sufficient to infer > >> the need of pinning for device? > >> > > I don't follow this. But I have the feeling that the subject of my > > original mail is way more accurate if I would have said just "memory > > maps". > > I think the iotlb layer in vhost-vdpa just provides the abstraction for > mapping, not pinning. Although in some case mapping implicitly relies on > pinning for DMA purpose, it doesn't have to tie to that in uAPI > semantics. We can do explicit on-demand pinning for cases for e.g. > warming up device at live migration destination. > > > > > > I still consider the way to fix it is to actually delegate that to the > > kernel vdpa, so it can choose if
Re: Reducing vdpa migration downtime because of memory pin / maps
On 6/7/23 01:08, Eugenio Perez Martin wrote: On Wed, Jun 7, 2023 at 12:43 AM Si-Wei Liu wrote: Sorry for reviving this old thread, I lost the best timing to follow up on this while I was on vacation. I have been working on this and found out some discrepancy, please see below. On 4/5/23 04:37, Eugenio Perez Martin wrote: Hi! As mentioned in the last upstream virtio-networking meeting, one of the factors that adds more downtime to migration is the handling of the guest memory (pin, map, etc). At this moment this handling is bound to the virtio life cycle (DRIVER_OK, RESET). In that sense, the destination device waits until all the guest memory / state is migrated to start pinning all the memory. The proposal is to bind it to the char device life cycle (open vs close), Hmmm, really? If it's the life cycle for char device, the next guest / qemu launch on the same vhost-vdpa device node won't make it work. Maybe my sentence was not accurate, but I think we're on the same page here. Two qemu instances opening the same char device at the same time are not allowed, and vhost_vdpa_release clean all the maps. So the next qemu that opens the char device should see a clean device anyway. I mean the pin can't be done at the time of char device open, where the user address space is not known/bound yet. The earliest point possible for pinning would be until the vhost_attach_mm() call from SET_OWNER is done. Actually I think the counterpart vhost_detach_mm() only gets handled in vhost_vdpa_release() at device close time is a resulting artifact and amiss of today's vhost protocol - the opposite RESET_OWNER call is not made mandatory hence only seen implemented in vhost-net device today. One qemu instance could well exec(3) another new qemu instance to live upgrade itself while keeping all emulated devices and guest alive. The current vhost design simply prohibits this from happening. so all the guest memory can be pinned for all the guest / qemu lifecycle. I think to tie pinning to guest / qemu process life cycle makes more sense. Essentially this pinning part needs to be decoupled from the iotlb mapping abstraction layer, and can / should work as a standalone uAPI. Such that QEMU at the destination may launch and pin all guest's memory as needed without having to start the device, while awaiting any incoming migration request. Though problem is, there's no existing vhost uAPI that could properly serve as the vehicle for that. SET_OWNER / SET_MEM_TABLE / RESET_OWNER seems a remote fit.. Any objection against introducing a new but clean vhost uAPI for pinning guest pages, subject to guest's life cycle? I think that to pin or not pin memory maps should be a kernel decision, not to be driven by qemu. It's kernel decision for sure. I am with this part. I'm not against it if needed, but let me know if the current "clean at close" address your concerns. To better facilitate QEMU exec (live update) case, I propose we add new vhost uAPI pair for explicit pinning request - which would live with user mm's, or more precisely qemu instance's lifecycle. Another concern is the use_va stuff, originally it tags to the device level and is made static at the time of device instantiation, which is fine. But others to come just find a new home at per-group level or per-vq level struct. Hard to tell whether or not pinning is actually needed for the latter use_va friends, as they are essentially tied to the virtio life cycle or feature negotiation. While guest / Qemu starts way earlier than that. Perhaps just ignore those sub-device level use_va usages? Presumably !use_va at the device level is sufficient to infer the need of pinning for device? I don't follow this. But I have the feeling that the subject of my original mail is way more accurate if I would have said just "memory maps". I think the iotlb layer in vhost-vdpa just provides the abstraction for mapping, not pinning. Although in some case mapping implicitly relies on pinning for DMA purpose, it doesn't have to tie to that in uAPI semantics. We can do explicit on-demand pinning for cases for e.g. warming up device at live migration destination. I still consider the way to fix it is to actually delegate that to the kernel vdpa, so it can choose if a particular ASID needs the pin or not. But let me know if I missed something. You can disregard this for now. I will discuss that further with you guys while bind_mm and per-group use_va stuffs are landed. Thanks! -Siwei Thanks! Regards, -Siwei This has two main problems: * At this moment the reset semantics forces the vdpa device to unmap all the memory. So this change needs a vhost vdpa feature flag. * This may increase the initialization time. Maybe we can delay it if qemu is not the destination of a LM. Anyway I think this should be done as an optimization on top. Any ideas or comments in this regard? Thanks!
Re: Reducing vdpa migration downtime because of memory pin / maps
On Wed, Jun 7, 2023 at 12:43 AM Si-Wei Liu wrote: > > Sorry for reviving this old thread, I lost the best timing to follow up > on this while I was on vacation. I have been working on this and found > out some discrepancy, please see below. > > On 4/5/23 04:37, Eugenio Perez Martin wrote: > > Hi! > > > > As mentioned in the last upstream virtio-networking meeting, one of > > the factors that adds more downtime to migration is the handling of > > the guest memory (pin, map, etc). At this moment this handling is > > bound to the virtio life cycle (DRIVER_OK, RESET). In that sense, the > > destination device waits until all the guest memory / state is > > migrated to start pinning all the memory. > > > > The proposal is to bind it to the char device life cycle (open vs > > close), > > Hmmm, really? If it's the life cycle for char device, the next guest / > qemu launch on the same vhost-vdpa device node won't make it work. > Maybe my sentence was not accurate, but I think we're on the same page here. Two qemu instances opening the same char device at the same time are not allowed, and vhost_vdpa_release clean all the maps. So the next qemu that opens the char device should see a clean device anyway. > > > so all the guest memory can be pinned for all the guest / qemu > > lifecycle. > > I think to tie pinning to guest / qemu process life cycle makes more > sense. Essentially this pinning part needs to be decoupled from the > iotlb mapping abstraction layer, and can / should work as a standalone > uAPI. Such that QEMU at the destination may launch and pin all guest's > memory as needed without having to start the device, while awaiting any > incoming migration request. Though problem is, there's no existing vhost > uAPI that could properly serve as the vehicle for that. SET_OWNER / > SET_MEM_TABLE / RESET_OWNER seems a remote fit.. Any objection against > introducing a new but clean vhost uAPI for pinning guest pages, subject > to guest's life cycle? > I think that to pin or not pin memory maps should be a kernel decision, not to be driven by qemu. I'm not against it if needed, but let me know if the current "clean at close" address your concerns. > Another concern is the use_va stuff, originally it tags to the device > level and is made static at the time of device instantiation, which is > fine. But others to come just find a new home at per-group level or > per-vq level struct. Hard to tell whether or not pinning is actually > needed for the latter use_va friends, as they are essentially tied to > the virtio life cycle or feature negotiation. While guest / Qemu starts > way earlier than that. Perhaps just ignore those sub-device level use_va > usages? Presumably !use_va at the device level is sufficient to infer > the need of pinning for device? > I don't follow this. But I have the feeling that the subject of my original mail is way more accurate if I would have said just "memory maps". I still consider the way to fix it is to actually delegate that to the kernel vdpa, so it can choose if a particular ASID needs the pin or not. But let me know if I missed something. Thanks! > Regards, > -Siwei > > > > > > This has two main problems: > > * At this moment the reset semantics forces the vdpa device to unmap > > all the memory. So this change needs a vhost vdpa feature flag. > > * This may increase the initialization time. Maybe we can delay it if > > qemu is not the destination of a LM. Anyway I think this should be > > done as an optimization on top. > > > > Any ideas or comments in this regard? > > > > Thanks! > > >
Re: Reducing vdpa migration downtime because of memory pin / maps
Sorry for reviving this old thread, I lost the best timing to follow up on this while I was on vacation. I have been working on this and found out some discrepancy, please see below. On 4/5/23 04:37, Eugenio Perez Martin wrote: Hi! As mentioned in the last upstream virtio-networking meeting, one of the factors that adds more downtime to migration is the handling of the guest memory (pin, map, etc). At this moment this handling is bound to the virtio life cycle (DRIVER_OK, RESET). In that sense, the destination device waits until all the guest memory / state is migrated to start pinning all the memory. The proposal is to bind it to the char device life cycle (open vs close), Hmmm, really? If it's the life cycle for char device, the next guest / qemu launch on the same vhost-vdpa device node won't make it work. so all the guest memory can be pinned for all the guest / qemu lifecycle. I think to tie pinning to guest / qemu process life cycle makes more sense. Essentially this pinning part needs to be decoupled from the iotlb mapping abstraction layer, and can / should work as a standalone uAPI. Such that QEMU at the destination may launch and pin all guest's memory as needed without having to start the device, while awaiting any incoming migration request. Though problem is, there's no existing vhost uAPI that could properly serve as the vehicle for that. SET_OWNER / SET_MEM_TABLE / RESET_OWNER seems a remote fit.. Any objection against introducing a new but clean vhost uAPI for pinning guest pages, subject to guest's life cycle? Another concern is the use_va stuff, originally it tags to the device level and is made static at the time of device instantiation, which is fine. But others to come just find a new home at per-group level or per-vq level struct. Hard to tell whether or not pinning is actually needed for the latter use_va friends, as they are essentially tied to the virtio life cycle or feature negotiation. While guest / Qemu starts way earlier than that. Perhaps just ignore those sub-device level use_va usages? Presumably !use_va at the device level is sufficient to infer the need of pinning for device? Regards, -Siwei This has two main problems: * At this moment the reset semantics forces the vdpa device to unmap all the memory. So this change needs a vhost vdpa feature flag. * This may increase the initialization time. Maybe we can delay it if qemu is not the destination of a LM. Anyway I think this should be done as an optimization on top. Any ideas or comments in this regard? Thanks!
Re: Reducing vdpa migration downtime because of memory pin / maps
On Wed, Apr 12, 2023 at 8:19 AM Jason Wang wrote: > > On Wed, Apr 12, 2023 at 1:56 PM Jason Wang wrote: > > > > On Tue, Apr 11, 2023 at 8:34 PM Eugenio Perez Martin > > wrote: > > > > > > On Wed, Apr 5, 2023 at 1:37 PM Eugenio Perez Martin > > > wrote: > > > > > > > > Hi! > > > > > > > > As mentioned in the last upstream virtio-networking meeting, one of > > > > the factors that adds more downtime to migration is the handling of > > > > the guest memory (pin, map, etc). At this moment this handling is > > > > bound to the virtio life cycle (DRIVER_OK, RESET). In that sense, the > > > > destination device waits until all the guest memory / state is > > > > migrated to start pinning all the memory. > > > > > > > > The proposal is to bind it to the char device life cycle (open vs > > > > close), so all the guest memory can be pinned for all the guest / qemu > > > > lifecycle. > > > > > > > > This has two main problems: > > > > * At this moment the reset semantics forces the vdpa device to unmap > > > > all the memory. So this change needs a vhost vdpa feature flag. > > > > * This may increase the initialization time. Maybe we can delay it if > > > > qemu is not the destination of a LM. Anyway I think this should be > > > > done as an optimization on top. > > > > > > > > > > Expanding on this we could reduce the pinning even more now that vring > > > supports VA [1] with the emulated CVQ. > > > > Note that VA for hardware means the device needs to support page fault > > through either PRI or vendor specific interface. > > > > > > > > Something like: > > > - Add VHOST_VRING_GROUP_CAN_USE_VA ioctl to check if a given VQ group > > > capability. Passthrough devices with emulated CVQ would return false > > > for the dataplane and true for the control vq group. > > We don't even need this actually, since the pinning is not visible to > the userspace. Userspace can only see the IOTLB abstraction actually. > > We can invent a group->use_va, then when we attach AS to a group that > can use va, we can avoid the pinning. > That would solve one part for sure, but SVQ will keep translating HVA to SVQ IOVA, and then the kernel needs to translate it back. With VHOST_VRING_GROUP_CAN_USE_VA, the SVQ and the kernel skip all translation. Thanks! > Thanks > > > > - If that is true, qemu does not need to map and translate addresses > > > for CVQ but to directly provide VA for buffers. This avoids pinning, > > > translations, etc in this case. > > > > For CVQ yes, but we only avoid the pinning for CVQ not others. > > > > Thanks > > > > > > > > Thanks! > > > > > > [1] > > > https://lore.kernel.org/virtualization/20230404131326.44403-2-sgarz...@redhat.com/ > > > >
Re: Reducing vdpa migration downtime because of memory pin / maps
On Wed, Apr 12, 2023 at 1:56 PM Jason Wang wrote: > > On Tue, Apr 11, 2023 at 8:34 PM Eugenio Perez Martin > wrote: > > > > On Wed, Apr 5, 2023 at 1:37 PM Eugenio Perez Martin > > wrote: > > > > > > Hi! > > > > > > As mentioned in the last upstream virtio-networking meeting, one of > > > the factors that adds more downtime to migration is the handling of > > > the guest memory (pin, map, etc). At this moment this handling is > > > bound to the virtio life cycle (DRIVER_OK, RESET). In that sense, the > > > destination device waits until all the guest memory / state is > > > migrated to start pinning all the memory. > > > > > > The proposal is to bind it to the char device life cycle (open vs > > > close), so all the guest memory can be pinned for all the guest / qemu > > > lifecycle. > > > > > > This has two main problems: > > > * At this moment the reset semantics forces the vdpa device to unmap > > > all the memory. So this change needs a vhost vdpa feature flag. > > > * This may increase the initialization time. Maybe we can delay it if > > > qemu is not the destination of a LM. Anyway I think this should be > > > done as an optimization on top. > > > > > > > Expanding on this we could reduce the pinning even more now that vring > > supports VA [1] with the emulated CVQ. > > Note that VA for hardware means the device needs to support page fault > through either PRI or vendor specific interface. > > > > > Something like: > > - Add VHOST_VRING_GROUP_CAN_USE_VA ioctl to check if a given VQ group > > capability. Passthrough devices with emulated CVQ would return false > > for the dataplane and true for the control vq group. We don't even need this actually, since the pinning is not visible to the userspace. Userspace can only see the IOTLB abstraction actually. We can invent a group->use_va, then when we attach AS to a group that can use va, we can avoid the pinning. Thanks > > - If that is true, qemu does not need to map and translate addresses > > for CVQ but to directly provide VA for buffers. This avoids pinning, > > translations, etc in this case. > > For CVQ yes, but we only avoid the pinning for CVQ not others. > > Thanks > > > > > Thanks! > > > > [1] > > https://lore.kernel.org/virtualization/20230404131326.44403-2-sgarz...@redhat.com/ > >
Re: Reducing vdpa migration downtime because of memory pin / maps
On Tue, Apr 11, 2023 at 8:34 PM Eugenio Perez Martin wrote: > > On Wed, Apr 5, 2023 at 1:37 PM Eugenio Perez Martin > wrote: > > > > Hi! > > > > As mentioned in the last upstream virtio-networking meeting, one of > > the factors that adds more downtime to migration is the handling of > > the guest memory (pin, map, etc). At this moment this handling is > > bound to the virtio life cycle (DRIVER_OK, RESET). In that sense, the > > destination device waits until all the guest memory / state is > > migrated to start pinning all the memory. > > > > The proposal is to bind it to the char device life cycle (open vs > > close), so all the guest memory can be pinned for all the guest / qemu > > lifecycle. > > > > This has two main problems: > > * At this moment the reset semantics forces the vdpa device to unmap > > all the memory. So this change needs a vhost vdpa feature flag. > > * This may increase the initialization time. Maybe we can delay it if > > qemu is not the destination of a LM. Anyway I think this should be > > done as an optimization on top. > > > > Expanding on this we could reduce the pinning even more now that vring > supports VA [1] with the emulated CVQ. Note that VA for hardware means the device needs to support page fault through either PRI or vendor specific interface. > > Something like: > - Add VHOST_VRING_GROUP_CAN_USE_VA ioctl to check if a given VQ group > capability. Passthrough devices with emulated CVQ would return false > for the dataplane and true for the control vq group. > - If that is true, qemu does not need to map and translate addresses > for CVQ but to directly provide VA for buffers. This avoids pinning, > translations, etc in this case. For CVQ yes, but we only avoid the pinning for CVQ not others. Thanks > > Thanks! > > [1] > https://lore.kernel.org/virtualization/20230404131326.44403-2-sgarz...@redhat.com/ >
Re: Reducing vdpa migration downtime because of memory pin / maps
On Wed, Apr 5, 2023 at 1:37 PM Eugenio Perez Martin wrote: > > Hi! > > As mentioned in the last upstream virtio-networking meeting, one of > the factors that adds more downtime to migration is the handling of > the guest memory (pin, map, etc). At this moment this handling is > bound to the virtio life cycle (DRIVER_OK, RESET). In that sense, the > destination device waits until all the guest memory / state is > migrated to start pinning all the memory. > > The proposal is to bind it to the char device life cycle (open vs > close), so all the guest memory can be pinned for all the guest / qemu > lifecycle. > > This has two main problems: > * At this moment the reset semantics forces the vdpa device to unmap > all the memory. So this change needs a vhost vdpa feature flag. > * This may increase the initialization time. Maybe we can delay it if > qemu is not the destination of a LM. Anyway I think this should be > done as an optimization on top. > Expanding on this we could reduce the pinning even more now that vring supports VA [1] with the emulated CVQ. Something like: - Add VHOST_VRING_GROUP_CAN_USE_VA ioctl to check if a given VQ group capability. Passthrough devices with emulated CVQ would return false for the dataplane and true for the control vq group. - If that is true, qemu does not need to map and translate addresses for CVQ but to directly provide VA for buffers. This avoids pinning, translations, etc in this case. Thanks! [1] https://lore.kernel.org/virtualization/20230404131326.44403-2-sgarz...@redhat.com/
Re: Reducing vdpa migration downtime because of memory pin / maps
On Tue, Apr 11, 2023 at 2:29 PM Eugenio Perez Martin wrote: > > On Tue, Apr 11, 2023 at 4:26 AM Jason Wang wrote: > > > > On Mon, Apr 10, 2023 at 5:05 PM Eugenio Perez Martin > > wrote: > > > > > > On Mon, Apr 10, 2023 at 5:22 AM Jason Wang wrote: > > > > > > > > On Mon, Apr 10, 2023 at 11:17 AM Longpeng (Mike, Cloud Infrastructure > > > > Service Product Dept.) wrote: > > > > > > > > > > > > > > > > > > > > 在 2023/4/10 10:14, Jason Wang 写道: > > > > > > On Wed, Apr 5, 2023 at 7:38 PM Eugenio Perez Martin > > > > > > wrote: > > > > > >> > > > > > >> Hi! > > > > > >> > > > > > >> As mentioned in the last upstream virtio-networking meeting, one of > > > > > >> the factors that adds more downtime to migration is the handling of > > > > > >> the guest memory (pin, map, etc). At this moment this handling is > > > > > >> bound to the virtio life cycle (DRIVER_OK, RESET). In that sense, > > > > > >> the > > > > > >> destination device waits until all the guest memory / state is > > > > > >> migrated to start pinning all the memory. > > > > > >> > > > > > >> The proposal is to bind it to the char device life cycle (open vs > > > > > >> close), so all the guest memory can be pinned for all the guest / > > > > > >> qemu > > > > > >> lifecycle. > > > > > >> > > > > > >> This has two main problems: > > > > > >> * At this moment the reset semantics forces the vdpa device to > > > > > >> unmap > > > > > >> all the memory. So this change needs a vhost vdpa feature flag. > > > > > > > > > > > > Is this true? I didn't find any codes to unmap the memory in > > > > > > vhost_vdpa_set_status(). > > > > > > > > > > > > > > > > It could depend on the vendor driver, for example, the vdpasim would > > > > > do > > > > > something like that. > > > > > > > > > > vhost_vdpa_set_status->vdpa_reset->vdpasim_reset->vdpasim_do_reset->vhost_iotlb_reset > > > > > > > > This looks like a bug. Or I wonder if any user space depends on this > > > > behaviour, if yes, we really need a new flag then. > > > > > > > > > > My understanding was that we depend on this for cases like qemu > > > crashes. We don't do an unmap(-1ULL) or anything like that to make > > > sure the device is clean when we bind a second qemu to the same > > > device. That's why I think that close() should clean them. > > > > In vhost_vdpa_release() we do: > > > > vhost_vdpa_release() > > vhost_vdpa_cleanup() > > for_each_as() > > vhost_vdpa_remove_as() > > vhost_vdpa_iotlb_unmap(0ULL, 0ULL - 1) > > vhost_vdpa_free_domain() > > > > Anything wrong here? > > > > No, I think we just trusted in different pre-existing cleanup points > "semantics". > > > Conceptually, the address mapping is not a part of the abstraction for > > a virtio device now. So resetting the memory mapping during virtio > > device reset seems wrong. > > > > I agree. So then no change in the kernel should be needed but to > revert this cleanup on device reset. I think you meant simulator, then yes. > I guess we should document it > ops->reset just in case? Probably. Thanks > > Thanks! >
Re: Reducing vdpa migration downtime because of memory pin / maps
On Tue, Apr 11, 2023 at 4:26 AM Jason Wang wrote: > > On Mon, Apr 10, 2023 at 5:05 PM Eugenio Perez Martin > wrote: > > > > On Mon, Apr 10, 2023 at 5:22 AM Jason Wang wrote: > > > > > > On Mon, Apr 10, 2023 at 11:17 AM Longpeng (Mike, Cloud Infrastructure > > > Service Product Dept.) wrote: > > > > > > > > > > > > > > > > 在 2023/4/10 10:14, Jason Wang 写道: > > > > > On Wed, Apr 5, 2023 at 7:38 PM Eugenio Perez Martin > > > > > wrote: > > > > >> > > > > >> Hi! > > > > >> > > > > >> As mentioned in the last upstream virtio-networking meeting, one of > > > > >> the factors that adds more downtime to migration is the handling of > > > > >> the guest memory (pin, map, etc). At this moment this handling is > > > > >> bound to the virtio life cycle (DRIVER_OK, RESET). In that sense, the > > > > >> destination device waits until all the guest memory / state is > > > > >> migrated to start pinning all the memory. > > > > >> > > > > >> The proposal is to bind it to the char device life cycle (open vs > > > > >> close), so all the guest memory can be pinned for all the guest / > > > > >> qemu > > > > >> lifecycle. > > > > >> > > > > >> This has two main problems: > > > > >> * At this moment the reset semantics forces the vdpa device to unmap > > > > >> all the memory. So this change needs a vhost vdpa feature flag. > > > > > > > > > > Is this true? I didn't find any codes to unmap the memory in > > > > > vhost_vdpa_set_status(). > > > > > > > > > > > > > It could depend on the vendor driver, for example, the vdpasim would do > > > > something like that. > > > > > > > > vhost_vdpa_set_status->vdpa_reset->vdpasim_reset->vdpasim_do_reset->vhost_iotlb_reset > > > > > > This looks like a bug. Or I wonder if any user space depends on this > > > behaviour, if yes, we really need a new flag then. > > > > > > > My understanding was that we depend on this for cases like qemu > > crashes. We don't do an unmap(-1ULL) or anything like that to make > > sure the device is clean when we bind a second qemu to the same > > device. That's why I think that close() should clean them. > > In vhost_vdpa_release() we do: > > vhost_vdpa_release() > vhost_vdpa_cleanup() > for_each_as() > vhost_vdpa_remove_as() > vhost_vdpa_iotlb_unmap(0ULL, 0ULL - 1) > vhost_vdpa_free_domain() > > Anything wrong here? > No, I think we just trusted in different pre-existing cleanup points "semantics". > Conceptually, the address mapping is not a part of the abstraction for > a virtio device now. So resetting the memory mapping during virtio > device reset seems wrong. > I agree. So then no change in the kernel should be needed but to revert this cleanup on device reset. I guess we should document it ops->reset just in case? Thanks!
Re: Reducing vdpa migration downtime because of memory pin / maps
On Mon, Apr 10, 2023 at 5:05 PM Eugenio Perez Martin wrote: > > On Mon, Apr 10, 2023 at 5:22 AM Jason Wang wrote: > > > > On Mon, Apr 10, 2023 at 11:17 AM Longpeng (Mike, Cloud Infrastructure > > Service Product Dept.) wrote: > > > > > > > > > > > > 在 2023/4/10 10:14, Jason Wang 写道: > > > > On Wed, Apr 5, 2023 at 7:38 PM Eugenio Perez Martin > > > > wrote: > > > >> > > > >> Hi! > > > >> > > > >> As mentioned in the last upstream virtio-networking meeting, one of > > > >> the factors that adds more downtime to migration is the handling of > > > >> the guest memory (pin, map, etc). At this moment this handling is > > > >> bound to the virtio life cycle (DRIVER_OK, RESET). In that sense, the > > > >> destination device waits until all the guest memory / state is > > > >> migrated to start pinning all the memory. > > > >> > > > >> The proposal is to bind it to the char device life cycle (open vs > > > >> close), so all the guest memory can be pinned for all the guest / qemu > > > >> lifecycle. > > > >> > > > >> This has two main problems: > > > >> * At this moment the reset semantics forces the vdpa device to unmap > > > >> all the memory. So this change needs a vhost vdpa feature flag. > > > > > > > > Is this true? I didn't find any codes to unmap the memory in > > > > vhost_vdpa_set_status(). > > > > > > > > > > It could depend on the vendor driver, for example, the vdpasim would do > > > something like that. > > > > > > vhost_vdpa_set_status->vdpa_reset->vdpasim_reset->vdpasim_do_reset->vhost_iotlb_reset > > > > This looks like a bug. Or I wonder if any user space depends on this > > behaviour, if yes, we really need a new flag then. > > > > My understanding was that we depend on this for cases like qemu > crashes. We don't do an unmap(-1ULL) or anything like that to make > sure the device is clean when we bind a second qemu to the same > device. That's why I think that close() should clean them. In vhost_vdpa_release() we do: vhost_vdpa_release() vhost_vdpa_cleanup() for_each_as() vhost_vdpa_remove_as() vhost_vdpa_iotlb_unmap(0ULL, 0ULL - 1) vhost_vdpa_free_domain() Anything wrong here? Conceptually, the address mapping is not a part of the abstraction for a virtio device now. So resetting the memory mapping during virtio device reset seems wrong. Thanks > Or maybe > even open(). > > The only other option I see is to remove the whole vhost-vdpa device > every time, or am I missing something? > > Thanks! >
Re: Reducing vdpa migration downtime because of memory pin / maps
On Mon, Apr 10, 2023 at 5:22 AM Jason Wang wrote: > > On Mon, Apr 10, 2023 at 11:17 AM Longpeng (Mike, Cloud Infrastructure > Service Product Dept.) wrote: > > > > > > > > 在 2023/4/10 10:14, Jason Wang 写道: > > > On Wed, Apr 5, 2023 at 7:38 PM Eugenio Perez Martin > > > wrote: > > >> > > >> Hi! > > >> > > >> As mentioned in the last upstream virtio-networking meeting, one of > > >> the factors that adds more downtime to migration is the handling of > > >> the guest memory (pin, map, etc). At this moment this handling is > > >> bound to the virtio life cycle (DRIVER_OK, RESET). In that sense, the > > >> destination device waits until all the guest memory / state is > > >> migrated to start pinning all the memory. > > >> > > >> The proposal is to bind it to the char device life cycle (open vs > > >> close), so all the guest memory can be pinned for all the guest / qemu > > >> lifecycle. > > >> > > >> This has two main problems: > > >> * At this moment the reset semantics forces the vdpa device to unmap > > >> all the memory. So this change needs a vhost vdpa feature flag. > > > > > > Is this true? I didn't find any codes to unmap the memory in > > > vhost_vdpa_set_status(). > > > > > > > It could depend on the vendor driver, for example, the vdpasim would do > > something like that. > > > > vhost_vdpa_set_status->vdpa_reset->vdpasim_reset->vdpasim_do_reset->vhost_iotlb_reset > > This looks like a bug. Or I wonder if any user space depends on this > behaviour, if yes, we really need a new flag then. > My understanding was that we depend on this for cases like qemu crashes. We don't do an unmap(-1ULL) or anything like that to make sure the device is clean when we bind a second qemu to the same device. That's why I think that close() should clean them. Or maybe even open(). The only other option I see is to remove the whole vhost-vdpa device every time, or am I missing something? Thanks!
Re: Reducing vdpa migration downtime because of memory pin / maps
On Mon, Apr 10, 2023 at 11:17 AM Longpeng (Mike, Cloud Infrastructure Service Product Dept.) wrote: > > > > 在 2023/4/10 10:14, Jason Wang 写道: > > On Wed, Apr 5, 2023 at 7:38 PM Eugenio Perez Martin > > wrote: > >> > >> Hi! > >> > >> As mentioned in the last upstream virtio-networking meeting, one of > >> the factors that adds more downtime to migration is the handling of > >> the guest memory (pin, map, etc). At this moment this handling is > >> bound to the virtio life cycle (DRIVER_OK, RESET). In that sense, the > >> destination device waits until all the guest memory / state is > >> migrated to start pinning all the memory. > >> > >> The proposal is to bind it to the char device life cycle (open vs > >> close), so all the guest memory can be pinned for all the guest / qemu > >> lifecycle. > >> > >> This has two main problems: > >> * At this moment the reset semantics forces the vdpa device to unmap > >> all the memory. So this change needs a vhost vdpa feature flag. > > > > Is this true? I didn't find any codes to unmap the memory in > > vhost_vdpa_set_status(). > > > > It could depend on the vendor driver, for example, the vdpasim would do > something like that. > > vhost_vdpa_set_status->vdpa_reset->vdpasim_reset->vdpasim_do_reset->vhost_iotlb_reset This looks like a bug. Or I wonder if any user space depends on this behaviour, if yes, we really need a new flag then. Thanks > > > Thanks > > > >> * This may increase the initialization time. Maybe we can delay it if > >> qemu is not the destination of a LM. Anyway I think this should be > >> done as an optimization on top. > >> > >> Any ideas or comments in this regard? > >> > >> Thanks! > >> > > > > . >
Re: Reducing vdpa migration downtime because of memory pin / maps
在 2023/4/10 10:14, Jason Wang 写道: On Wed, Apr 5, 2023 at 7:38 PM Eugenio Perez Martin wrote: Hi! As mentioned in the last upstream virtio-networking meeting, one of the factors that adds more downtime to migration is the handling of the guest memory (pin, map, etc). At this moment this handling is bound to the virtio life cycle (DRIVER_OK, RESET). In that sense, the destination device waits until all the guest memory / state is migrated to start pinning all the memory. The proposal is to bind it to the char device life cycle (open vs close), so all the guest memory can be pinned for all the guest / qemu lifecycle. This has two main problems: * At this moment the reset semantics forces the vdpa device to unmap all the memory. So this change needs a vhost vdpa feature flag. Is this true? I didn't find any codes to unmap the memory in vhost_vdpa_set_status(). It could depend on the vendor driver, for example, the vdpasim would do something like that. vhost_vdpa_set_status->vdpa_reset->vdpasim_reset->vdpasim_do_reset->vhost_iotlb_reset Thanks * This may increase the initialization time. Maybe we can delay it if qemu is not the destination of a LM. Anyway I think this should be done as an optimization on top. Any ideas or comments in this regard? Thanks! .
Re: Reducing vdpa migration downtime because of memory pin / maps
On Wed, Apr 5, 2023 at 7:38 PM Eugenio Perez Martin wrote: > > Hi! > > As mentioned in the last upstream virtio-networking meeting, one of > the factors that adds more downtime to migration is the handling of > the guest memory (pin, map, etc). At this moment this handling is > bound to the virtio life cycle (DRIVER_OK, RESET). In that sense, the > destination device waits until all the guest memory / state is > migrated to start pinning all the memory. > > The proposal is to bind it to the char device life cycle (open vs > close), so all the guest memory can be pinned for all the guest / qemu > lifecycle. > > This has two main problems: > * At this moment the reset semantics forces the vdpa device to unmap > all the memory. So this change needs a vhost vdpa feature flag. Is this true? I didn't find any codes to unmap the memory in vhost_vdpa_set_status(). Thanks > * This may increase the initialization time. Maybe we can delay it if > qemu is not the destination of a LM. Anyway I think this should be > done as an optimization on top. > > Any ideas or comments in this regard? > > Thanks! >
