Hi Zhenzhong, On 1/18/24 08:10, Duan, Zhenzhong wrote: > Hi Eric, > >> -----Original Message----- >> From: Eric Auger <eric.au...@redhat.com> >> Cc: m...@redhat.com; c...@redhat.com >> Subject: [RFC 0/7] VIRTIO-IOMMU/VFIO: Fix host iommu geometry handling >> for hotplugged devices >> >> In [1] we attempted to fix a case where a VFIO-PCI device protected >> with a virtio-iommu was assigned to an x86 guest. On x86 the physical >> IOMMU may have an address width (gaw) of 39 or 48 bits whereas the >> virtio-iommu used to expose a 64b address space by default. >> Hence the guest was trying to use the full 64b space and we hit >> DMA MAP failures. To work around this issue we managed to pass >> usable IOVA regions (excluding the out of range space) from VFIO >> to the virtio-iommu device. This was made feasible by introducing >> a new IOMMU Memory Region callback dubbed iommu_set_iova_regions(). >> This latter gets called when the IOMMU MR is enabled which >> causes the vfio_listener_region_add() to be called. >> >> However with VFIO-PCI hotplug, this technique fails due to the >> race between the call to the callback in the add memory listener >> and the virtio-iommu probe request. Indeed the probe request gets >> called before the attach to the domain. So in that case the usable >> regions are communicated after the probe request and fail to be >> conveyed to the guest. To be honest the problem was hinted by >> Jean-Philippe in [1] and I should have been more careful at >> listening to him and testing with hotplug :-( > It looks the global virtio_iommu_config.bypass is never cleared in guest. > When guest virtio_iommu driver enable IOMMU, should it clear this > bypass attribute? > If it could be cleared in viommu_probe(), then qemu will call > virtio_iommu_set_config() then virtio_iommu_switch_address_space_all() > to enable IOMMU MR. Then both coldplugged and hotplugged devices will work.
this field is iommu wide while the probe applies on a one device.In general I would prefer not to be dependent on the MR enablement. We know that the device is likely to be protected and we can collect its requirements beforehand. > > Intel iommu has a similar bit in register GCMD_REG.TE, when guest > intel_iommu driver probe set it, on qemu side, vtd_address_space_refresh_all() > is called to enable IOMMU MRs. interesting. Would be curious to get Jean Philippe's pov. > >> For coldplugged device the technique works because we make sure all >> the IOMMU MR are enabled once on the machine init done: 94df5b2180 >> ("virtio-iommu: Fix 64kB host page size VFIO device assignment") >> for granule freeze. But I would be keen to get rid of this trick. >> >> Using an IOMMU MR Ops is unpractical because this relies on the IOMMU >> MR to have been enabled and the corresponding vfio_listener_region_add() >> to be executed. Instead this series proposes to replace the usage of this >> API by the recently introduced PCIIOMMUOps: ba7d12eb8c ("hw/pci: >> modify >> pci_setup_iommu() to set PCIIOMMUOps"). That way, the callback can be >> called earlier, once the usable IOVA regions have been collected by >> VFIO, without the need for the IOMMU MR to be enabled. >> >> This looks cleaner. In the short term this may also be used for >> passing the page size mask, which would allow to get rid of the >> hacky transient IOMMU MR enablement mentionned above. >> >> [1] [PATCH v4 00/12] VIRTIO-IOMMU/VFIO: Don't assume 64b IOVA space >> https://lore.kernel.org/all/20231019134651.842175-1- >> eric.au...@redhat.com/ >> >> [2] https://lore.kernel.org/all/20230929161547.GB2957297@myrica/ >> >> Extra Notes: >> With that series, the reserved memory regions are communicated on time >> so that the virtio-iommu probe request grabs them. However this is not >> sufficient. In some cases (my case), I still see some DMA MAP failures >> and the guest keeps on using IOVA ranges outside the geometry of the >> physical IOMMU. This is due to the fact the VFIO-PCI device is in the >> same iommu group as the pcie root port. Normally the kernel >> iova_reserve_iommu_regions (dma-iommu.c) is supposed to call >> reserve_iova() >> for each reserved IOVA, which carves them out of the allocator. When >> iommu_dma_init_domain() gets called for the hotplugged vfio-pci device >> the iova domain is already allocated and set and we don't call >> iova_reserve_iommu_regions() again for the vfio-pci device. So its >> corresponding reserved regions are not properly taken into account. > I suspect there is same issue with coldplugged devices. If those devices > are in same group, get iova_reserve_iommu_regions() is only called > for first device. But other devices's reserved regions are missed. Correct > > Curious how you make passthrough device and pcie root port under same > group. > When I start a x86 guest with passthrough device, I see passthrough > device and pcie root port are in different group. > > -[0000:00]-+-00.0 > +-01.0 > +-02.0 > +-03.0-[01]----00.0 > > /sys/kernel/iommu_groups/3/devices: > 0000:00:03.0 > /sys/kernel/iommu_groups/7/devices: > 0000:01:00.0 > > My qemu cmdline: > -device pcie-root-port,id=root0,slot=0 > -device vfio-pci,host=6f:01.0,id=vfio0,bus=root0 I just replayed the scenario: - if you have a coldplugged vfio-pci device, the pci root port and the passthroughed device end up in different iommu groups. On my end I use ioh3420 but you confirmed that's the same for the generic pcie-root-port - however if you hotplug the vfio-pci device that's a different story: they end up in the same group. Don't ask me why. I tried with both virtio-iommu and intel iommu and I end up with the same topology. That looks really weird to me. I initially thought this was an ACS issue but I am now puzzled. Thanks! Eric > > Thanks > Zhenzhong > >> This is not trivial to fix because theoretically the 1st attached >> devices could already have allocated IOVAs within the reserved regions >> of the second device. Also we are somehow hijacking the reserved >> memory regions to model the geometry of the physical IOMMU so not sure >> any attempt to fix that upstream will be accepted. At the moment one >> solution is to make sure assigned devices end up in singleton group. >> Another solution is to work on a different approach where the gaw >> can be passed as an option to the virtio-iommu device, similarly at >> what is done with intel iommu. >> >> This series can be found at: >> https://github.com/eauger/qemu/tree/hotplug-resv-rfc >> >> >> Eric Auger (7): >> hw/pci: Introduce PCIIOMMUOps::set_host_iova_regions >> hw/pci: Introduce pci_device_iommu_bus >> vfio/pci: Pass the usable IOVA ranges through PCIIOMMUOps >> virtio-iommu: Implement PCIIOMMUOps set_host_resv_regions >> virtio-iommu: Remove the implementation of iommu_set_iova_ranges >> hw/vfio: Remove memory_region_iommu_set_iova_ranges() call >> memory: Remove IOMMU MR iommu_set_iova_range API >> >> include/exec/memory.h | 32 ------- >> include/hw/pci/pci.h | 16 ++++ >> hw/pci/pci.c | 16 ++++ >> hw/vfio/common.c | 10 -- >> hw/vfio/pci.c | 27 ++++++ >> hw/virtio/virtio-iommu.c | 201 ++++++++++++++++++++------------------- >> system/memory.c | 13 --- >> 7 files changed, 160 insertions(+), 155 deletions(-) >> >> -- >> 2.41.0