> From: Jason Wang <jasow...@redhat.com> > Sent: Thursday, October 15, 2020 4:41 PM > > > On 2020/10/15 ??3:58, Tian, Kevin wrote: > >> From: Jason Wang <jasow...@redhat.com> > >> Sent: Thursday, October 15, 2020 2:52 PM > >> > >> > >> On 2020/10/14 ??11:08, Tian, Kevin wrote: > >>>> From: Jason Wang <jasow...@redhat.com> > >>>> Sent: Tuesday, October 13, 2020 2:22 PM > >>>> > >>>> > >>>> On 2020/10/12 ??4:38, Tian, Kevin wrote: > >>>>>> From: Jason Wang <jasow...@redhat.com> > >>>>>> Sent: Monday, September 14, 2020 12:20 PM > >>>>>> > >>>>> [...] > >>>>> > If it's possible, I would suggest a generic uAPI instead of > >>>>> a VFIO > >>>>>> specific one. > >>>>>> > >>>>>> Jason suggest something like /dev/sva. There will be a lot of > >>>>>> other subsystems that could benefit from this (e.g vDPA). > >>>>>> > >>>>>> Have you ever considered this approach? > >>>>>> > >>>>> Hi, Jason, > >>>>> > >>>>> We did some study on this approach and below is the output. It's a > >>>>> long writing but I didn't find a way to further abstract w/o > >>>>> losing necessary context. Sorry about that. > >>>>> > >>>>> Overall the real purpose of this series is to enable IOMMU nested > >>>>> translation capability with vSVA as one major usage, through below > >>>>> new uAPIs: > >>>>> 1) Report/enable IOMMU nested translation capability; > >>>>> 2) Allocate/free PASID; > >>>>> 3) Bind/unbind guest page table; > >>>>> 4) Invalidate IOMMU cache; > >>>>> 5) Handle IOMMU page request/response (not in this series); > >>>>> 1/3/4) is the minimal set for using IOMMU nested translation, with > >>>>> the other two optional. For example, the guest may enable vSVA on > >>>>> a device without using PASID. Or, it may bind its gIOVA page table > >>>>> which doesn't require page fault support. Finally, all operations > >>>>> can be applied to either physical device or subdevice. > >>>>> > >>>>> Then we evaluated each uAPI whether generalizing it is a good > >>>>> thing both in concept and regarding to complexity. > >>>>> > >>>>> First, unlike other uAPIs which are all backed by iommu_ops, PASID > >>>>> allocation/free is through the IOASID sub-system. > >>>> A question here, is IOASID expected to be the single management > >>>> interface for PASID? > >>> yes > >>> > >>>> (I'm asking since there're already vendor specific IDA based PASID > >>>> allocator e.g amdgpu_pasid_alloc()) > >>> That comes before IOASID core was introduced. I think it should be > >>> changed to use the new generic interface. Jacob/Jean can better > >>> comment if other reason exists for this exception. > >> > >> If there's no exception it should be fixed. > >> > >> > >>>>> From this angle > >>>>> we feel generalizing PASID management does make some sense. > >>>>> First, PASID is just a number and not related to any device before > >>>>> it's bound to a page table and IOMMU domain. Second, PASID is a > >>>>> global resource (at least on Intel VT-d), > >>>> I think we need a definition of "global" here. It looks to me for > >>>> vt-d the PASID table is per device. > >>> PASID table is per device, thus VT-d could support per-device PASIDs > >>> in concept. > >> > >> I think that's the requirement of PCIE spec which said PASID + RID > >> identifies the process address space ID. > >> > >> > >>> However on Intel platform we require PASIDs to be managed in > >>> system-wide (cross host and guest) when combining vSVA, SIOV, SR-IOV > >>> and ENQCMD together. > >> > >> Any reason for such requirement? (I'm not familiar with ENQCMD, but > >> my understanding is that vSVA, SIOV or SR-IOV doesn't have the > >> requirement for system-wide PASID). > > ENQCMD is a new instruction to allow multiple processes submitting > > workload to one shared workqueue. Each process has an unique PASID > > saved in a MSR, which is included in the ENQCMD payload to indicate > > the address space when the CPU sends to the device. As one process > > might issue ENQCMD to multiple devices, OS-wide PASID allocation is > > required both in host and guest side. > > > > When executing ENQCMD in the guest to a SIOV device, the guest > > programmed value in the PASID_MSR must be translated to a host PASID > > value for proper function/isolation as PASID represents the address > > space. The translation is done through a new VMCS PASID translation > > structure (per-VM, and 1:1 mapping). From this angle the host PASIDs > > must be allocated 'globally' cross all assigned devices otherwise it > > may lead to 1:N mapping when a guest process issues ENQCMD to multiple > > assigned devices/subdevices. > > > > There will be a KVM forum session for this topic btw. > > > Thanks for the background. Now I see the restrict comes from ENQCMD. > > > > > >> > >>> Thus the host creates only one 'global' PASID namespace but do use > >>> per-device PASID table to assure isolation between devices on Intel > >>> platforms. But ARM does it differently as Jean explained. > >>> They have a global namespace for host processes on all host-owned > >>> devices (same as Intel), but then per-device namespace when a device > >>> (and its PASID table) is assigned to userspace. > >>> > >>>> Another question, is this possible to have two DMAR hardware > >>>> unit(at least I can see two even in my laptop). In this case, is > >>>> PASID still a global resource? > >>> yes > >>> > >>>>> while having separate VFIO/ > >>>>> VDPA allocation interfaces may easily cause confusion in > >>>>> userspace, e.g. which interface to be used if both VFIO/VDPA devices > >>>>> exist. > >>>>> Moreover, an unified interface allows centralized control over how > >>>>> many PASIDs are allowed per process. > >>>> Yes. > >>>> > >>>> > >>>>> One unclear part with this generalization is about the permission. > >>>>> Do we open this interface to any process or only to those which > >>>>> have assigned devices? If the latter, what would be the mechanism > >>>>> to coordinate between this new interface and specific passthrough > >>>>> frameworks? > >>>> I'm not sure, but if you just want a permission, you probably can > >>>> introduce new capability (CAP_XXX) for this. > >>>> > >>>> > >>>>> A more tricky case, vSVA support on ARM (Eric/Jean please > >>>>> correct me) plans to do per-device PASID namespace which is built > >>>>> on a bind_pasid_table iommu callback to allow guest fully manage > >>>>> its PASIDs on a given passthrough device. > >>>> I see, so I think the answer is to prepare for the namespace > >>>> support from the start. (btw, I don't see how namespace is handled > >>>> in current IOASID module?) > >>> The PASID table is based on GPA when nested translation is enabled > >>> on ARM SMMU. This design implies that the guest manages PASID table > >>> thus PASIDs instead of going through host-side API on assigned > >>> device. From this angle we don't need explicit namespace in the host > >>> API. Just need a way to control how many PASIDs a process is allowed > >>> to allocate in the global namespace. btw IOASID module already has > >>> 'set' concept per-process and PASIDs are managed per-set. Then the > >>> quota control can be easily introduced in the 'set' level. > >>> > >>>>> I'm not sure > >>>>> how such requirement can be unified w/o involving passthrough > >>>>> frameworks, or whether ARM could also switch to global PASID > >>>>> style... > >>>>> > >>>>> Second, IOMMU nested translation is a per IOMMU domain capability. > >>>>> Since IOMMU domains are managed by VFIO/VDPA > >>>>> (alloc/free domain, attach/detach device, set/get domain > >>>>> attribute, etc.), reporting/enabling the nesting capability is an > >>>>> natural extension to the domain uAPI of existing passthrough frameworks. > >>>>> Actually, VFIO already includes a nesting enable interface even > >>>>> before this series. So it doesn't make sense to generalize this > >>>>> uAPI out. > >>>> So my understanding is that VFIO already: > >>>> > >>>> 1) use multiple fds > >>>> 2) separate IOMMU ops to a dedicated container fd (type1 iommu) > >>>> 3) provides API to associated devices/group with a container > >>>> > >>>> And all the proposal in this series is to reuse the container fd. > >>>> It should be possible to replace e.g type1 IOMMU with a unified module. > >>> yes, this is the alternative option that I raised in the last paragraph. > >>> > >>>>> Then the tricky part comes with the remaining operations (3/4/5), > >>>>> which are all backed by iommu_ops thus effective only within an > >>>>> IOMMU domain. To generalize them, the first thing is to find a way > >>>>> to associate the sva_FD (opened through generic /dev/sva) with an > >>>>> IOMMU domain that is created by VFIO/VDPA. The second thing is to > >>>>> replicate {domain<->device/subdevice} association in /dev/sva path > >>>>> because some operations (e.g. page fault) is triggered/handled per > >>>>> device/subdevice. > >>>> Is there any reason that the #PF can not be handled via SVA fd? > >>> using per-device FDs or multiplexing all fault info through one > >>> sva_FD is just an implementation choice. The key is to mark faults > >>> per device/ subdevice thus anyway requires a userspace-visible > >>> handle/tag to represent device/subdevice and the domain/device > >>> association must be constructed in this new path. > >> > >> I don't get why it requires a userspace-visible handle/tag. The > >> binding between SVA fd and device fd could be done either explicitly > >> or implicitly. So userspace know which (sub)device that this SVA fd is for. > >> > >> > >>>>> Therefore, /dev/sva must provide both per- domain and > >>>>> per-device uAPIs similar to what VFIO/VDPA already does. Moreover, > >>>>> mapping page fault to subdevice requires pre- registering > >>>>> subdevice fault data to IOMMU layer when binding guest page table, > >>>>> while such fault data can be only retrieved from parent driver > >>>>> through VFIO/VDPA. > >>>>> > >>>>> However, we failed to find a good way even at the 1st step about > >>>>> domain association. The iommu domains are not exposed to the > >>>>> userspace, and there is no 1:1 mapping between domain and device. > >>>>> In VFIO, all devices within the same VFIO container share the > >>>>> address space but they may be organized in multiple IOMMU domains > >>>>> based on their bus type. How (should we let) the userspace know > >>>>> the domain information and open an sva_FD for each domain is the > >>>>> main problem here. > >>>> The SVA fd is not necessarily opened by userspace. It could be get > >>>> through subsystem specific uAPIs. > >>>> > >>>> E.g for vDPA if a vDPA device contains several vSVA-capable > >>>> domains, we > >> can: > >>>> 1) introduce uAPI for userspace to know the number of vSVA-capable > >>>> domain > >>>> 2) introduce e.g VDPA_GET_SVA_FD to get the fd for each > >>>> vSVA-capable domain > >>> and also new interface to notify userspace when a domain disappears > >>> or a device is detached? > >> > >> You need to deal with this case even in VFIO, isn't it? > > No. VFIO doesn't expose domain knowledge to userspace. > > > Neither did the above API I think. > > > > > >> > >>> Finally looks we are creating a completely set of new subsystem > >>> specific uAPIs just for generalizing another set of subsystem > >>> specific uAPIs. Remember after separating PASID mgmt. > >>> out then most of remaining vSVA uAPIs are simpler wrapper of IOMMU > >>> API. Replicating them is much easier logic than developing a new > >>> glue mechanism in each subsystem. > >> > >> As discussed, the point is more than just simple generalizing. It's > >> about the limitation of current uAPI. So I have the following questions: > >> > >> Do we want a single PASID to be used by more than one devices? > > Yes. > > > >> If yes, do we want those devices to share I/O page tables? > > Yes. > > > >> If yes, which uAPI is used to program the shared I/O page tables? > >> > > Page table binding needs to be done per-device, so the userspace will > > use VFIO uAPI for VFIO device and vDPA uAPI for vDPA device. > > > Any design considerations for this, I think it should be done per PASID > instead > (consider PASID is a global resource)?
per device and per PASID. you may have a look from the below arch. PASID table is per device, the binding of page table are set to PASID table entry. " In VT-d implementation, PASID table is per device and maintained in the host. Guest PASID table is shadowed in VMM where virtual IOMMU is emulated. .-------------. .---------------------------. | vIOMMU | | Guest process CR3, FL only| | | '---------------------------' .----------------/ | PASID Entry |--- PASID cache flush - '-------------' | | | V | | CR3 in GPA '-------------' Guest ------| Shadow |--------------------------|-------- v v v Host .-------------. .----------------------. | pIOMMU | | Bind FL for GVA-GPA | | | '----------------------' .----------------/ | | PASID Entry | V (Nested xlate) '----------------\.------------------------------. | | |SL for GPA-HPA, default domain| | | '------------------------------' '-------------' Where: - FL = First level/stage one page tables - SL = Second level/stage two page tables " copied from https://lwn.net/Articles/807506/ > > > The binding request is initiated by the virtual IOMMU, when capturing > > guest attempt of binding page table to a virtual PASID entry for a > > given device. > > > And for L2 page table programming, if PASID is use by both e.g VFIO and > vDPA, user need to choose one of uAPI to build l2 mappings? for L2 page table mappings, it's done by VFIO MAP/UNMAP. for vdpa, I guess it is tlb flush. so you are right. Keeping L1/L2 page table management in a single uAPI set is also a reason for my current series which extends VFIO for L1 management. Regards, Yi Liu > Thanks > > > > > > Thanks > > Kevin > > > >> > >>>>> In the end we just realized that doing such generalization doesn't > >>>>> really lead to a clear design and instead requires tight coordination > >>>>> between /dev/sva and VFIO/VDPA for almost every new uAPI > >>>>> (especially about synchronization when the domain/device > >>>>> association is changed or when the device/subdevice is being reset/ > >>>>> drained). Finally it may become a usability burden to the userspace > >>>>> on proper use of the two interfaces on the assigned device. > >>>>> > >>>>> Based on above analysis we feel that just generalizing PASID mgmt. > >>>>> might be a good thing to look at while the remaining operations are > >>>>> better being VFIO/VDPA specific uAPIs. anyway in concept those are > >>>>> just a subset of the page table management capabilities that an > >>>>> IOMMU domain affords. Since all other aspects of the IOMMU domain > >>>>> is managed by VFIO/VDPA already, continuing this path for new nesting > >>>>> capability sounds natural. There is another option by generalizing the > >>>>> entire IOMMU domain management (sort of the entire vfio_iommu_ > >>>>> type1), but it's unclear whether such intrusive change is worthwhile > >>>>> (especially when VFIO/VDPA already goes different route even in legacy > >>>>> mapping uAPI: map/unmap vs. IOTLB). > >>>>> > >>>>> Thoughts? > >>>> I'm ok with starting with a unified PASID management and consider the > >>>> unified vSVA/vIOMMU uAPI later. > >>>> > >>> Glad to see that we have consensus here. :) > >>> > >>> Thanks > >>> Kevin _______________________________________________ iommu mailing list iommu@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/iommu