Messages in this thread | | | Date | Thu, 23 Mar 2023 11:13:48 -0700 | From | Nicolin Chen <> | Subject | Re: [PATCH v1 14/14] iommu/arm-smmu-v3: Add arm_smmu_cache_invalidate_user |
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On Thu, Mar 23, 2023 at 09:16:51AM -0300, Jason Gunthorpe wrote: > On Wed, Mar 22, 2023 at 01:43:59PM -0700, Nicolin Chen wrote: > > > > So I think for ARM we want to reflect the physical SMMU instances into > > > vSMMU instances and that feels best done by having a unique S2 > > > iommu_domain for each SMMU instance. Then we know that an invalidation > > > for a SMMU instance is delivered to that S2's singular CMDQ and things > > > like vCMDQ become possible. > > > > In that environment, do we still need a VMID unification? > > If each S2 is per-smmu-instance then the VMID can be local to the SMMU > instance
It sounds like related to the multi-SMMU instance too? Anyway, it's good to think we that have a way out from requiring this VMID unification.
> > > > Our approach of setting up a stage-2 mapping in QEMU is to > > > > map the entire guest memory. I don't see a point in having > > > > a separate S2 domain, even if there are multiple instances? > > > > > > And then this is the drawback, we don't really want to have duplicated > > > S2 page tables in the system for every stage 2. > > > > > > Maybe we have made a mistake by allowing the S2 to be an unmanaged > > > domain. Perhaps we should create the S2 out of an unmanaged domain > > > like the S1. > > > > > > Then the rules could be > > > - Unmanaged domain can be used with every smmu instance, only one > > > copy of the page table. The ASID in the iommu_domain is > > > kernel-global > > > - S2 domain is a child of a shared unmanaged domain. It can be used > > > only with the SMMU it is associated with, it has a per-SMMU VM ID > > > - S1 domain is a child of a S2 domain, it can be used only with the > > > SMMU it's S2 is associated with, just because > > > > The actual S2 pagetable has to stay at the unmanaged domain > > for IOAS_MAP, while we maintain an s2_cfg data structure in > > the shadow S2 domain per SMMU instance that has its own VMID > > but a shared S2 page table pointer? > > Yes > > > Hmm... Feels very complicated to me. How does that help? > > It de-duplicates the page table across multiple SMMU instances.
Oh. So that the s2_cfg data structures can have a shared S2 IOPT while having different VMIDs. This would be a big rework. It changes the two-domain design for nesting. Should we do this at a later stage when supporting multi-SMMU instance or now? And I am not sure Intel would need this...
> > > So, I have been exploring a different approach by creating an > > > > internal multiplication inside VCMDQ... > > > > > > How can that work? > > > > > > You'd have to have the guest VM to know to replicate to different > > > vCMDQ's? Which isn't the standard SMMU programming model anymore.. > > > > VCMDQ has multiple VINTFs (Virtual Interfaces) that's supposed > > to be used by the host to expose to multiple VMs. > > > > In a multi-SMMU environment, every single SMMU+VCMDQ instance > > would have one VINTF only that contains one or more VCMDQs. In > > this case, passthrough devices behind different physical SMMU > > instances are straightforwardly attached to different vSMMUs. > > Yes, this is the obvious simple impementation > > > However, if we can't have multiple vSMMU instances, the guest > > VM (its HW) would enable multiple VINTFs corresponding to the > > number of physical SMMU/VCMDQ instances, for devices to attach > > accordingly. That means I need to figure out a way to pin the > > devices onto those VINTFs, by somehow passing their physical > > SMMU IDs. > > And a way to request the correctly bound vCMDQ from the guest as well. > Sounds really messsy, I'd think multi-smmu is the much cleaner choice
Yes. I agree, we would need the entire QEMU community to give consent to change that though.
Thanks! Nicolin
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