>-----Original Message----- >From: Peter Xu <pet...@redhat.com> >Sent: Tuesday, June 6, 2023 11:42 PM >Subject: Re: [PATCH v2 2/4] intel_iommu: Fix a potential issue in VFIO dirty >page sync > ... >> >> a/include/exec/memory.h b/include/exec/memory.h index >> >> c3661b2276c7..eecc3eec6702 100644 >> >> --- a/include/exec/memory.h >> >> +++ b/include/exec/memory.h >> >> @@ -142,6 +142,10 @@ struct IOMMUTLBEntry { >> >> * events (e.g. VFIO). Both notifications must be accurate so that >> >> * the shadow page table is fully in sync with the guest view. >> >> * >> >> + * Besides MAP, there is a special use case called FULL_MAP which >> >> + * requests notification for all the existent mappings (e.g. VFIO >> >> + * dirty page sync). >> > >> >Why do we need FULL_MAP? Can we simply reimpl MAP? >> >> Sorry, I just realized IOMMU_NOTIFIER_FULL_MAP is confusing. >> Maybe IOMMU_NOTIFIER_MAP_FAST_PATH could be a bit more accurate. >> >> IIUC, currently replay() is called from two paths, one is VFIO device >> address space switch which walks over the IOMMU page table to setup >> initial mapping and cache it in IOVA tree. The other is VFIO dirty >> sync which walks over the IOMMU page table to notify the mapping, >> because we already cache the mapping in IOVA tree and VFIO dirty sync >> is protected by BQL, so I think it's fine to pick mapping from IOVA >> tree directly instead of walking over IOMMU page table. That's the >> reason of FULL_MAP (IOMMU_NOTIFIER_MAP_FAST_PATH better). >> >> About "reimpl MAP", do you mean to walk over IOMMU page table to >> notify all existing MAP events without checking with the IOVA tree for >> difference? If you prefer, I'll rewrite an implementation this way. > >We still need to maintain iova tree. IIUC that's the major complexity of vt-d >emulation, because we have that extra cache layer to sync with the real guest >iommu pgtables.
Can't agree more, looks only intel-iommu and virtio-iommu implemented such optimization for now. > >But I think we were just wrong to also notify in the unmap_all() procedure. > >IIUC the right thing to do (keeping replay() the interface as-is, per it used >to be >defined) is we should replace the unmap_all() to only evacuate the iova tree >(keeping all host mappings untouched, IOW, don't notify UNMAP), and do a >full resync there, which will notify all existing mappings as MAP. Then we >don't interrupt with any existing mapping if there is (e.g. for the dirty sync >case), meanwhile we keep sync too to latest (for moving a vfio device into an >existing iommu group). > >Do you think that'll work for us? Yes, I think I get your point. Below simple change will work in your suggested way, do you agree? @@ -3825,13 +3833,10 @@ static void vtd_iommu_replay(IOMMUMemoryRegion *iommu_mr, IOMMUNotifier *n) IntelIOMMUState *s = vtd_as->iommu_state; uint8_t bus_n = pci_bus_num(vtd_as->bus); VTDContextEntry ce; + DMAMap map = { .iova = 0, .size = HWADDR_MAX } - /* - * The replay can be triggered by either a invalidation or a newly - * created entry. No matter what, we release existing mappings - * (it means flushing caches for UNMAP-only registers). - */ - vtd_address_space_unmap(vtd_as, n); + /* replay is protected by BQL, page walk will re-setup IOVA tree safely */ + iova_tree_remove(as->iova_tree, map); if (vtd_dev_to_context_entry(s, bus_n, vtd_as->devfn, &ce) == 0) { trace_vtd_replay_ce_valid(s->root_scalable ? "scalable mode" : Thanks Zhenzhong