Re: [RFC v2 1/1] memory: Delete assertion in memory_region_unregister_iommu_notifier

On 2020/7/10 下午9:30, Peter Xu wrote: > On Fri, Jul 10, 2020 at 02:34:11PM +0800, Jason Wang wrote: > > On 2020/7/9 下午10:10, Peter Xu wrote: > > > On Thu, Jul 09, 2020 at 01:58:33PM +0800, Jason Wang wrote: > > > > > > - If we care the performance, it's better to implement the MAP event for > > > > > > vhost, otherwise it could be a lot of IOTLB miss > > > > > I feel like these are two things. > > > > > > > > > > So far what we are talking about is whether vt-d should have knowledge about > > > > > what kind of events one iommu notifier is interested in. I still think we > > > > > should keep this as answered in question 1. > > > > > > > > > > The other question is whether we want to switch vhost from UNMAP to MAP/UNMAP > > > > > events even without vDMA, so that vhost can establish the mapping even before > > > > > IO starts. IMHO it's doable, but only if the guest runs DPDK workloads. When > > > > > the guest is using dynamic iommu page mappings, I feel like that can be even > > > > > slower, because then the worst case is for each IO we'll need to vmexit twice: > > > > > > > > > > - The first vmexit caused by an invalidation to MAP the page tables, so vhost > > > > > will setup the page table before IO starts > > > > > > > > > > - IO/DMA triggers and completes > > > > > > > > > > - The second vmexit caused by another invalidation to UNMAP the page tables > > > > > > > > > > So it seems to be worse than when vhost only uses UNMAP like right now. At > > > > > least we only have one vmexit (when UNMAP). We'll have a vhost translate() > > > > > request from kernel to userspace, but IMHO that's cheaper than the vmexit. > > > > Right but then I would still prefer to have another notifier. > > > > > > > > Since vtd_page_walk has nothing to do with device IOTLB. IOMMU have a > > > > dedicated command for flushing device IOTLB. But the check for > > > > vtd_as_has_map_notifier is used to skip the device which can do demand > > > > paging via ATS or device specific way. If we have two different notifiers, > > > > vhost will be on the device iotlb notifier so we don't need it at all? > > > But we can still have iommu notifier that only registers to UNMAP even after we > > > introduce dev-iotlb notifier? We don't want to do page walk for them as well. > > > TCG should be the only one so far, but I don't know.. maybe there can still be > > > new ones? > > > > I think you're right. But looking at the codes, it looks like the check of > > vtd_as_has_map_notifier() was only used in: > > > > 1) vtd_iommu_replay() > > 2) vtd_iotlb_page_invalidate_notify() (PSI) > > > > For the replay, it's expensive anyhow. For PSI, I think it's just about one > > or few mappings, not sure it will have obvious performance impact. > > > > And I had two questions: > > > > 1) The codes doesn't check map for DSI or GI, does this match what spec > > said? (It looks to me the spec is unclear in this part) > Both DSI/GI should cover maps too? E.g. vtd_sync_shadow_page_table() in > vtd_iotlb_domain_invalidate(). I meant the code doesn't check whether there's an MAP notifier :)

> > > 2) for the replay() I don't see other implementations (either spapr or > > generic one) that did unmap (actually they skip unmap explicitly), any > > reason for doing this in intel IOMMU? > I could be wrong, but I'd guess it's because vt-d implemented the caching mode > by leveraging the same invalidation strucuture, so it's harder to make all > things right (IOW, we can't clearly identify MAP with UNMAP when we receive an > invalidation request, because MAP/UNMAP requests look the same). > > I didn't check others, but I believe spapr is doing it differently by using > some hypercalls to deliver IOMMU map/unmap requests, which seems a bit close to > what virtio-iommu is doing. Anyway, the point is if we have explicit MAP/UNMAP > from the guest, logically the replay indeed does not need to do any unmap > because we don't need to call replay() on an already existing device but only > for e.g. hot plug. But this looks conflict with what memory_region_iommu_replay( ) did, for IOMMU that doesn't have a replay method, it skips UNMAP request:     for (addr = 0; addr < memory_region_size(mr); addr += granularity) {         iotlb = imrc->translate(iommu_mr, addr, IOMMU_NONE, n->iommu_idx);         if (iotlb.perm != IOMMU_NONE) {             n->notify(n, &iotlb);         } I guess there's no knowledge of whether guest have an explicit MAP/UMAP for this generic code. Or replay implies that guest doesn't have explicit MAP/UNMAP?

(btw, the code shortcut the memory_region_notify_one(), not sure the reason)

> VT-d does not have that clear interface, so VT-d needs to > maintain its own mapping structures, and also vt-d is using the same replay & > page_walk operations to sync all these structures, which complicated the vt-d > replay a bit. With that, we assume replay() can be called anytime on a device, > and we won't notify duplicated MAPs to lower layer like vfio if it is mapped > before. At the meantime, since we'll compare the latest mapping with the one > we cached in the iova tree, UNMAP becomes possible too. AFAIK vtd_iommu_replay() did a completely UNMAP:     /*      * 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); Since it doesn't do any comparison with iova tree. Will this cause unnecessary UNMAP to be sent to VFIO?