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

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)

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?