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Mon, 13 Jul 2020 04:04:18 +0000 (UTC) Subject: Re: [RFC v2 1/1] memory: Delete assertion in memory_region_unregister_iommu_notifier To: Peter Xu References: <20200702154540.GI40675@xz-x1> <34fe0e55-c0ae-8e56-462b-6281b6cca4f5@redhat.com> <20200703130338.GD6677@xz-x1> <20200707195429.GF88106@xz-x1> <5004a059-6eb0-4ef3-40b7-94dfbf9ec08f@redhat.com> <20200708141657.GA199122@xz-x1> <14b1ca26-448d-0feb-7529-6546809aaa59@redhat.com> <20200709141037.GF199122@xz-x1> <20200710133005.GL199122@xz-x1> From: Jason Wang Message-ID: <05bb512c-ca0a-e80e-1eed-446e918ad729@redhat.com> Date: Mon, 13 Jul 2020 12:04:16 +0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 MIME-Version: 1.0 In-Reply-To: <20200710133005.GL199122@xz-x1> Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=207.211.31.120; envelope-from=jasowang@redhat.com; helo=us-smtp-1.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/07/13 00:04:38 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] [fuzzy] X-Spam_score_int: -40 X-Spam_score: -4.1 X-Spam_bar: ---- X-Spam_report: (-4.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-1, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H2=-1, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Peter Maydell , Yan Zhao , "Michael S. Tsirkin" , "libvir-list@redhat.com" , Juan Quintela , qemu-devel@nongnu.org, =?UTF-8?Q?Eugenio_P=c3=a9rez?= , Eric Auger , Paolo Bonzini Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" 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? Thanks > > Thanks, >