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Thu, 16 Jul 2020 02:54:32 +0000 (UTC) Subject: Re: [RFC v2 1/1] memory: Delete assertion in memory_region_unregister_iommu_notifier To: Peter Xu References: <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> <05bb512c-ca0a-e80e-1eed-446e918ad729@redhat.com> <20200716010005.GA535743@xz-x1> From: Jason Wang Message-ID: Date: Thu, 16 Jul 2020 10:54:31 +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: <20200716010005.GA535743@xz-x1> Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=jasowang@redhat.com 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=205.139.110.120; envelope-from=jasowang@redhat.com; helo=us-smtp-1.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/07/15 19:36:06 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 2020/7/16 上午9:00, Peter Xu wrote: > On Mon, Jul 13, 2020 at 12:04:16PM +0800, Jason Wang wrote: >> 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 :) > It's actually checked, because it loops over vtd_as_with_notifiers, and only > MAP notifiers register to that. :) I may miss something but I don't find the code to block UNMAP notifiers? vhost_iommu_region_add()     memory_region_register_iommu_notifier()         memory_region_update_iommu_notify_flags()             imrc->notify_flag_changed()                 vtd_iommu_notify_flag_changed() ? > > But I agree with you that it should be cleaner to introduce the dev-iotlb > notifier type. Yes, it can solve the issues of duplicated UNMAP issue of vhost. >> >>>> 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? > I think it matches exactly with a hot plug case? Note that when IOMMU_NONE > could also mean the translation does not exist. So it's actually trying to map > everything that can be translated and then notify(). Yes, so the question is what's the assumption before calling memory_region_iommu_replay(). If it assumes an empty mapping, there's probably no need for unamp. > >> (btw, the code shortcut the memory_region_notify_one(), not sure the reason) > I think it's simply because memory_region_notify_one() came later. :) Ok, that explains. > >> >>> 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? > I feel like that can be removed now, but needs some testings... Probably, but need to answer the above question about replay first. Thanks > > Thanks, >