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[70.31.27.79]) by smtp.gmail.com with ESMTPSA id d16-20020a05620a241000b006fa43e139b5sm7513712qkn.59.2022.12.06.14.09.49 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 06 Dec 2022 14:09:50 -0800 (PST) Date: Tue, 6 Dec 2022 17:09:49 -0500 From: Peter Xu To: Eric Auger Cc: Jason Wang , qemu-devel@nongnu.org, "Michael S . Tsirkin" , Yi Liu , Alex Williamson Subject: Re: [PATCH] intel-iommu: Document iova_tree Message-ID: References: <20221201162501.3864692-1-peterx@redhat.com> <6189ef29-fe38-bc74-aff1-034eb28ab031@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: Received-SPF: pass client-ip=170.10.133.124; envelope-from=peterx@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, 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=-0.001, 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: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org On Tue, Dec 06, 2022 at 05:28:01PM +0100, Eric Auger wrote: > > > On 12/6/22 17:05, Peter Xu wrote: > > On Tue, Dec 06, 2022 at 02:16:32PM +0100, Eric Auger wrote: > >> Hi Peter, > >> On 12/6/22 00:28, Peter Xu wrote: > >>> On Mon, Dec 05, 2022 at 12:23:20PM +0800, Jason Wang wrote: > >>>> On Fri, Dec 2, 2022 at 12:25 AM Peter Xu wrote: > >>>>> It seems not super clear on when iova_tree is used, and why. Add a rich > >>>>> comment above iova_tree to track why we needed the iova_tree, and when we > >>>>> need it. > >>>>> > >>>>> Suggested-by: Jason Wang > >>>>> Signed-off-by: Peter Xu > >>>>> --- > >>>>> include/hw/i386/intel_iommu.h | 30 +++++++++++++++++++++++++++++- > >>>>> 1 file changed, 29 insertions(+), 1 deletion(-) > >>>>> > >>>>> diff --git a/include/hw/i386/intel_iommu.h b/include/hw/i386/intel_iommu.h > >>>>> index 46d973e629..8d130ab2e3 100644 > >>>>> --- a/include/hw/i386/intel_iommu.h > >>>>> +++ b/include/hw/i386/intel_iommu.h > >>>>> @@ -109,7 +109,35 @@ struct VTDAddressSpace { > >>>>> QLIST_ENTRY(VTDAddressSpace) next; > >>>>> /* Superset of notifier flags that this address space has */ > >>>>> IOMMUNotifierFlag notifier_flags; > >>>>> - IOVATree *iova_tree; /* Traces mapped IOVA ranges */ > >>>>> + /* > >>>>> + * @iova_tree traces mapped IOVA ranges. > >>>>> + * > >>>>> + * The tree is not needed if no MAP notifiers is registered with > >>>>> + * current VTD address space, because all UNMAP (including iotlb or > >>>>> + * dev-iotlb) events can be transparently delivered to !MAP iommu > >>>>> + * notifiers. > >>>> So this means the UNMAP notifier doesn't need to be as accurate as > >>>> MAP. (Should we document it in the notifier headers)? > >>> Yes. > >>> > >>>> For MAP[a, b] MAP[b, c] we can do a UNMAP[a. c]. > >>> IIUC a better way to say this is, for MAP[a, b] we can do an UNMAP[a-X, > >>> b+Y] as long as the range covers [a, b]? > >>> > >>>>> + * > >>>>> + * The tree OTOH is required for MAP typed iommu notifiers for a few > >>>>> + * reasons. > >>>>> + * > >>>>> + * Firstly, there's no way to identify whether an PSI event is MAP or > >>>>> + * UNMAP within the PSI message itself. Without having prior knowledge > >>>>> + * of existing state vIOMMU doesn't know whether it should notify MAP > >>>>> + * or UNMAP for a PSI message it received. > >>>>> + * > >>>>> + * Secondly, PSI received from guest driver (or even a large PSI can > >>>>> + * grow into a DSI at least with Linux intel-iommu driver) can be > >>>>> + * larger in range than the newly mapped ranges for either MAP or UNMAP > >>>>> + * events. > >>>> Yes, so I think we need a document that the UNMAP handler should be > >>>> prepared for this. > >>> How about I squash below into this same patch? > >>> > >>> diff --git a/include/exec/memory.h b/include/exec/memory.h > >>> index 91f8a2395a..c83bd11a68 100644 > >>> --- a/include/exec/memory.h > >>> +++ b/include/exec/memory.h > >>> @@ -129,6 +129,24 @@ struct IOMMUTLBEntry { > >>> /* > >>> * Bitmap for different IOMMUNotifier capabilities. Each notifier can > >>> * register with one or multiple IOMMU Notifier capability bit(s). > >>> + * > >>> + * Normally there're two use cases for the notifiers: > >>> + * > >>> + * (1) When the device needs accurate synchronizations of the vIOMMU page > >> accurate synchronizations sound too vague & subjective to me. > > Suggestions? > Well I would say: > when the notified device maintains a shadow page table and must to be s/to// > notified on each guest MAP (page table entry creation) and UNMAP > (invalidation) events (VFIO). Both notifications must be accurate so > that the shadow page table is fully in sync with the guest view. Thanks, I'll try to squash this into the new version. > > > >>> + * tables, it needs to register with both MAP|UNMAP notifies (which > >>> + * is defined as IOMMU_NOTIFIER_IOTLB_EVENTS below). As long as MAP > >>> + * events are registered, the notifications will be accurate but > >>> + * there's overhead on synchronizing the guest vIOMMU page tables. > >>> + * > >>> + * (2) When the device doesn't need accurate synchronizations of the > >>> + * vIOMMU page tables (when the device can both cache translations > >>> + * and requesting to translate dynamically during DMA process), it > when the notified device maintains a cache of IOMMU translations (IOTLB) > and is able to fill that cache by requesting translations from the > vIOMMU through a protocol similar to ATS. In that case the notified > device only needs to register an UNMAP notifier. In that case the unmap > notifications are allower to be wider than the strict necessary. Same here. > > However the problem is since you need to satisfy the VFIO use case, how > do you detect when you are allowed to invalidate more that the strict > necessary? We detect that by checking whether the vtd_as has map notifier registered. Please feel free to have a look at all sites of vtd_as_has_map_notifier(). We maintain the iova tree only for MAP case currently. -- Peter Xu