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bh=UWPYT268JJ7duWbT5LFMEPo6jzoSsBJomX4dyYL3XNg=; b=fFlH0BEmQQZlcxFJKLCS/d/5PfGt/NxKqbCKsQCAPZkWQ1/54ufBdKuK8ES9PVwb+tW+hy xsAUaTQabZVAPWU1hJkstIBLx/eZru5srJZxJPb/q0ivbUd9GJj68v8RiXfqLR9Au7T3Ut FsAZAhuNSDZf6lHbwzo4CsqCSeb7O/Q= X-MC-Unique: 2P-et4l6MbWkoinlDb1Igw-1 Subject: [PULL 15/17] docs/devel: Document VFIO device dirty page tracking From: Alex Williamson To: qemu-devel@nongnu.org Cc: alex.williamson@redhat.com, avihaih@nvidia.com, clg@redhat.com, joao.m.martins@oracle.com Date: Tue, 07 Mar 2023 11:53:45 -0700 Message-ID: <167821522548.619792.7501558238966897784.stgit@omen> In-Reply-To: <167821508699.619792.1719671327865445814.stgit@omen> References: <167821508699.619792.1719671327865445814.stgit@omen> User-Agent: StGit/1.5.dev2+g9ce680a52bd9 MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable X-Scanned-By: MIMEDefang 3.1 on 10.11.54.2 Received-SPF: pass (zohomail.com: domain of gnu.org designates 209.51.188.17 as permitted sender) client-ip=209.51.188.17; envelope-from=qemu-devel-bounces+importer2=patchew.org@nongnu.org; helo=lists.gnu.org; Received-SPF: pass client-ip=170.10.133.124; envelope-from=alex.williamson@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+importer2=patchew.org@nongnu.org Sender: qemu-devel-bounces+importer2=patchew.org@nongnu.org X-ZohoMail-DKIM: pass (identity @redhat.com) X-ZM-MESSAGEID: 1678215243608100001 From: Avihai Horon Adjust the VFIO dirty page tracking documentation and add a section to describe device dirty page tracking. Signed-off-by: Avihai Horon Signed-off-by: Joao Martins Reviewed-by: C=C3=A9dric Le Goater Link: https://lore.kernel.org/r/20230307125450.62409-16-joao.m.martins@orac= le.com Signed-off-by: Alex Williamson --- docs/devel/vfio-migration.rst | 46 ++++++++++++++++++++++++++++---------= ---- 1 file changed, 31 insertions(+), 15 deletions(-) diff --git a/docs/devel/vfio-migration.rst b/docs/devel/vfio-migration.rst index c214c73e2818..1b68ccf11529 100644 --- a/docs/devel/vfio-migration.rst +++ b/docs/devel/vfio-migration.rst @@ -59,22 +59,37 @@ System memory dirty pages tracking ---------------------------------- =20 A ``log_global_start`` and ``log_global_stop`` memory listener callback in= forms -the VFIO IOMMU module to start and stop dirty page tracking. A ``log_sync`` -memory listener callback marks those system memory pages as dirty which are -used for DMA by the VFIO device. The dirty pages bitmap is queried per -container. All pages pinned by the vendor driver through external APIs hav= e to -be marked as dirty during migration. When there are CPU writes, CPU dirty = page -tracking can identify dirtied pages, but any page pinned by the vendor dri= ver -can also be written by the device. There is currently no device or IOMMU -support for dirty page tracking in hardware. +the VFIO dirty tracking module to start and stop dirty page tracking. A +``log_sync`` memory listener callback queries the dirty page bitmap from t= he +dirty tracking module and marks system memory pages which were DMA-ed by t= he +VFIO device as dirty. The dirty page bitmap is queried per container. + +Currently there are two ways dirty page tracking can be done: +(1) Device dirty tracking: +In this method the device is responsible to log and report its DMAs. This +method can be used only if the device is capable of tracking its DMAs. +Discovering device capability, starting and stopping dirty tracking, and +syncing the dirty bitmaps from the device are done using the DMA logging u= API. +More info about the uAPI can be found in the comments of the +``vfio_device_feature_dma_logging_control`` and +``vfio_device_feature_dma_logging_report`` structures in the header file +linux-headers/linux/vfio.h. + +(2) VFIO IOMMU module: +In this method dirty tracking is done by IOMMU. However, there is currentl= y no +IOMMU support for dirty page tracking. For this reason, all pages are +perpetually marked dirty, unless the device driver pins pages through exte= rnal +APIs in which case only those pinned pages are perpetually marked dirty. + +If the above two methods are not supported, all pages are perpetually mark= ed +dirty by QEMU. =20 By default, dirty pages are tracked during pre-copy as well as stop-and-co= py -phase. So, a page pinned by the vendor driver will be copied to the destin= ation -in both phases. Copying dirty pages in pre-copy phase helps QEMU to predic= t if -it can achieve its downtime tolerances. If QEMU during pre-copy phase keeps -finding dirty pages continuously, then it understands that even in stop-an= d-copy -phase, it is likely to find dirty pages and can predict the downtime -accordingly. +phase. So, a page marked as dirty will be copied to the destination in both +phases. Copying dirty pages in pre-copy phase helps QEMU to predict if it = can +achieve its downtime tolerances. If QEMU during pre-copy phase keeps findi= ng +dirty pages continuously, then it understands that even in stop-and-copy p= hase, +it is likely to find dirty pages and can predict the downtime accordingly. =20 QEMU also provides a per device opt-out option ``pre-copy-dirty-page-track= ing`` which disables querying the dirty bitmap during pre-copy phase. If it is s= et to @@ -89,7 +104,8 @@ phase of migration. In that case, the unmap ioctl return= s any dirty pages in that range and QEMU reports corresponding guest physical pages dirty. Duri= ng stop-and-copy phase, an IOMMU notifier is used to get a callback for mapped pages and then dirty pages bitmap is fetched from VFIO IOMMU modules for t= hose -mapped ranges. +mapped ranges. If device dirty tracking is enabled with vIOMMU, live migra= tion +will be blocked. =20 Flow of state changes during Live migration =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D