docs/devel: Document VFIO device dirty page tracking

Adjust the VFIO dirty page tracking documentation and add a section to
describe device dirty page tracking.

Signed-off-by: Avihai Horon <avihaih@nvidia.com>
Link: https://lore.kernel.org/r/20230222174915.5647-21-avihaih@nvidia.com


Signed-off-by: Alex Williamson <alex.williamson@redhat.com>

docs/devel/vfio-migration.rst

@@ -71,22 +71,37 @@ System memory dirty pages tracking
 ----------------------------------
 
 A ``log_global_start`` and ``log_global_stop`` memory listener callback informs
-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 have 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 driver
-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 the
+dirty tracking module and marks system memory pages which were DMA-ed by the
+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 uAPI.
+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 currently no
+IOMMU support for dirty page tracking. For this reason, all pages are
+perpetually marked dirty, unless the device driver pins pages through external
+APIs in which case only those pinned pages are perpetually marked dirty.
+
+If the above two methods are not supported, all pages are perpetually marked
+dirty by QEMU.
 
 By default, dirty pages are tracked during pre-copy as well as stop-and-copy
-phase. So, a page pinned by the vendor driver 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
-finding dirty pages continuously, then it understands that even in stop-and-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 finding
+dirty pages continuously, then it understands that even in stop-and-copy phase,
+it is likely to find dirty pages and can predict the downtime accordingly.
 
 QEMU also provides a per device opt-out option ``pre-copy-dirty-page-tracking``
 which disables querying the dirty bitmap during pre-copy phase. If it is set to
@@ -97,10 +112,9 @@ System memory dirty pages tracking when vIOMMU is enabled
 ---------------------------------------------------------
 
 With vIOMMU, an IO virtual address range can get unmapped while in pre-copy
-phase of migration. In that case, the unmap ioctl returns any dirty pages in
-that range and QEMU reports corresponding guest physical pages dirty. During
-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 those
+phase of migration. In that case, dirty page bitmap for this range is queried
+and synced with QEMU. During stop-and-copy phase, an IOMMU notifier is used to
+get a callback for mapped pages and then dirty page bitmap is fetched for those
 mapped ranges.
 
 Flow of state changes during Live migration