On Thu, 11 Mar 2021 00:50:09 +0530
Tarun Gupta <targupta@nvidia.com> wrote:
Document interfaces used for VFIO device migration. Added flow of state changes
during live migration with VFIO device. Tested by building docs with the new
vfio-migration.rst file.
v2:
- Included the new vfio-migration.rst file in index.rst
- Updated dirty page tracking section, also added details about
'pre-copy-dirty-page-tracking' opt-out option.
- Incorporated comments around wording of doc.
Signed-off-by: Tarun Gupta <targupta@nvidia.com>
Signed-off-by: Kirti Wankhede <kwankhede@nvidia.com>
---
MAINTAINERS | 1 +
docs/devel/index.rst | 1 +
docs/devel/vfio-migration.rst | 135 ++++++++++++++++++++++++++++++++++
3 files changed, 137 insertions(+)
create mode 100644 docs/devel/vfio-migration.rst
(...)
diff --git a/docs/devel/vfio-migration.rst b/docs/devel/vfio-migration.rst
new file mode 100644
index 0000000000..6196fb132c
--- /dev/null
+++ b/docs/devel/vfio-migration.rst
@@ -0,0 +1,135 @@
+=====================
+VFIO device Migration
+=====================
Maybe add an introductory sentence or two describing the general
approach? I.e. we have a general framework, and specific support for
devices needs to be hooked up.
+
+VFIO devices use an iterative approach for migration because certain VFIO
+devices (e.g. GPU) have large amount of data to be transfered. The iterative
+pre-copy phase of migration allows for the guest to continue whilst the VFIO
+device state is transferred to the destination, this helps to reduce the total
+downtime of the VM. VFIO devices can choose to skip the pre-copy phase of
+migration by returning pending_bytes as zero during the pre-copy phase.
What about something like:
"Migration of VFIO devices consists of two phases: the optional
pre-copy phase, and the stop-and-copy phase. The pre-copy phase is
iterative and allows to accommodate VFIO devices that have a large
amount of data that needs to be transferred. The iterative pre-copy
phase..."
+
+A detailed description of the UAPI for VFIO device migration can be found in
+the comment for the ``vfio_device_migration_info`` structure in the header
+file linux-headers/linux/vfio.h.
+
+VFIO device hooks for iterative approach:
+
+* A ``save_setup`` function that sets up the migration region, sets _SAVING
+ flag in the VFIO device state and informs the VFIO IOMMU module to start
+ dirty page tracking.
+
+* A ``load_setup`` function that sets up the migration region on the
+ destination and sets _RESUMING flag in the VFIO device state.
+
+* A ``save_live_pending`` function that reads pending_bytes from the vendor
+ driver, which indicates the amount of data that the vendor driver has yet to
+ save for the VFIO device.
+
+* A ``save_live_iterate`` function that reads the VFIO device's data from the
+ vendor driver through the migration region during iterative phase.
+
+* A ``save_live_complete_precopy`` function that resets _RUNNING flag from the
+ VFIO device state, saves the device config space, if any, and iteratively
+ copies the remaining data for the VFIO device untill the vendor driver
s/untill/until/
+ indicates that no data remains (pending bytes is zero).
+
+* A ``load_state`` function that loads the config section and the data
+ sections that are generated by the save functions above
+
+* ``cleanup`` functions for both save and load that perform any migration
+ related cleanup, including unmapping the migration region
+
+A VM state change handler is registered to change the VFIO device state when
+the VM state changes.
+
+Similarly, a migration state change notifier is registered to get a
+notification on migration state change. These states are translated to VFIO
s/to/to the corresponding/
+device state and conveyed to vendor driver.
s/to/to the/
(...)
+Postcopy
+========
+
+Postcopy migration is not supported for VFIO devices.
s/is not/is currently not/ ?