On Mon, Jun 24, 2024 at 10:38:51 -0700, Jon Kohler wrote:
Add plumbing for QEMU's switchover-ack migration capability, which helps lower the downtime during VFIO migrations. This capability is enabled by default as long as both the source and destination support it.
Note: switchover-ack depends on the return path capability, so this may not be used when VIR_MIGRATE_TUNNELLED flag is set.
Extensive details about the qemu switchover-ack implementation are available in the qemu series v6 cover letter [1] where the highlight is the extreme reduction in guest visible downtime. In addition to the original test results below, I saw a roughly ~20% reduction in downtime for VFIO VGPU devices at minimum.
=== Test results ===
The below table shows the downtime of two identical migrations. In the first migration swithcover ack is disabled and in the second it is enabled. The migrated VM is assigned with a mlx5 VFIO device which has 300MB of device data to be migrated.
+----------------------+-----------------------+----------+ | Switchover ack | VFIO device data size | Downtime | +----------------------+-----------------------+----------+ | Disabled | 300MB | 1900ms | | Enabled | 300MB | 420ms | +----------------------+-----------------------+----------+
Switchover ack gives a roughly 4.5 times improvement in downtime. The 1480ms difference is time that is used for resource allocation for the VFIO device in the destination. Without switchover ack, this time is spent when the source VM is stopped and thus the downtime is much higher. With switchover ack, the time is spent when the source VM is still running.
[1] https://patchwork.kernel.org/project/qemu-devel/cover/20230621111201.29729-1...
Signed-off-by: Jon Kohler <jon@nutanix.com> Cc: Alex Williamson <alex.williamson@redhat.com> Cc: Avihai Horon <avihaih@nvidia.com> Cc: Markus Armbruster <armbru@redhat.com> Cc: Peter Xu <peterx@redhat.com> Cc: YangHang Liu <yanghliu@redhat.com> --- v1 - https://lists.libvirt.org/archives/list/devel@lists.libvirt.org/thread/2XCWP... v1 -> v2: - Addressed comments to simplify approach (Daniel, Jiri) --- src/qemu/qemu_migration.h | 1 + src/qemu/qemu_migration_params.c | 8 +++++++- src/qemu/qemu_migration_params.h | 1 + 3 files changed, 9 insertions(+), 1 deletion(-)
diff --git a/src/qemu/qemu_migration.h b/src/qemu/qemu_migration.h index ed62fd4a91..cd89e100e1 100644 --- a/src/qemu/qemu_migration.h +++ b/src/qemu/qemu_migration.h @@ -62,6 +62,7 @@ VIR_MIGRATE_NON_SHARED_SYNCHRONOUS_WRITES | \ VIR_MIGRATE_POSTCOPY_RESUME | \ VIR_MIGRATE_ZEROCOPY | \ + VIR_MIGRATE_SWITCHOVER_ACK | \ 0)
/* All supported migration parameters and their types. */
This is a leftover from v1, it's no longer needed. I removed this hunk and pushed the patch. Thanks. Reviewed-by: Jiri Denemark <jdenemar@redhat.com>