Re: [PATCH 07/14] migration: Make switchover-ack re-usable

[Fabiano Rosas <farosas@suse.de>]

Avihai Horon <avihaih@nvidia.com> writes:

> Switchover-ack is a mechanism to synchronize between source and
> destination QEMU during migration to prevent the source from switching
> over prematurely.
>
> VFIO uses switchover-ack to ensure switchover happens only after
> destination side has loaded the precopy initial bytes. This is important
> for VFIO, as otherwise downtime could be impacted and be higher.
>
> In its current state, switchover-ack is a one-time mechanism, meaning
> that switchover is acked only once and past that another ACK cannot be
> requested again. This was sufficient until now, as VFIO precopy initial
> bytes was defined to be monotonically decreasing. Thus, when precopy
> initial bytes reached zero for all VFIO devices, a single ACK would be
> sent and its validity would hold.
>
> However, now the new VFIO_PRECOPY_INFO_REINIT feature allows precopy
> initial bytes to be re-initialized during precopy. Specifically, it
> means that initial bytes can grow after reaching zero, which would
> invalidate a previously sent switchover ACK.
>
> To solve this, make switchover-ack reusable and allow devices to request
> another switchover ACK when needed.
>
> To avoid scattering them all over, switchover ACKs are requested through
> a new request_switchover_ack handler which is called in specific places.
>
> Since now switchover ACK can be requested for a specific device and in
> different times, make switchover ACK per-device (instead of a single ACK
> for all devices) and let source side do the pending ACKs accounting.
>
> Keep the legacy switchover-ack mechanism for backward compatibility and
> turn it on by a compatibility property for older machines. Enable the
> property until VFIO implements the new switchover-ack.
>
> Signed-off-by: Avihai Horon <avihaih@nvidia.com>

Reviewed-by: Fabiano Rosas <farosas@suse.de>