Re: [PATCH] qemu: add support for qemu switchover-ack
On Tue, Jun 18, 2024 at 16:14:29 +0100, Daniel P. Berrangé wrote:
On Tue, Jun 18, 2024 at 08:06:06AM -0700, Jon Kohler wrote:
> diff --git a/include/libvirt/libvirt-domain.h b/include/libvirt/libvirt-domain.h
> index 2f5b01bbfe..9543629f30 100644
> --- a/include/libvirt/libvirt-domain.h
> +++ b/include/libvirt/libvirt-domain.h
> @@ -1100,6 +1100,17 @@ typedef enum {
> * Since: 8.5.0
> */
> VIR_MIGRATE_ZEROCOPY = (1 << 20),
> +
> + /* Use switchover ack migration capability to reduce downtime on VFIO
> + * device migration. This prevents the source from stopping the VM and
> + * completing the migration until an ACK is received from the destination
> + * that it's OK to do so. Thus, a VFIO device can make sure that its
> + * initial bytes were sent and loaded in the destination before the
> + * source VM is stopped.
> + *
> + * Since: 10.5.0
> + */
> + VIR_MIGRATE_SWITCHOVER_ACK = (1 << 21),
> } virDomainMigrateFlags;
Do we really need a flag for this ? Is there a credible scenario
in which this flag works, and yet shouldn't be used by libvirt ?
IOW, can we just "do the right thing" and always enable this,
except for TUNNELLED mode.
I discussed this capability some time ago with Peter (I think) and if
IIRC there was some downside when the capability is enabled for domains
that do not use VFIO. I don't remember exactly what it was about, but
perhaps introducing an extra delay in migration switchover? Peter, can
you add the details, please?
That said, I think we should be able to do the right thing anyway and
enable this capability only for domains that use VFIO.
Jirka