Re: [PATCH v5 2/9] migration: Implement switchover ack logic

[Avihai Horon]

On 08/06/2023 21:32, Alex Williamson wrote:
> External email: Use caution opening links or attachments
>
>
> On Tue, 6 Jun 2023 15:12:13 +0300
> Avihai Horon <avihaih@nvidia.com> wrote:
> > On 06/06/2023 1:06, Alex Williamson wrote:
> > > On Tue, 30 May 2023 17:48:14 +0300
> > > Avihai Horon <avihaih@nvidia.com> wrote:
> > > > @@ -2700,6 +2713,20 @@ static void migration_update_counters(MigrationState *s,
> > > >                                  bandwidth, s->threshold_size);
> > > >    }
> > > >
> > > > +static bool migration_can_switchover(MigrationState *s)
> > > > +{
> > > > +    if (!migrate_switchover_ack()) {
> > > > +        return true;
> > > > +    }
> > > > +
> > > > +    /* No reason to wait for switchover ACK if VM is stopped */
> > > > +    if (!runstate_is_running()) {
> > > > +        return true;
> > > > +    }
> > > Is it possible for QEMU to force the migration to continue regardless
> > > of receiving an ack from the target and is this the check that would
> > > allow that?
> > Yes. If you stop the source VM then migration will not wait for an ACK
> > to do the switchover.
> >
> > > It seems that we don't know the downtime allowed for the VM in any of
> > > this, nor do we know how much time the target device will require to
> > > generate an ack, but we could certainly have conditions where the
> > > priority is moving the VM from the source host regardless of the
> > > resulting downtime.
> > In such cases you can keep the switchover-ack capability off.
> How is that accomplished?

You simply don't enable the switchover-ack migration capability (it is 
disabled by default).

> > > Also does the return path requirement preclude offline migration or
> > > does the above again take care of that if we pause the VM for an
> > > offline migration (ex. save to and restore from file)?
> > I suppose that by offline migration you mean migration where you stop
> > the source VM first and then do migration?
> Yes.
>
> > If so, offline migration should work and in that case we don't care
> > about the ACK as downtime is not a concern.
> >
> > However, migrating to a file doesn't work with return-path, as you don't
> > have the destination side responding to the source via the return path.
> > For this reason, using return-path when migrating to a file doesn't make
> > sense.
> So we require return-path for switchover-ack, but switchover-ack is
> only required for pre-copy, therefore why do we require return-path for
> an offline migration?

We don't. See below.

> If there's a way to turn off switchover-ack capability, is there also a
> way to turn off return-path and therefore enable migration to file?

Yes.

By default, return-path and switchover-ack migration capabilities are 
disabled.

So for an offline migration nothing needs to be done -- you simply run 
migration.

For an online migration, you can choose to use switchover-ack or not.
If you want to use switchover-ack, then you need to enable return-path 
and switchover-ack capabilities first and then run migration.
If you don't want (e.g., you don't have a VFIO device assigned to the 
VM, so there is no reason to), then you keep return-path and 
switchover-ack capabilities disabled.

I hope that's clear now.

Thanks.