Re: [PATCH v3 07/10] migration: add new migration state wait-unplug

[Jens Freimann]

On Fri, Oct 11, 2019 at 06:11:33PM +0100, Dr. David Alan Gilbert wrote:
> * Jens Freimann (address@hidden) wrote:
> > This patch adds a new migration state called wait-unplug.  It is entered
> > after the SETUP state and will transition into ACTIVE once all devices
> > were succesfully unplugged from the guest.
> >
> > So if a guest doesn't respond or takes long to honor the unplug request
> > the user will see the migration state 'wait-unplug'.
> >
> > In the migration thread we query failover devices if they're are still
> > pending the guest unplug. When all are unplugged the migration
> > continues. We give it a defined number of iterations including small
> > waiting periods before we proceed.
> >
> > Signed-off-by: Jens Freimann <address@hidden>
[..]
> > @@ -3260,6 +3271,27 @@ static void *migration_thread(void *opaque)
> >
> >      qemu_savevm_state_setup(s->to_dst_file);
> >
> > +    migrate_set_state(&s->state, MIGRATION_STATUS_SETUP,
> > +                      MIGRATION_STATUS_WAIT_UNPLUG);
>
> I think I'd prefer if you only went into this state if you had any
> devices that were going to need unplugging.

Sure, that makes sense. I'll change it.

> > +    while (i < FAILOVER_UNPLUG_RETRIES &&
> > +           s->state == MIGRATION_STATUS_WAIT_UNPLUG) {
> > +        i++;
> > +        qemu_sem_timedwait(&s->wait_unplug_sem, FAILOVER_GUEST_UNPLUG_WAIT);
> > +        all_unplugged = qemu_savevm_state_guest_unplug_pending();
> > +        if (all_unplugged) {
> > +            break;
> > +        }
> > +    }
> > +
> > +    if (all_unplugged) {
> > +        migrate_set_state(&s->state, MIGRATION_STATUS_WAIT_UNPLUG,
> > +                MIGRATION_STATUS_ACTIVE);
> > +    } else {
> > +        migrate_set_state(&s->state, MIGRATION_STATUS_WAIT_UNPLUG,
> > +                          MIGRATION_STATUS_CANCELLING);
> > +    }
>
> I think you can get rid of both the timeout and the count and just make
> sure that migrate_cancel works at this point.

I see, I need to add the new state to migration_is_setup_or_active() or
a cancel won't work.  

> This pushes the problem up a layer, which I think is fine.

Seems good to me. To be clear, you're saying I should just poll on
the device unplugged state? Like

        while (s->state == MIGRATION_STATUS_WAIT_UNPLUG &&
               !qemu_savevm_state_guest_unplug_pending()) {
_            /* This block intentionally left blank */
        }

regards,
Jens