* Paolo Bonzini ([email protected]) wrote:
>
>
> On 03/03/2017 13:00, Dr. David Alan Gilbert wrote:
> > Ouch that's pretty nasty; I remember Paolo explaining to me a while ago that
> > their were times when run_on_cpu would have to drop the BQL and I worried
> > about it,
> > but this is the 1st time I've seen an error due to it.
> >
> > Do you know what the migration state was at that point? Was it
> > MIGRATION_STATUS_CANCELLING?
> > I'm thinking perhaps we should stop 'cont' from continuing while migration
> > is in
> > MIGRATION_STATUS_CANCELLING. Do we send an event when we hit CANCELLED -
> > so that
> > perhaps libvirt could avoid sending the 'cont' until then?
>
> No, there's no event, though I thought libvirt would poll until
> "query-migrate" returns the cancelled state. Of course that is a small
> consolation, because a segfault is unacceptable.
I think you might get an event if you set the new migrate capability called
'events' on!
void migrate_set_state(int *state, int old_state, int new_state)
{
if (atomic_cmpxchg(state, old_state, new_state) == old_state) {
trace_migrate_set_state(new_state);
migrate_generate_event(new_state);
}
}
static void migrate_generate_event(int new_state)
{
if (migrate_use_events()) {
qapi_event_send_migration(new_state, &error_abort);
}
}
That event feature went in sometime after 2.3.0.
> One possibility is to suspend the monitor in qmp_migrate_cancel and
> resume it (with add_migration_state_change_notifier) when we hit the
> CANCELLED state. I'm not sure what the latency would be between the end
> of migrate_fd_cancel and finally reaching CANCELLED.
I don't like suspending monitors; it can potentially take quite a significant
time to do a cancel.
How about making 'cont' fail if we're in CANCELLING?
I'd really love to see the 'run_on_cpu' being more careful about the BQL;
we really need all of the rest of the devices to stay quiesced at times.
Dave
> Paolo
--
Dr. David Alan Gilbert / [email protected] / Manchester, UK
