Re: [Qemu-devel] [Bug?] BQL about live migration

[Paolo Bonzini <pbonzini@redhat.com>]

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.

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.

Paolo