Re: [PATCH 0/5] Introduce 'yank' oob qmp command to recover from hanging qemu

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Daniel P. Berrangé (berrange@redhat.com) wrote:
> On Tue, May 12, 2020 at 11:32:06AM +0200, Lukas Straub wrote:
> > On Mon, 11 May 2020 16:46:45 +0100
> > "Dr. David Alan Gilbert" <dgilbert@redhat.com> wrote:
> > 
> > > * Daniel P. Berrangé (berrange@redhat.com) wrote: 
> > > > ...
> > > > That way if QEMU does get stuck, you can start by tearing down the
> > > > least distruptive channel. eg try tearing down the migration connection
> > > > first (which shouldn't negatively impact the guest), and only if that
> > > > doesn't work then, move on to tear down the NBD connection (which risks
> > > > data loss)  
> > > 
> > > I wonder if a different way would be to make all network connections
> > > register with yank, but then make yank take a list of connections to
> > > shutdown(2).
> > 
> > Good Idea. We could name the connections (/yank callbacks) in the
> > form "nbd:<node-name>", "chardev:<chardev-name>" and "migration"
> > (and add "netdev:...", etc. in the future). Then make yank take a
> > list of connection names as you suggest and silently ignore connections
> > that don't exist. And maybe even add a 'query-yank' oob command returning
> > a list of registered connections so the management application can do
> > pattern matching if it wants.
> 
> Yes, that would make the yank command much more flexible in how it can
> be used.
> 
> As an alternative to using formatted strings like this, it could be
> modelled more explicitly in QAPI
> 
>   { 'struct':  'YankChannels',
>     'data': { 'chardev': [ 'string' ],
>               'nbd': ['string'],
> 	      'migration': bool } }
> 
> In this example, 'chardev' would accept a list of chardev IDs which
> have it enabled, 'nbd' would accept a list of block node IDs which
> have it enabled, and migration is a singleton on/off.

Do we already have a QOM object name for each of these things?
Is that nbd/blockdevice unique - i.e. can you have multiple nbd clients
on the same node?

> The benefit of this modelling is that you can introspect QEMU
> to discover what classes of channels support being yanked by
> this QEMU build, as well as what instances are configured to
> be yanked. ie you can distinguish between a QEMU that doesn't
> support yanking network devices, from a QEMU that does support
> yanking network devices, but doesn't have it enabled for any
> network device instances.

What do we need to make it introspectable like that?

Dave

> Regards,
> Daniel
> -- 
> |: https://berrange.com      -o-    https://www.flickr.com/photos/dberrange :|
> |: https://libvirt.org         -o-            https://fstop138.berrange.com :|
> |: https://entangle-photo.org    -o-    https://www.instagram.com/dberrange :|
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK