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

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

* Kevin Wolf (kwolf@redhat.com) wrote:
> Am 12.05.2020 um 11:43 hat Daniel P. Berrangé geschrieben:
> > 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.
> 
> I'm generally not a big fan of silently ignoring things. Is there a
> specific requirement to do it in this case, or can management
> applications be expected to know which connections exist?
> 
> > 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.
> 
> Of course, it also means that the yank code needs to know about every
> single object that supports the operation, whereas if you only have
> strings, the objects could keep registering their connection with a
> generic function like yank_register_function() in this version.
> 
> I'm not sure if the additional complexity is worth the benefits.

I tend to agree; although we do have to ensure we either use an existing
naming scheme (e.g. QOM object names?) or make sure we've got a well
defined list of prefixes.

Dave

> 
> > 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.
> 
> This is true, though I think Lukas' suggestion with query-yank should be
> as good in practice (you can't check before creating the NBD device
> then, but would you actually want to do this?).
> 
> And if all else fails, we can still add a few more feature flags to the
> schema...
> 
> Kevin
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK