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

[Kevin Wolf]

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" <address@hidden> wrote:
> > 
> > > * Daniel P. Berrangé (address@hidden) 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.

> 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