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

[Lukas Straub]

On Tue, 12 May 2020 10:43:37 +0100
Daniel P. Berrangé <address@hidden> wrote:

> On Tue, May 12, 2020 at 11:32:06AM +0200, Lukas Straub wrote:
> > ... 
> > 
> > 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.

With the new command, the yank feature can then be enabled unconditionally on 
the instances.

> 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.
> 
> Regards,
> Daniel

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