Re: [PATCH] vl: Add -set options to device opts dict when using JSON syntax for -device

[Daniel P . Berrangé]

On Wed, Dec 22, 2021 at 09:22:47AM +0100, Gerd Hoffmann wrote:
> On Tue, Dec 21, 2021 at 04:40:28PM +0100, Markus Armbruster wrote:
> > Paolo Bonzini <pbonzini@redhat.com> writes:
> > 
> > > On 12/21/21 13:58, Markus Armbruster wrote:
> > >>> Is this a regression?  I suspect commit 5dacda5167 "vl: Enable JSON
> > >>> syntax for -device" (v6.2.0).
> > >> 
> > >> Obviously not a regression: everything that used to work still works.
> > >
> > > FWIW I think -set should be deprecated.  I'm not aware of any
> > > particularly useful use of it.  There are a couple in the QEMU tests
> > > (in vhost-user-test and in qemu-iotests 068), but in both cases the
> > > code would be easier to follow without; patches can be dusted off if
> > > desired.
> > 
> > -set has its uses, but they're kind of obscure.  When you want to use
> > some canned configuration with slight modifications, then -readconfig
> > canned.cfg -set ... is nicer than editing a copy of canned.cfg.
> 
> Simliar: configure stuff not supported by libvirt:
> 
>   <qemu:commandline>
>     <qemu:arg value='-set'/>
>     <qemu:arg value='device.video0.guestdebug=1'/>
>   </qemu:commandline>
> 
> There will always be a gap between qemu and libvirt, even if most of
> them are temporary only (while developing a new feature).  I think we
> need some way to deal with this kind of tweaks when moving to QAPI-based
> machine setup.  Possibly not in qemu, maybe it's easier to add new
> '<qemu:set device=... property=... value=...>' syntax to libvirt.

Yes, I'd suggest we get

   <qemu:device alias="video0" name="guestdebug" value="1/>

and then libvirt can use it to add  'guestdebug: 1' directly to the 
JSON it generates, avoiding -set entirely.

Regards,
Daniel
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