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

[Damien Hedde]

On 12/22/21 09:22, 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.
>
> take care,
>    Gerd

Can the set feature be handled by libvirt ?
I mean, libvirt could do the merge itself because if I understand it 
correctly, the snippset just say:
please add/override the "guestdebug=1" key/value pair to the 'video0' 
device command option.

In QAPI, otherwise, we have qom-set, but it will happens after the 
device has been created, so it don't work for all properties.

--
Damien