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

[Gerd Hoffmann]

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