[libvirt PATCH 10/11] domain_capabilities: Add blockers attribute for CPU models

[Jiri Denemark]

> On Tue, Oct 04, 2022 at 07:35:31PM +0200, Jiri Denemark wrote:
> > On Tue, Oct 04, 2022 at 17:34:34 +0100, Daniel P. Berrangé wrote:
> > > On Tue, Oct 04, 2022 at 04:28:53PM +0200, Jiri Denemark wrote:
> > > > We already show whether a specific CPU model is usable on the current
> > > > host without modification via the 'usable' attribute of each CPU model.
> > > > But it may be useful to actually see what features are blocking each CPU
> > > > model from being usable. Especially when we already fetch the info from
> > > > QEMU and propagating it to domain capabilities XML is all we need to do.
> > > 
> > > > diff --git a/tests/domaincapsdata/qemu_4.2.0-q35.x86_64.xml b/tests/domaincapsdata/qemu_4.2.0-q35.x86_64.xml
> > > > index dab12e5888..8ca9e8d2b2 100644
> > > > --- a/tests/domaincapsdata/qemu_4.2.0-q35.x86_64.xml
> > > > +++ b/tests/domaincapsdata/qemu_4.2.0-q35.x86_64.xml
> > > > @@ -63,7 +63,7 @@
> > > >      <mode name='custom' supported='yes'>
> > > >        <model usable='yes' vendor='unknown'>qemu64</model>
> > > >        <model usable='yes' vendor='unknown'>qemu32</model>
> > > > -      <model usable='no' vendor='AMD'>phenom</model>
> > > > +      <model usable='no' vendor='AMD' blockers='mmxext,fxsr_opt,3dnowext,3dnow,sse4a,npt'>phenom</model>
> > > 
> > > This is an XML design anti-pattern, because it invents a data format
> > > inside the attribute which the caller then has to further parse.
> > > 
> > > If we want to expose this, it needs to be with child elements IMHO,
> > > but yes it will be more much more verbose.
> > 
> > You're absolutely right, but that's the only option we have I'm afraid.
> > Mixing subelements and text nodes is a much worse anti-pattern. I wish
> > the model name was in an attribute, but it isn't and having
> > 
> >     <model usable='no' vendor='AMD'>
> >       <blocker name='mmxext'/>
> >       phenom
> >     </model>
> > 
> > is just insane :-(
> 
> True, I wonder if there's a different approach to the overall problem
> that would be better.

Actually a third option just came to my mind. It's not ideal either, but
at least it would be a proper XML :-)

    <mode name='custom' supported='yes'>
      <model usable='yes' vendor='unknown'>qemu64</model>
      <model usable='no' vendor='AMD'>phenom</model>
      <blockers model='phenom'>
        <feature name='mmxext'/>
        <feature name='fxsr_opt'/>
        ...
      </blockers>
      <model ...>...</model>
      ...
    </mode>

Jirka