Re: [Qemu-devel] [PATCH v3 3/3] Change other funcitons referring to feature_word_info[]

[Eric Blake]

On 09/05/2018 11:44 AM, Eduardo Habkost wrote:
> On Wed, Sep 05, 2018 at 10:32:33AM -0500, Eric Blake wrote:
> > On 09/05/2018 09:10 AM, Eduardo Habkost wrote:
> > > Question to the QAPI schema maintainers below:
> >
> > > >    ##
> > > > -{ 'struct': 'X86CPUFeatureWordInfo',
> > > > -  'data': { 'cpuid-input-eax': 'int',
> > > > -            '*cpuid-input-ecx': 'int',
> > > > -            'cpuid-register': 'X86CPURegister32',
> > > > +{ 'struct': 'X86CPUIDFeatureWordInfo',
> > > > +  'data': { 'input-eax': 'int',
> > > > +            '*input-ecx': 'int',
> > > > +            'register': 'X86CPURegister32',
> > > >                'features': 'int' } }
> >
> > You are renaming the struct (which is not visible over the wire), as well as
> > renaming members within the struct (which is visible, if this type appears
> > on the wire).
> >
> > However, 'grep FeatureWordInfo qapi/qapi-introspect.c' has no hits either
> > before or after this patch (well, first you have to build with my
> > currently-pending patch as part of Markus' pull request for this trick to
> > work, https://lists.gnu.org/archive/html/qemu-devel/2018-08/msg05968.html).
> > Or, even without my patch, grepping for 'input-eax' has no hits as a member
> > name in any struct.  Which means that there are no QMP commands currently
> > exposing this struct over the wire.
>
> There is one: qom-get.

Oh, right, the nasty exception that is not visible to introspection :(

> > > AFAICS, this will break the existing libvirt code: it will make
> > > qemuMonitorJSONParseCPUx86Features() error out because it won't
> > > find the "input-eax" field on all X86CPUFeatureWordInfo elements.
> >
> > No, this change can't break libvirt, since I just proved there is no QMP
> > command that libvirt could be using that either supplies an input member or
> > expects an output member named "input-eax" in the first place.
>
> I'm sure it will break libvirt.  libvirt uses
> "qom-get path=<cpu-path> property=feature-words" to get
> X86FeatureWordInfo data, and it expects the returned data to
> have a "input-eax" field.

Yeah, since this type is used in the qom-get backdoor that evades 
introspection, it's even more important that you follow the 
backwards-compatibility rules and not rename or delete any 
previously-mandatory members, since libvirt CAN'T introspect if such 
changes have happened.

--
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org