Re: [Qemu-devel] [libvirt] [PATCH qom-cpu 6/9] target-i386: Add "feature-words" property

[Eduardo Habkost]

On Mon, Apr 22, 2013 at 02:37:06PM -0600, Eric Blake wrote:
> On 04/22/2013 01:00 PM, Eduardo Habkost wrote:
> > This property will be useful for libvirt, as libvirt already has logic
> > based on low-level feature bits (not feature names), so it will be
> > really easy to convert the current libvirt logic to something using the
> > "feature-words" property.
> > 
> > The property will have two main use cases:
> >  - Checking host capabilities, by checking the features of the "host"
> >    CPU model
> >  - Checking which features are enabled on each CPU model
> > 
> > Example output:
> > 
> >   $ ./QMP/qmp --path=/tmp/m qom-get --path=/machine/unattached/device[1] 
> > --property=feature-words
> 
> If I'm not mistaken, the QMP counterpart that libvirt will use is:
> 
> { "execute":"qom-get",
>   "arguments": { "path":"/machine/unattached/device[1]",
>                  "property":"feature-words" } }

Yes, but note that libvirt needs to look for the X86CPU objects, to find
the actual path as there's no guarantee that it will always be
/machine/unattached/devices[1]. Maybe the CPU hotplug work will end up
offering a predictable path for the X86CPU objects, but this is not
available yet.

> 
> >   item[0].cpuid-register: EDX
> >   item[0].cpuid-input-eax: 2147483658
> >   item[0].features: 0
> >   item[1].cpuid-register: EAX
> >   item[1].cpuid-input-eax: 1073741825
> >   item[1].features: 0
> >   item[2].cpuid-register: EDX
> >   item[2].cpuid-input-eax: 3221225473
> >   item[2].features: 0
> >   item[3].cpuid-register: ECX
> >   item[3].cpuid-input-eax: 2147483649
> >   item[3].features: 101
> >   item[4].cpuid-register: EDX
> >   item[4].cpuid-input-eax: 2147483649
> >   item[4].features: 563346425
> >   item[5].cpuid-register: EBX
> >   item[5].cpuid-input-eax: 7
> >   item[5].features: 0
> >   item[5].cpuid-input-ecx: 0
> >   item[6].cpuid-register: ECX
> >   item[6].cpuid-input-eax: 1
> >   item[6].features: 2155880449
> >   item[7].cpuid-register: EDX
> >   item[7].cpuid-input-eax: 1
> >   item[7].features: 126614521
> 
> And this would then be returned as a JSON array containing struct
> members looking like this:
> 
> > +{ 'type': 'X86CPUFeatureWordInfo',
> > +  'data': { 'cpuid-input-eax': 'int',
> > +            '*cpuid-input-ecx': 'int',
> > +            'cpuid-register': 'X86CPURegister32',
> > +            'features': 'int' } }

Yes.

> 
> Looks reasonable (and better than what we've had in the past!), although
> I'll let Jiri Denemark give final say on whether it meets libvirt's needs.

Thanks for the feedback!

-- 
Eduardo