Re: [Qemu-devel] [PATCH v4 11/15] target-s390x: New QMP command query-cpu-model

[Michael Mueller <mimu@linux.vnet.ibm.com>]

On Wed, 1 Apr 2015 10:01:13 -0300
Eduardo Habkost <ehabkost@redhat.com> wrote:

> On Tue, Mar 31, 2015 at 10:09:09PM +0200, Michael Mueller wrote:
> > On Tue, 31 Mar 2015 15:35:26 -0300
> > Eduardo Habkost <ehabkost@redhat.com> wrote:
> > 
> > > On Mon, Mar 30, 2015 at 04:28:24PM +0200, Michael Mueller wrote:
> > > > This patch implements a new QMP request named 'query-cpu-model'.
> > > > It returns the cpu model of cpu 0 and its backing accelerator.
> > > > 
> > > > request:
> > > >   {"execute" : "query-cpu-model" }
> > > > 
> > > > answer:
> > > >   {"return" : {"name": "2827-ga2", "accel": "kvm" }}
> > > > 
> > > > Alias names are resolved to their respective machine type and GA names
> > > > already during cpu instantiation. Thus, also a cpu model like 'host'
> > > > which is implemented as alias will return its normalized cpu model name.
> > > > 
> > > > Furthermore the patch implements the following function:
> > > > 
> > > > - s390_cpu_models_used(), returns true if S390 cpu models are in use
> > > > 
> > > > Signed-off-by: Michael Mueller <mimu@linux.vnet.ibm.com>
> > > > ---
> > > [...]
> > > > +static inline char *strdup_s390_cpu_name(S390CPUClass *cc)
> > > > +{
> > > > +    return g_strdup_printf("%04x-ga%u", cc->proc.type, cc->mach.ga);
> > > > +}
> > > 
> > > How exactly is this information going to be used by clients? If getting
> > > the correct type and ga values is important for them, maybe you could
> > > add them as integer fields, instead of requiring clients to parse the
> > > CPU model name?
> > 
> > The consumer don't need to parse the name, it is just important for them to have
> > distinctive names that correlate with the names returned by query-cpu-definitions.
> > Once the name of an active guest is known, e.g. ("2827-ga2", "kvm") a potential
> > migration target can be verified, i.e. its query-cpu-definitions answer for "kvm"
> > has to contain "2827-ga2" with the attribute runnable set to true. With that mechanism
> > also the largest common denominator can be calculated. That model will be used then.
> 
> Understood. So the point is to really have a name that can be found at
> query-cpu-definitions. Makes sense.
> 
> (BTW, if you reused strdup_s390_cpu_name() inside
> s390_cpu_compare_class_name() too, you would automatically ensure that
> query-cpus, query-cpu-definitions and s390_cpu_class_by_name() will
> always agree with each other).

I have to verify but it seems to make sense from reading... I will do that if it works. :-)

> 
> > 
> > I also changed the above mentioned routine to map the cpu model none case:
> > 
> > static inline char *strdup_s390_cpu_name(S390CPUClass *cc)
> > {
> >     if (cpuid(cc->proc)) {
> >         return g_strdup_printf("%04x-ga%u", cc->proc.type, cc->mach.ga);
> >     } else {
> >         return g_strdup("none");
> >     }
> > }
> 
> What about:
> 
>   static const char *s390_cpu_name(S390CPUClass *cc)
>   {
>       return cc->model_name;
>   }
> 
> And then you can just set cc->model_name=_name inside S390_PROC_DEF (and
> set it to "none" inside s390_cpu_class_init()).
> 

Wouldn't that store redundant information... but it would at least shift the work into the
initialization phase and do the format just once per model.

> I wonder if this class->model_name conversion could be made generic
> inside the CPU class. We already have a CPU::class_by_name() method, so
> it makes sense to have the opposite function too.
> 
> (But I wouldn't mind making this s390-specific first, and converted
> later to generic code if appropriate).

ok

> 
> > 
> > This implicitly will fail a comparison for cpu model ("none", "kvm") as that will
> > never be part of the query-cpu-definitions answer.
> 
> I am not sure I follow. If ("none", "kvm") is never in the list, is
> "-cpu none -machine accel=kvm" always an invalid use case?

Not directly invalid as "-cpu none" will be the same as omitting the -cpu option.
KVM will setup the vcpu properties withou any QEMU control to whatever the hosting
machine and the kvm kernel code offers. That will allow to run QEMU against a KVM
version that is not aware of the s390 cpu model ioctls.

> 
> (I don't understand completely the meaning of "-cpu none" yet. How does
> the CPU look like for the guest in this case? Is it possible to
> live-migrate when using -cpu none?)

And yes, that does not make sense in a migration context. The answer on query-cpu-model
(or query-cpus) will be ("none", "kvm") and that will never match a runnable model.
The guest cpu will be derived from the hosting system and the kvm kernel as it is currently
without the cpu model interface. 

I hope I made it better to understand now...

Michael

>