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

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

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.

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");
    }
}

This implicitly will fail a comparison for cpu model ("none", "kvm") as that will
never be part of the query-cpu-definitions answer.

I actually applied a couple of your suggestions like:

- test for NULL skipped after strdup_s390_cpu_name()
- strdup_s390_cpu_name() now also handles none cpu model case
- omit runnable and is-default field from query-cpu-definitions
  answer when they are false
- global variable cpu_models_used dropped
- function s390_cpu_models_used() dropped
- routine query-cpu-definitions has a single code path now

Only the integration of the ACCEL_ID with the cpu state in cpu_generic_init() and
the change for the query-cpus implementation is under construction. I hope to resend
the patches by tomorrow evening.

Thanks,
Michael 

>