Re: [Qemu-devel] [PATCH V7 0/2] Add option to configure guest vPMU

[Andrew Jones]

On Mon, Oct 24, 2016 at 11:39:59PM -0500, Wei Huang wrote:
> >> V6->V7:
> >>   * change has_pmu variable type from OnOffAuto to Boolean
> >>   * only add "pmu" property to CPU under kvm mode, default ON
> > 
> > Hmm, if we don't allow the property with TCG then switching a guest from
> > KVM to TCG will require more than just an accelerator switch. That's a
> > bit annoying and I think we'd have to teach it to libvirt too. I'd prefer
> > 
> >  -M virt-2.8,accel=tcg -cpu cortex-a57         NO     NO
> >  -M virt-2.8,accel=tcg -cpu cortex-a57,pmu=off NO     NO
> >  -M virt-2.8,accel=tcg -cpu cortex-a57,pmu=on  NO     "Warning: PMU not
> > yet supported with TCG" (or something)
> 
> I am fine with this request. But note that, if we enforce
> pmu-default=ON, we can't tell "-cpu cortex-a57" apart from
> "cortex-a57,pmu=on", implying that we have to print the warning msg for
> the case of "cortex-a57" as well (this was why we switch to tri-state
> before). To solve this problem, we have to switch from pmu-default=ON to
> pmu-default=OFF under TCG mode, something like:
> 
>     if (arm_feature(&cpu->env, ARM_FEATURE_PMU)) {
>         qdev_property_add_static(DEVICE(obj),
>             &arm_cpu_has_pmu_property, &error_abort);
>         if (!kvm_enabled())
>             object_property_set_bool(obj, false, "pmu", NULL);
>     }
> 
> Then we do:
>     if (cpu->has_pmu && !kvm_enabled()) {
>         cpu->has_pmu = false;
>         if (!pmu_warned && !qtest_enabled()) {
>             error_report("warning: pmu not supported under TCG");
>             pmu_warned = true;
>         }
>     }
> 
> This will work. Are you and Peter OK with this solution?
>

Yes, I prefer it to not being able to easily switch a command line from
KVM to TCG. Granted, right now we mostly need to run with -cpu host for
KVM, which also needs to be switched for TCG. However that will hopefully
change with Shannon's work.

Thanks,
drew