Re: KVM: x86/vPMU/AMD: Can we detect PMU is off for a VM?

[Jim Mattson]

On Thu, Nov 9, 2023 at 10:18 AM Konstantin Khorenko
<khorenko@xxxxxxxxxxxxx> wrote:
>
> Hi All,
>
> as a followup for my patch: i have noticed that
> currently Intel kernel code provides an ability to detect if PMU is totally disabled for a VM
> (pmu->version == 0 in this case), but for AMD code pmu->version is never 0,
> no matter if PMU is enabled or disabled for a VM (i mean <pmu state='off'/> in the VM config which
> results in "-cpu pmu=off" qemu option).
>
> So the question is - is it possible to enhance the code for AMD to also honor PMU VM setting or it is
> impossible by design?

The AMD architectural specification prior to AMD PMU v2 does not allow
one to describe a CPU (via CPUID or MSRs) that has fewer than 4
general purpose PMU counters. While AMD PMU v2 does allow one to
describe such a CPU, legacy software that knows nothing of AMD PMU v2
can expect four counters regardless.

Having said that, KVM does provide a per-VM capability for disabling
the virtual PMU: KVM_CAP_PMU_CAPABILITY(KVM_PMU_CAP_DISABLE). See
section 8.35 in Documentation/virt/kvm/api.rst.