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

["Denis V. Lunev" <den@virtuozzo.com>]

On 11/9/23 23:52, Jim Mattson wrote:
> On Thu, Nov 9, 2023 at 10:18 AM Konstantin Khorenko
> <khorenko@virtuozzo.com> 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.
But this means in particular that QEMU should immediately
use this KVM_PMU_CAP_DISABLE if this capability is supported and 
PMU=off. I am not seeing this code thus I believe that we have missed 
this. I think that this change worth adding. We will measure the impact 
:-) Den