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

[Dongli Zhang]

On 11/9/23 3:46 PM, Denis V. Lunev wrote:
> On 11/9/23 23:52, Jim Mattson wrote:
>> 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.
> 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
> 

I used to have a patch to use KVM_PMU_CAP_DISABLE in QEMU, but that did not draw
many developers' attention.

https://lore.kernel.org/qemu-devel/20230621013821.6874-2-dongli.zhang@xxxxxxxxxx/

It is time to first re-send that again.

Dongli Zhang