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

Denis V. Lunev
2023-11-13T09:31:18Z

On 11/10/23 01:01, Dongli Zhang wrote:
>
> 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@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
>>
> 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@oracle.com/
>
> It is time to first re-send that again.
>
> Dongli Zhang
We have checked that setting KVM_PMU_CAP_DISABLE really helps.
Konstantin has done this and this is good. On the other hand, looking
into these patches I disagree with them. We should not introduce new
option for QEMU. If PMU is disabled, i.e. we assume that pmu=off passed
in the command line, we should set KVM_PMU_CAP_DISABLE for that virtual
machine. Den