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

[Dongli Zhang <dongli.zhang@oracle.com>]

Hi Denis,

On 11/13/23 01:31, Denis V. Lunev wrote:
> 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://urldefense.com/v3/__https://lore.kernel.org/qemu-devel/20230621013821.6874-2-dongli.zhang@oracle.com/__;!!ACWV5N9M2RV99hQ!McSH2M-kuHmzAwTuXKxrjLkrdJoPqML6cY_Ndc-8k9LRQ7D1V9bSBRQPwHqtx9XCVLK3uzdsMaxyfwve$
>> 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

Can I assume you meant pmu=off, that is, cpu->enable_pmu in QEMU?

In my opinion, cpu->enable_pmu indicates the option to control the cpu features.
It may be used by any accelerators, and it is orthogonal to the KVM cap.


The KVM_PMU_CAP_DISABLE is only specific to the KVM accelerator.


That's why I had introduced a new option, to allow to configure the VM in my
dimensions.

It means one dimension to AMD, but two for Intel: to disable PMU via cpuid, or
KVM cap.

Anyway, this is KVM mailing list, and I may initiate the discussion in QEMU list.

Thank you very much!

Dongli Zhang