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

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Message-ID: <12aa9054-73cd-44d3-ba76-f3b59a2bdda3@virtuozzo.com>
Date:   Mon, 13 Nov 2023 10:31:09 +0100
From:   "Denis V. Lunev" <den@...tuozzo.com>
To:     Dongli Zhang <dongli.zhang@...cle.com>,
        Jim Mattson <jmattson@...gle.com>,
        Konstantin Khorenko <khorenko@...tuozzo.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H . Peter Anvin" <hpa@...or.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Alexander Ivanov <alexander.ivanov@...tuozzo.com>
Subject: Re: KVM: x86/vPMU/AMD: Can we detect PMU is off for a VM?

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@...tuozzo.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