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@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://urldefense.com/v3/__https://lore.kernel.org/qemu-devel/20230621013821.6874-2-dongli.zhang@xxxxxxxxxx/__;!!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
