On 11/13/23 06:42, Denis V. Lunev wrote: > On 11/13/23 15:14, Dongli Zhang wrote: >> 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 > with the option pmu='off' it is expected that PMU should be > off for the guest. At the moment (without this KVM capability) > we can disable PMU for Intel only and thus have performance > degradation on AMD. > > This option disables PMU and thus normally when we are > running KVM guest and wanting PMU to be off it would > be required to > * disable CPUID leaf for Intel > * set KVM_PMU_CAP_DISABLE for both processors This would be quite natural and > transparent for the libvirt. Alexander will prepare the patch today or tomorrow > for the discussion. Den That is what I had implemented in the v1 of patch. https://lore.kernel.org/all/20221119122901.2469-3-dongli.zhang@xxxxxxxxxx/ However, I changed that after people suggested introduce a new property. Dongli Zhang
