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

[Denis V. Lunev]

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://lore.kernel.org/qemu-devel/20230621013821.6874-2-dongli.zhang@xxxxxxxxxx/
>
> 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