Re: [PATCH v6 04/16] KVM: x86/pmu: Set MSR_IA32_MISC_ENABLE_EMON bit when vPMU is enabled

[Venkatesh Srinivas]

On Wed, May 12, 2021 at 7:50 PM Xu, Like <like.xu@xxxxxxxxx> wrote:
>
> On 2021/5/12 23:18, Sean Christopherson wrote:
> > On Wed, May 12, 2021, Xu, Like wrote:
> >> Hi Venkatesh Srinivas,
> >>
> >> On 2021/5/12 9:58, Venkatesh Srinivas wrote:
> >>> On 5/10/21, Like Xu <like.xu@xxxxxxxxxxxxxxx> wrote:
> >>>> On Intel platforms, the software can use the IA32_MISC_ENABLE[7] bit to
> >>>> detect whether the processor supports performance monitoring facility.
> >>>>
> >>>> It depends on the PMU is enabled for the guest, and a software write
> >>>> operation to this available bit will be ignored.
> >>> Is the behavior that writes to IA32_MISC_ENABLE[7] are ignored (rather than #GP)
> >>> documented someplace?
> >> The bit[7] behavior of the real hardware on the native host is quite
> >> suspicious.
> > Ugh.  Can you file an SDM bug to get the wording and accessibility updated?  The
> > current phrasing is a mess:
> >
> >    Performance Monitoring Available (R)
> >    1 = Performance monitoring enabled.
> >    0 = Performance monitoring disabled.
> >
> > The (R) is ambiguous because most other entries that are read-only use (RO), and
> > the "enabled vs. disabled" implies the bit is writable and really does control
> > the PMU.  But on my Haswell system, it's read-only.
>
> On your Haswell system, does it cause #GP or just silent if you change this
> bit ?
>
> > Assuming the bit is supposed
> > to be a read-only "PMU supported bit", the SDM should be:
> >
> >    Performance Monitoring Available (RO)
> >    1 = Performance monitoring supported.
> >    0 = Performance monitoring not supported.

Can't speak to Haswell, but on Apollo Lake/Goldmont, this bit is _not_
set natively
and we get a #GP when attempting to set it, even though the PMU is available.

Should this bit be conditional on the host having it set?

> >
> > And please update the changelog to explain the "why" of whatever the behavior
> > ends up being.  The "what" is obvious from the code.
>
> Thanks for your "why" comment.
>
> >
> >> To keep the semantics consistent and simple, we propose ignoring write
> >> operation in the virtualized world, since whether or not to expose PMU is
> >> configured by the hypervisor user space and not by the guest side.
> > Making up our own architectural behavior because it's convient is not a good
> > idea.
>
> Sometime we do change it.
>
> For example, the scope of some msrs may be "core level share"
> but we likely keep it as a "thread level" variable in the KVM out of
> convenience.
>
> >
> >>>> diff --git a/arch/x86/kvm/vmx/pmu_intel.c b/arch/x86/kvm/vmx/pmu_intel.c
> >>>> index 9efc1a6b8693..d9dbebe03cae 100644
> >>>> --- a/arch/x86/kvm/vmx/pmu_intel.c
> >>>> +++ b/arch/x86/kvm/vmx/pmu_intel.c
> >>>> @@ -488,6 +488,7 @@ static void intel_pmu_refresh(struct kvm_vcpu *vcpu)
> >>>>            if (!pmu->version)
> >>>>                    return;
> >>>>
> >>>> +  vcpu->arch.ia32_misc_enable_msr |= MSR_IA32_MISC_ENABLE_EMON;
> > Hmm, normally I would say overwriting the guest's value is a bad idea, but if
> > the bit really is a read-only "PMU supported" bit, then this is the correct
> > behavior, albeit weird if userspace does a late CPUID update (though that's
> > weird no matter what).
> >
> >>>>            perf_get_x86_pmu_capability(&x86_pmu);
> >>>>
> >>>>            pmu->nr_arch_gp_counters = min_t(int, eax.split.num_counters,
> >>>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> >>>> index 5bd550eaf683..abe3ea69078c 100644
> >>>> --- a/arch/x86/kvm/x86.c
> >>>> +++ b/arch/x86/kvm/x86.c
> >>>> @@ -3211,6 +3211,7 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct
> >>>> msr_data *msr_info)
> >>>>                    }
> >>>>                    break;
> >>>>            case MSR_IA32_MISC_ENABLE:
> >>>> +          data &= ~MSR_IA32_MISC_ENABLE_EMON;
> > However, this is not.  If it's a read-only bit, then toggling the bit should
> > cause a #GP.
>
> The proposal here is trying to make it as an
> unchangeable bit and don't make it #GP if guest changes it.
>
> It may different from the host behavior but
> it doesn't cause potential issue if some guest code
> changes it during the use of performance monitoring.
>
> Does this make sense to you or do you want to
> keep it strictly the same as the host side?
>
> >
> >>>>                    if (!kvm_check_has_quirk(vcpu->kvm, KVM_X86_QUIRK_MISC_ENABLE_NO_MWAIT)
> >>>> &&
> >>>>                        ((vcpu->arch.ia32_misc_enable_msr ^ data) &
> >>>> MSR_IA32_MISC_ENABLE_MWAIT)) {
> >>>>                            if (!guest_cpuid_has(vcpu, X86_FEATURE_XMM3))
> >>>> --
>