Re: [PATCH v2 15/16] KVM: x86/pmu: Advertise PerfMon version 5 on Intel hosts

["Chen, Zide" <zide.chen@intel.com>]

On 10/28/2026 4:59 PM, Jim Mattson wrote:
> On Thu, Aug 27, 2026 at 3:48 PM Zide Chen <zide.chen@intel.com> wrote:
>>
>> From: Dapeng Mi <dapeng1.mi@linux.intel.com>
>>
>> KVM currently caps the guest PerfMon version at 2 for all Intel
>> platforms. Now that KVM emulates the basic architectural PMU features
>> introduced in PerfMon versions 3 through 5, raise the guest PerfMon
>> version to 5. Features that require additional emulation support, e.g.
>> architectural LBR will be enabled separately.
>>
>> PerfMon versions 3 and 4 continue to be exposed as version 2 because
>> KVM cannot safely virtualize AnyThread due to cross-VM information
>> leakage. PerfMon v5 is supported because it provides the
>> AnyThread-deprecated capability, allowing KVM to indicate that
>> AnyThread is deprecated.
>>
>> When enable_mediated_pmu is disabled or in the non-Intel paths, KVM
>> retains the existing cap of version 2.
>>
>> Signed-off-by: Dapeng Mi <dapeng1.mi@linux.intel.com>
>> Signed-off-by: Zide Chen <zide.chen@intel.com>
>> ---
>> v2:
>> - Cap vPMU v3/v4 to v2, as AnyThread cannot be safely virtualized.
>>   (Sashiko)
>> ---
>>  arch/x86/kvm/pmu.c | 16 +++++++++++++++-
>>  1 file changed, 15 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/kvm/pmu.c b/arch/x86/kvm/pmu.c
>> index 4cb7c796daac..1a7be7d73b62 100644
>> --- a/arch/x86/kvm/pmu.c
>> +++ b/arch/x86/kvm/pmu.c
>> @@ -175,7 +175,21 @@ void kvm_init_pmu_capability(struct kvm_pmu_ops *pmu_ops)
>>         }
>>
>>         memcpy(&kvm_pmu_cap, &kvm_host_pmu, sizeof(kvm_host_pmu));
>> -       kvm_pmu_cap.version = min(kvm_pmu_cap.version, 2);
>> +
>> +       if (is_intel && enable_mediated_pmu) {
>> +               kvm_pmu_cap.version = min(kvm_pmu_cap.version, 5);
>> +
>> +               /*
>> +                * PMU v3/v4 has AnyThread support, which KVM cannot safely
>> +                * virtualize. Expose PMU v2 instead of advertising a partially
>> +                * functional PMU v3/v4 to guests.
>> +                */
>> +               if (kvm_pmu_cap.version == 3 || kvm_pmu_cap.version == 4)
>> +                       kvm_pmu_cap.version = 2;
> 
> Nit: on Intel, enable_mediated_pmu already requires host_pmu->version >= 4
> (see intel_is_mediated_pmu_supported()), so kvm_pmu_cap.version == 3 is
> impossible when (is_intel && enable_mediated_pmu) is true. Hence, I think this
> can be simplified to:
> 
>         if (is_intel && enable_mediated_pmu && kvm_pmu_cap.version >= 5)
>                 kvm_pmu_cap.version = 5;
>         else
>                 kvm_pmu_cap.version = min(kvm_pmu_cap.version, 2);


Nice simplification. You are right that version == 3 is dead code. I
will adopt this simplified code in v3.