Re: [PATCH v2 14/16] KVM: x86/pmu: Ignore AnyThread bit if CPUID.0AH:EDX[15] is set

[Mi, Dapeng]

Reviewed-by: Dapeng Mi <dapeng1.mi@xxxxxxxxxxxxxxx>


On 8/28/2026 6:37 AM, Zide Chen wrote:
> Intel PerfMon v5 introduces the ANYTHREAD_DEPRECATION capability
> (CPUID.0AH:EDX[15]) to indicate that AnyThread counting is deprecated
> and that writes to the AnyThread bit in IA32_PERFEVTSELx are ignored.
>
> When ANYTHREAD_DEPRECATION is present in the guest CPUID, emulate the
> architectural behavior and silently ignore writes to the AnyThread bit
> instead of injecting #GP.
>
> Continue to inject #GP when ANYTHREAD_DEPRECATION is not present in the
> guest CPUID, e.g. for PerfMon v3/v4 guests or when the capability is
> not configured for a PerfMon v5+ guest.
>
> Advertise CPUID.0AH:EDX[15] only for PMU version 5+ guests, since the
> bit doesn't exist below version 5.
>
> Signed-off-by: Zide Chen <zide.chen@xxxxxxxxx>
> ---
> v2:
> - Advertise anythread_deprecated only if kvm_pmu_cap.version >= 5.
> - Add the missing AnyThr bit handling for IA32_FIXED_CTR_CTRL.
> - Update the comments to be clearer.
> ---
>  arch/x86/kvm/cpuid.c         |  2 +-
>  arch/x86/kvm/vmx/pmu_intel.c | 19 +++++++++++++++++++
>  2 files changed, 20 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 63201d4ea4a1..ef208906b3ee 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -1542,7 +1542,7 @@ static inline int __do_cpuid_func(struct kvm_cpuid_array *array, u32 function)
>  					    KVM_MAX_NR_FIXED_COUNTERS);
>  		edx.split.bit_width_fixed = kvm_pmu_cap.bit_width_fixed;
>  
> -		if (kvm_pmu_cap.version)
> +		if (kvm_pmu_cap.version >= 5)
>  			edx.split.anythread_deprecated = 1;
>  
>  		entry->eax = eax.full;
> diff --git a/arch/x86/kvm/vmx/pmu_intel.c b/arch/x86/kvm/vmx/pmu_intel.c
> index c7472778137a..599c69dfa6e3 100644
> --- a/arch/x86/kvm/vmx/pmu_intel.c
> +++ b/arch/x86/kvm/vmx/pmu_intel.c
> @@ -446,6 +446,12 @@ static int intel_pmu_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>  		if (data & pmu->fixed_ctr_ctrl_rsvd)
>  			return 1;
>  
> +		/*
> +		 * This is to emulate ANYTHREAD_DEPRECATION. Guests without
> +		 * support for it would have had the write rejected by
> +		 * pmu->fixed_ctr_ctrl_rsvd above.
> +		 */
> +		data &= ~intel_pmu_fixed_ctrl_bits(pmu, INTEL_FIXED_0_ANYTHREAD);
>  		if (pmu->fixed_ctr_ctrl != data)
>  			reprogram_fixed_counters(pmu, data);
>  		break;
> @@ -494,6 +500,8 @@ static int intel_pmu_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>  			if (data & eventsel_rsvd)
>  				return 1;
>  
> +			/* See above comments for MSR_CORE_PERF_FIXED_CTR_CTRL. */
> +			data &= ~ARCH_PERFMON_EVENTSEL_ANY;
>  			if (data != pmc->eventsel) {
>  				pmc->eventsel = data;
>  				pmc->eventsel_hw = data;
> @@ -603,6 +611,17 @@ static void intel_pmu_refresh(struct kvm_vcpu *vcpu)
>  	pmu->pmc_exists64 = (BIT_ULL(nr_gp_counters) - 1) &
>  			    kvm_pmu_cap.cntr_mask64;
>  
> +	/*
> +	 * AnyThread counting is not supported by KVM due to cross-VM
> +	 * information leakage concerns on SMT cores. Therefore, AnyThread
> +	 * remains unavailable for PerfMon v3/v4 guests, where AnyThread
> +	 * deprecation is not enumerated.
> +	 */
> +	if (pmu->version >= 5 && edx.split.anythread_deprecated) {
> +		pmu->eventsel_rsvd &= ~ARCH_PERFMON_EVENTSEL_ANY;
> +		fixed_enable_bits |= INTEL_FIXED_0_ANYTHREAD;
> +	}
> +
>  	entry = kvm_find_cpuid_entry_index(vcpu, 7, 0);
>  	if (entry &&
>  	    (boot_cpu_has(X86_FEATURE_HLE) || boot_cpu_has(X86_FEATURE_RTM)) &&