Re: [PATCH v8 7/8] KVM: x86/pmu: Emulate RDPMC on performance metrics

On 8/17/2026 2:01 PM, [email protected] wrote:
> Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
> - [Medium] Metrics Clear Mode is not emulated, causing guest application crashes (#GP) when RDPMC is intercepted.
> --
> 
> --- Patch [7]: [PATCH v8 7/8] KVM: x86/pmu: Emulate RDPMC on performance metrics ---
> commit 5022d9b3f098d7ca5a383495c52fb9e9c119df3b
> Author: Zide Chen <[email protected]>
> 
> KVM: x86/pmu: Emulate RDPMC on performance metrics
> 
> This commit adds emulation for RDPMC performance metrics when RDPMC
> interception is enabled. It handles type 0x2000 but skips Metrics Clear Mode
> for simplicity.
> 
> [ ... ]
> 
>> diff --git a/arch/x86/kvm/vmx/pmu_intel.c b/arch/x86/kvm/vmx/pmu_intel.c
>> index bbf4afcffafc8..3ca61e4b0ba68 100644
>> --- a/arch/x86/kvm/vmx/pmu_intel.c
>> +++ b/arch/x86/kvm/vmx/pmu_intel.c
> [ ... ]
>> @@ -125,6 +126,19 @@ static int intel_emulate_rdpmc(struct kvm_vcpu *vcpu, unsigned int idx,
>>  		counters = pmu->gp_counters;
>>  		num_counters = pmu->nr_arch_gp_counters;
>>  		break;
>> +	case INTEL_RDPMC_METRICS:
>> +		if (!kvm_vcpu_has_perf_metrics(vcpu))
>> +			return 1;
>> +
>> +		/*
>> +		 * The index in ECX[15:0] is implementation specific, but no
>> +		 * platform currently supports a non-zero index.
>> +		 */
>> +		if (idx)
>> +			return 1;
>> +
>> +		*data = pmu->perf_metrics;
>> +		return 0;
>>  	default:
>>  		return 1;
> 
> [Severity: Medium]
> The commit message notes that Metrics Clear Mode is not supported for
> simplicity. However, if a guest application uses Metrics Clear Mode (setting
> bit 16 in ECX, making the type 0x20010000), it will fall through to this
> default case since it doesn't match INTEL_RDPMC_METRICS.
> 
> Returning 1 here causes KVM to inject a #GP. Does this cause unexpected
> crashes (like a SIGSEGV) for unprivileged guest userspace applications that
> legitimately try to use this architecturally valid mode when RDPMC is
> intercepted?

This is a false positive. Setting bit 16 in ECX is not indicating
Metrics Clear mode. Whether or not clear metrics, depends on
IA32_FIXED_CTR_CTRL.METRICS_CLEAR_EN[bit 14].

BTW, IA32_PERF_CAPABILITIES.RDPMC_METRICS_CLEAR[bit 19] is not exposed.
>>  	}
>