Re: [PATCH V10 05/18] KVM: x86/pmu: Set MSR_IA32_MISC_ENABLE_EMON bit when vPMU is enabled

[Liuxiangdong]

Hi, like and lingshan.

As said,  IA32_MISC_ENABLE[7] bit depends on the PMU is enabled for the 
guest, so a software
write openration to this bit will be ignored.

But, in this patch, all the openration that writes msr_ia32_misc_enable 
in guest could make this bit become 0.

Suppose:
When we start vm with "enable_pmu", vcpu->arch.ia32_misc_enable_msr may 
be 0x80 first.
And next, guest writes msr_ia32_misc_enable value 0x1.
What we want could be 0x81, but unfortunately, it will be 0x1 because of
"data &= ~MSR_IA32_MISC_ENABLE_EMON;"
And even if guest writes msr_ia32_misc_enable value 0x81, it will be 0x1 
also.


What we want is write operation will not change this bit. So, how about 
this?

--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3321,6 +3321,7 @@ 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;
+        data |= (vcpu->arch.ia32_misc_enable_msr & 
MSR_IA32_MISC_ENABLE_EMON);
         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))


Or is there anything in your design intention I don't understand?

Thanks!

Xiangdong Liu


On 2021/8/6 21:37, Zhu Lingshan wrote:
> From: Like Xu <like.xu@xxxxxxxxxxxxxxx>
>
> 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. The proposal to ignore
> the toggle in KVM is the way to go and that behavior matches bare metal.
>
> Cc: Yao Yuan <yuan.yao@xxxxxxxxx>
> Signed-off-by: Like Xu <like.xu@xxxxxxxxxxxxxxx>
> Reviewed-by: Venkatesh Srinivas <venkateshs@xxxxxxxxxxxx>
> Signed-off-by: Zhu Lingshan <lingshan.zhu@xxxxxxxxx>
> Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> ---
>   arch/x86/kvm/vmx/pmu_intel.c | 1 +
>   arch/x86/kvm/x86.c           | 1 +
>   2 files changed, 2 insertions(+)
>
> 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;
>   	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 efd11702465c..f6b6984e26ef 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -3321,6 +3321,7 @@ 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;
>   		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))