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

[Like Xu <like.xu.linux@gmail.com>]

On 8/11/2021 12:07 pm, Liuxiangdong wrote:
> 
> 
> On 2021/11/8 11:06, Like Xu wrote:
>> On 7/11/2021 6:14 pm, Liuxiangdong wrote:
>>> 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.
>>>
>>
>> Yes and thank you. The fix has been committed on my private tree for a long time.
>>
>>>
>>> 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))
>>>
>>>
>>
>> How about this for the final state considering PEBS enabling:
>>
>>     case MSR_IA32_MISC_ENABLE: {
>>         u64 old_val = vcpu->arch.ia32_misc_enable_msr;
>>         u64 pmu_mask = MSR_IA32_MISC_ENABLE_EMON |
>>             MSR_IA32_MISC_ENABLE_EMON;
>>
>          u64 pmu_mask = MSR_IA32_MISC_ENABLE_EMON |
>              MSR_IA32_MISC_ENABLE_EMON;
> 
> Repetitive "MSR_IA32_MISC_ENABLE_EMON" ?

Oops,
  		
	u64 pmu_mask = MSR_IA32_MISC_ENABLE_EMON |
			MSR_IA32_MISC_ENABLE_PEBS_UNAVAIL;

I'll send the fix after sync with Lingshan.

> 
>>         /* RO bits */
>>         if (!msr_info->host_initiated &&
>>             ((old_val ^ data) & MSR_IA32_MISC_ENABLE_PEBS_UNAVAIL))
>>             return 1;
>>
>>         /*
>>          * For a dummy user space, the order of setting vPMU capabilities and
>>          * initialising MSR_IA32_MISC_ENABLE is not strictly guaranteed, so to
>>          * avoid inconsistent functionality we keep the vPMU bits unchanged here.
>>          */
> Yes. It's a little clearer with comments.

Thanks for your feedback! Enjoy the feature.

>>         data &= ~pmu_mask;
>>         data |= old_val & pmu_mask;
>>         if (!kvm_check_has_quirk(vcpu->kvm, 
>> KVM_X86_QUIRK_MISC_ENABLE_NO_MWAIT) &&
>>             ((old_val ^ data) & MSR_IA32_MISC_ENABLE_MWAIT)) {
>>             if (!guest_cpuid_has(vcpu, X86_FEATURE_XMM3))
>>                 return 1;
>>             vcpu->arch.ia32_misc_enable_msr = data;
>>             kvm_update_cpuid_runtime(vcpu);
>>         } else {
>>             vcpu->arch.ia32_misc_enable_msr = data;
>>         }
>>         break;
>>     }
>>
>>> 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@linux.intel.com>
>>>>
>>>> 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@intel.com>
>>>> Signed-off-by: Like Xu <like.xu@linux.intel.com>
>>>> Reviewed-by: Venkatesh Srinivas <venkateshs@chromium.org>
>>>> Signed-off-by: Zhu Lingshan <lingshan.zhu@intel.com>
>>>> Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
>>>> ---
>>>>   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))
>>>
>