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

[Liuxiangdong <liuxiangdong5@huawei.com>]

On 2021/11/8 12:11, Like Xu wrote:
> 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;
>

Yes. bit[12] is also read-only, so we can keep this bit unchanged also.

And, because write operation will not change this bit by "pmu_mask", do 
we still need this if statement?

         /* RO bits */
         if (!msr_info->host_initiated &&
             ((old_val ^ data) & MSR_IA32_MISC_ENABLE_PEBS_UNAVAIL))
             return 1;

"(old_val ^ data) & MSR_IA32_MISC_ENABLE_PEBS_UNAVAIL" means some 
operation tries to change this bit,
so we cannot allow it.
But, if there is no this judgement, "pmu_mask" will still make this 
bit[12] no change.

The only difference is that we can not change other bit (except bit 12 
and bit 7) once "old_val[12] != data[12]" if there exists this statement
and we can change other bit if there is no judgement.

For both MSR_IA32_MISC_ENABLE_EMON and MSR_IA32_MISC_ENABLE_EMON are 
read-only, maybe we can keep
their behavioral consistency. Either both judge, or neither.

Do you think so?


> 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))
>>>>
>>