linux-kernel - Re: [PATCH V10 05/18] KVM: x86/pmu: Set MSR_IA32_MISC_ENABLE

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Message-ID: <5aa115ab-d22c-098d-0591-36c7ab15f8b6@gmail.com>
Date:   Mon, 8 Nov 2021 11:06:40 +0800
From:   Like Xu <like.xu.linux@...il.com>
To:     Liuxiangdong <liuxiangdong5@...wei.com>,
        Zhu Lingshan <lingshan.zhu@...el.com>
Cc:     seanjc@...gle.com, vkuznets@...hat.com, wanpengli@...cent.com,
        jmattson@...gle.com, joro@...tes.org, kan.liang@...ux.intel.com,
        ak@...ux.intel.com, wei.w.wang@...el.com, eranian@...gle.com,
        linux-kernel@...r.kernel.org, x86@...nel.org, kvm@...r.kernel.org,
        boris.ostrvsky@...cle.com, Yao Yuan <yuan.yao@...el.com>,
        Venkatesh Srinivas <venkateshs@...omium.org>,
        "Fangyi (Eric)" <eric.fangyi@...wei.com>,
        Xiexiangyou <xiexiangyou@...wei.com>
Subject: Re: [PATCH V10 05/18] KVM: x86/pmu: Set MSR_IA32_MISC_ENABLE_EMON bit
 when vPMU is enabled

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;

		/* 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.
		 */
		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@...ux.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@...el.com>
>> Signed-off-by: Like Xu <like.xu@...ux.intel.com>
>> Reviewed-by: Venkatesh Srinivas <venkateshs@...omium.org>
>> Signed-off-by: Zhu Lingshan <lingshan.zhu@...el.com>
>> Acked-by: Peter Zijlstra (Intel) <peterz@...radead.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))
>