Re: [PATCH v2 11/16] KVM: x86/pmu: Emulate the GLOBAL_STATUS_SET and GLOBAL_INUSE MSRs

["Mi, Dapeng" <dapeng1.mi@linux.intel.com>]

On 9/1/2026 10:20 PM, Chen, Zide wrote:
>
> On 9/1/2026 1:45 AM, Mi, Dapeng wrote:
>> On 8/28/2026 6:37 AM, Zide Chen wrote:
>>> Intel PerfMon v4 introduces IA32_PERF_GLOBAL_STATUS_SET (0x391) to
>>> allow software to set individual bits in the global status MSR. Reads
>>> of IA32_PERF_GLOBAL_STATUS_SET always return zero.
>>>
>>> IA32_PERF_GLOBAL_INUSE (0x392) is also introduced in v4, to track
>>> which counters and the PMI are currently claimed by other agents,
>>> allowing independent software agents to check counter availability
>>> without a shared scheduler arbitrating between them.
>>>
>>> IA32_PERF_GLOBAL_INUSE is an read-only MSR, and any write attempt
>>> results in a #GP.
>>>
>>> Neither MSR is part of the VM state, so they don't need to be
>>> advertised to userspace, nor saved and restored during live
>>> migration.
>>>
>>> Originally-by: Yang Weijiang <weijiang.yang@intel.com>
>>> Signed-off-by: Zide Chen <zide.chen@intel.com>
>>> ---
>>> v2:
>>> - Change intel_pmu_get_global_inuse() to return u64, to match the
>>>   surrounding code style.
>>> - Add the missing vmcs02 updates for these two MSRs.
>>> - Change "> 3" to ">= 4" to make the "v4-gated" more obvious and match
>>>   the existing code style.
>>> ---
>>>  arch/x86/include/asm/msr-index.h |  4 ++++
>>>  arch/x86/kvm/pmu.c               |  9 ++++++++
>>>  arch/x86/kvm/vmx/nested.c        |  2 ++
>>>  arch/x86/kvm/vmx/pmu_intel.c     | 37 ++++++++++++++++++++++++++++++++
>>>  arch/x86/kvm/vmx/vmx.c           |  4 ++++
>>>  5 files changed, 56 insertions(+)
>>>
>>> diff --git a/arch/x86/include/asm/msr-index.h b/arch/x86/include/asm/msr-index.h
>>> index 11b99d237e05..0b093cf41edf 100644
>>> --- a/arch/x86/include/asm/msr-index.h
>>> +++ b/arch/x86/include/asm/msr-index.h
>>> @@ -1240,6 +1240,10 @@
>>>  #define MSR_CORE_PERF_GLOBAL_CTRL	0x0000038f
>>>  #define MSR_CORE_PERF_GLOBAL_OVF_CTRL	0x00000390
>>>  #define MSR_CORE_PERF_GLOBAL_STATUS_SET	0x00000391
>>> +#define MSR_CORE_PERF_GLOBAL_INUSE	0x00000392
>>> +
>>> +/* Intel IA32_PERF_GLOBAL_INUSE MSR */
>>> +#define PERF_GLOBAL_INUSE_PMI_INUSE	BIT_ULL(63)
>>>  
>>>  #define MSR_PERF_METRICS		0x00000329
>>>  
>>> diff --git a/arch/x86/kvm/pmu.c b/arch/x86/kvm/pmu.c
>>> index 437a7bc49bf8..7c05cf5157bf 100644
>>> --- a/arch/x86/kvm/pmu.c
>>> +++ b/arch/x86/kvm/pmu.c
>>> @@ -832,6 +832,8 @@ bool kvm_pmu_is_valid_msr(struct kvm_vcpu *vcpu, u32 msr)
>>>  	case MSR_CORE_PERF_GLOBAL_CTRL:
>>>  	case MSR_CORE_PERF_GLOBAL_OVF_CTRL:
>>>  		return kvm_pmu_has_perf_global_ctrl(vcpu_to_pmu(vcpu));
>>> +	case MSR_CORE_PERF_GLOBAL_STATUS_SET:
>>> +		return vcpu_to_pmu(vcpu)->version >= 4;
>>>  	default:
>>>  		break;
>>>  	}
>>> @@ -865,6 +867,7 @@ int kvm_pmu_get_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>>>  	case MSR_AMD64_PERF_CNTR_GLOBAL_STATUS_CLR:
>>>  	case MSR_AMD64_PERF_CNTR_GLOBAL_STATUS_SET:
>>>  	case MSR_CORE_PERF_GLOBAL_OVF_CTRL:
>>> +	case MSR_CORE_PERF_GLOBAL_STATUS_SET:
>>>  		msr_info->data = 0;
>>>  		break;
>>>  	default:
>>> @@ -931,6 +934,12 @@ int kvm_pmu_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>>>  		if (!msr_info->host_initiated)
>>>  			pmu->global_status &= ~data;
>>>  		break;
>>> +	case MSR_CORE_PERF_GLOBAL_STATUS_SET:
>>> +		if (data & pmu->global_status_rsvd)
>>> +			return 1;
>>> +		if (!msr_info->host_initiated)
>>> +			pmu->global_status |= data;
>>> +		break;
>>>  	case MSR_AMD64_PERF_CNTR_GLOBAL_STATUS_SET:
>>>  		if (!msr_info->host_initiated)
>>>  			pmu->global_status |= data & ~pmu->global_status_rsvd;
>>> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
>>> index 0cff369982ae..ae7dd3636e70 100644
>>> --- a/arch/x86/kvm/vmx/nested.c
>>> +++ b/arch/x86/kvm/vmx/nested.c
>>> @@ -719,6 +719,8 @@ static void nested_vmx_merge_pmu_msr_bitmaps(struct kvm_vcpu *vcpu,
>>>  	nested_vmx_merge_msr_bitmaps_rw(MSR_CORE_PERF_GLOBAL_CTRL);
>>>  	nested_vmx_merge_msr_bitmaps_read(MSR_CORE_PERF_GLOBAL_STATUS);
>>>  	nested_vmx_merge_msr_bitmaps_write(MSR_CORE_PERF_GLOBAL_OVF_CTRL);
>>> +	nested_vmx_merge_msr_bitmaps_write(MSR_CORE_PERF_GLOBAL_STATUS_SET);
>>> +	nested_vmx_merge_msr_bitmaps_read(MSR_CORE_PERF_GLOBAL_INUSE);
>>>  }
>>>  
>>>  /*
>>> diff --git a/arch/x86/kvm/vmx/pmu_intel.c b/arch/x86/kvm/vmx/pmu_intel.c
>>> index 4df55a3e21da..3070fba2687f 100644
>>> --- a/arch/x86/kvm/vmx/pmu_intel.c
>>> +++ b/arch/x86/kvm/vmx/pmu_intel.c
>>> @@ -194,6 +194,8 @@ static bool intel_is_valid_msr(struct kvm_vcpu *vcpu, u32 msr)
>>>  	switch (msr) {
>>>  	case MSR_CORE_PERF_FIXED_CTR_CTRL:
>>>  		return kvm_pmu_has_perf_global_ctrl(pmu);
>>> +	case MSR_CORE_PERF_GLOBAL_INUSE:
>>> +		return pmu->version >= 4;
>>>  	case MSR_IA32_PEBS_ENABLE:
>>>  		ret = vcpu_get_perf_capabilities(vcpu) & PERF_CAP_PEBS_FORMAT;
>>>  		break;
>>> @@ -341,6 +343,38 @@ static bool intel_pmu_handle_lbr_msrs_access(struct kvm_vcpu *vcpu,
>>>  	return true;
>>>  }
>>>  
>>> +static u64 intel_pmu_get_global_inuse(struct kvm_vcpu *vcpu)
>>> +{
>>> +	struct kvm_pmu *pmu = vcpu_to_pmu(vcpu);
>>> +	unsigned long fixed_mask = kvm_fixed_pmc_mask(pmu);
>>> +	unsigned long gp_mask = kvm_gp_pmc_mask(pmu);
>>> +	bool pmi_inuse = false;
>>> +	u64 eventsel, data = 0;
>>> +	u32 fixed_ctrl;
>>> +	int i;
>>> +
>>> +	kvm_for_each_gp_counter(i, gp_mask) {
>>> +		eventsel = pmu->gp_counters[i].eventsel;
>>> +
>>> +		if (eventsel & ARCH_PERFMON_EVENTSEL_EVENT)
>> Why to check ARCH_PERFMON_EVENTSEL_EVENT instead of
>> ARCH_PERFMON_EVENTSEL_ENABLE here? Suppose only
>> ARCH_PERFMON_EVENTSEL_ENABLE is set, then the counter is in use.
> This implementation strictly follows the SDM, which checks the event
> bits, not the enable bit. This is also verified on bare metal.
>
> "IA32_PERF_GLOBAL_INUSE.PERFEVTSEL0_InUse[bit 0]: This bit reflects the
> logical state of (IA32_PERFEVTSEL0[7:0] != 0)."

Ok, thanks for double checking. 


>
> That's why, in the reply above to Sashiko, I noted that on bare metal
> without a vCPU scheduled in, after a perf event is closed, the counter
> could still show as in use. perf/x86 clears the enable bit while keeps
> the event bits set.

That's why I feel confusing. Currently perf/x86 doesn't clear the Evensel
field and only clears the enable bit, so IN_USE bit would be set forever
once an event ever used the counter even the GP counter is not enabled
right now.

It's deserved to add a comment here, I believe some persons have same
question with me.

Anyway, let's keep consistent with SDM here. Thanks.