Re: [PATCH v2 04/15] KVM: PMU: disable LBR handling if architectural LBR is available

["Yang, Weijiang" <weijiang.yang@intel.com>]

On 1/28/2023 4:10 AM, Sean Christopherson wrote:
> On Thu, Nov 24, 2022, Yang Weijiang wrote:
>> From: Paolo Bonzini <pbonzini@redhat.com>
>>
>> Traditional LBR is absent on CPU models that have architectural LBR, so
>> disable all processing of traditional LBR MSRs if they are not there.
>>
>> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
>> Signed-off-by: Yang Weijiang <weijiang.yang@intel.com>
>> ---
>>   arch/x86/kvm/vmx/pmu_intel.c | 32 ++++++++++++++++++++++----------
>>   1 file changed, 22 insertions(+), 10 deletions(-)
>>
>> diff --git a/arch/x86/kvm/vmx/pmu_intel.c b/arch/x86/kvm/vmx/pmu_intel.c
>> index e5cec07ca8d9..905673228932 100644
>> --- a/arch/x86/kvm/vmx/pmu_intel.c
>> +++ b/arch/x86/kvm/vmx/pmu_intel.c
>> @@ -170,19 +170,23 @@ static inline struct kvm_pmc *get_fw_gp_pmc(struct kvm_pmu *pmu, u32 msr)
>>   static bool intel_pmu_is_valid_lbr_msr(struct kvm_vcpu *vcpu, u32 index)
>>   {
>>   	struct x86_pmu_lbr *records = vcpu_to_lbr_records(vcpu);
>> -	bool ret = false;
>>   
>>   	if (!intel_pmu_lbr_is_enabled(vcpu))
>> -		return ret;
>> +		return false;
>>   
>> -	ret = (index == MSR_LBR_SELECT) || (index == MSR_LBR_TOS) ||
>> -		(index >= records->from && index < records->from + records->nr) ||
>> -		(index >= records->to && index < records->to + records->nr);
>> +	if (!guest_cpuid_has(vcpu, X86_FEATURE_ARCH_LBR) &&
> IIUC, the MSRs flat out don't exist _and_ KVM expects to passthrough MSRs to the
> guest, i.e. KVM should check host support, not guest support.  Probably a moot
> point from a functionality perspective since KVM shouldn't allow LBRs to shouldn't
> be enabled for the guest, but from a performance perspective, checking guest CPUID
> is slooow.


OK, I'll change the check.


>
> That brings me to point #2, which is that KVM needs to disallow enabling legacy
> LBRs on CPUs that support arch LBRs.  Again, IIUC, because KVM doesn't have the
> option to fallback to legacy LBRs,


Legacy LBR and Arch-lbr are exclusive on any platforms, on old 
platforms, legacy LBR is available,

on new platforms, e.g., SPR, arch-lbr is present, so we don't have 
fallback logic.


> that restriction needs to be treated as a bug
> fix.  I'll post a separate patch unless my understanding is wrong.
>
>> +	    (index == MSR_LBR_SELECT || index == MSR_LBR_TOS))
>> +		return true;
>>   
>> -	if (!ret && records->info)
>> -		ret = (index >= records->info && index < records->info + records->nr);
>> +	if ((index >= records->from && index < records->from + records->nr) ||
>> +	    (index >= records->to && index < records->to + records->nr))
>> +		return true;
>>   
>> -	return ret;
>> +	if (records->info && index >= records->info &&
>> +	    index < records->info + records->nr)
>> +		return true;
>> +
>> +	return false;
>>   }
>>   
>>   static bool intel_is_valid_msr(struct kvm_vcpu *vcpu, u32 msr)
>> @@ -702,6 +706,9 @@ static void vmx_update_intercept_for_lbr_msrs(struct kvm_vcpu *vcpu, bool set)
>>   			vmx_set_intercept_for_msr(vcpu, lbr->info + i, MSR_TYPE_RW, set);
>>   	}
>>   
>> +	if (guest_cpuid_has(vcpu, X86_FEATURE_ARCH_LBR))
> Similar to above, I really don't want to query guest CPUID in the VM-Enter path.
> If we establish the rule that LBRs can be enabled if and only if the correct type
> is enabled (traditional/legacy vs. arch), then this can simply check host support.

I understand your concerns, will try to use other efficient ways to 
check guest arch-lbr support.


>
>> +		return;
>> +
>>   	vmx_set_intercept_for_msr(vcpu, MSR_LBR_SELECT, MSR_TYPE_RW, set);
>>   	vmx_set_intercept_for_msr(vcpu, MSR_LBR_TOS, MSR_TYPE_RW, set);
>>   }
>> @@ -742,10 +749,12 @@ void vmx_passthrough_lbr_msrs(struct kvm_vcpu *vcpu)
>>   {
>>   	struct kvm_pmu *pmu = vcpu_to_pmu(vcpu);
>>   	struct lbr_desc *lbr_desc = vcpu_to_lbr_desc(vcpu);
>> +	bool lbr_enable = !guest_cpuid_has(vcpu, X86_FEATURE_ARCH_LBR) &&
>> +		(vmcs_read64(GUEST_IA32_DEBUGCTL) & DEBUGCTLMSR_LBR);
> Unnecessary guest CPUID lookup and VMCS read, i.e. this can be deferred to the
> !lbr_desc->event path.

OK


>
>>   
>>   	if (!lbr_desc->event) {
>>   		vmx_disable_lbr_msrs_passthrough(vcpu);
>> -		if (vmcs_read64(GUEST_IA32_DEBUGCTL) & DEBUGCTLMSR_LBR)
>> +		if (lbr_enable)
>>   			goto warn;
>>   		if (test_bit(INTEL_PMC_IDX_FIXED_VLBR, pmu->pmc_in_use))
>>   			goto warn;
>> @@ -768,7 +777,10 @@ void vmx_passthrough_lbr_msrs(struct kvm_vcpu *vcpu)
>>   
>>   static void intel_pmu_cleanup(struct kvm_vcpu *vcpu)
>>   {
>> -	if (!(vmcs_read64(GUEST_IA32_DEBUGCTL) & DEBUGCTLMSR_LBR))
>> +	bool lbr_enable = !guest_cpuid_has(vcpu, X86_FEATURE_ARCH_LBR) &&
>> +		(vmcs_read64(GUEST_IA32_DEBUGCTL) & DEBUGCTLMSR_LBR);
>> +
>> +	if (!lbr_enable)
>>   		intel_pmu_release_guest_lbr_event(vcpu);
>>   }
>>   
>> -- 
>> 2.27.0
>>