Re: [PATCH] KVM: VMX: Always treat MSR_IA32_PERF_CAPABILITIES as a valid PMU MSR

[Sean Christopherson]

On Thu, Jun 04, 2020 at 09:37:59AM +0800, Xu, Like wrote:
> On 2020/6/4 4:33, Sean Christopherson wrote:
> >Unconditionally return true when querying the validity of
> >MSR_IA32_PERF_CAPABILITIES so as to defer the validity check to
> >intel_pmu_{get,set}_msr(), which can properly give the MSR a pass when
> >the access is initiated from host userspace.
> Regardless of  the MSR is emulated or not, is it a really good assumption that
> the guest cpuids are not properly ready when we do initialization from host
> userspace
> ?

I don't know if I would call it a "good assumption" so much as a "necessary
assumption".  KVM_{GET,SET}_MSRS are allowed, and must function correctly,
if they're called prior to KVM_SET_CPUID{2}.

> >The MSR is emulated so
> >there is no underlying hardware dependency to worry about.
> >
> >Fixes: 27461da31089a ("KVM: x86/pmu: Support full width counting")
> >Cc: Like Xu <like.xu@xxxxxxxxxxxxxxx>
> >Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
> >---
> >
> >KVM selftests are completely hosed for me, everything fails on KVM_GET_MSRS.
> At least I tried "make --silent -C tools/testing/selftests/kvm run_tests"
> and how do I reproduce the "everything fails" for this issue ?

Hmm, I did nothing more than run the tests on a HSW system.

> Thanks,
> Like Xu
> >
> >  arch/x86/kvm/vmx/pmu_intel.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> >diff --git a/arch/x86/kvm/vmx/pmu_intel.c b/arch/x86/kvm/vmx/pmu_intel.c
> >index d33d890b605f..bdcce65c7a1d 100644
> >--- a/arch/x86/kvm/vmx/pmu_intel.c
> >+++ b/arch/x86/kvm/vmx/pmu_intel.c
> >@@ -181,7 +181,7 @@ static bool intel_is_valid_msr(struct kvm_vcpu *vcpu, u32 msr)
> >  		ret = pmu->version > 1;
> >  		break;
> >  	case MSR_IA32_PERF_CAPABILITIES:
> >-		ret = guest_cpuid_has(vcpu, X86_FEATURE_PDCM);
> >+		ret = 1;
> >  		break;
> >  	default:
> >  		ret = get_gp_pmc(pmu, msr, MSR_IA32_PERFCTR0) ||
>