linux-kernel - Re: [PATCH 2/2] KVM: x86: always allow host-initiated writes to PMU MSRs

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Message-ID: <YqEhMxlPxzP+CPSR@google.com>
Date:   Wed, 8 Jun 2022 22:22:43 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        likexu@...cent.com
Subject: Re: [PATCH 2/2] KVM: x86: always allow host-initiated writes to PMU
 MSRs

On Tue, May 31, 2022, Paolo Bonzini wrote:
> Whenever an MSR is part of KVM_GET_MSR_INDEX_LIST, it has to be always
> retrievable and settable with KVM_GET_MSR and KVM_SET_MSR.  Accept
> the PMU MSRs unconditionally in intel_is_valid_msr, if the access was
> host-initiated.

...so that userspace can explode in intel_get_msr() or intel_set_msr().  Selftests
that regurgitate MSRs are still failing.  The below "fixes" the issue, but I don't
know that it's actually a good idea.  I also haven't tried AMD.

diff --git a/arch/x86/kvm/vmx/pmu_intel.c b/arch/x86/kvm/vmx/pmu_intel.c
index 515ab6594333..fcb5224028a6 100644
--- a/arch/x86/kvm/vmx/pmu_intel.c
+++ b/arch/x86/kvm/vmx/pmu_intel.c
@@ -401,7 +401,7 @@ static int intel_pmu_get_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
                        return 0;
        }

-       return 1;
+       return !msr_info->host_initiated;
 }

 static int intel_pmu_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
@@ -497,7 +497,7 @@ static int intel_pmu_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
                        return 0;
        }

-       return 1;
+       return !msr_info->host_initiated;
 }

 static void setup_fixed_pmc_eventsel(struct kvm_pmu *pmu)