linux-kernel - Re: [PATCH 6/7] KVM: x86: Ignore benign host accesses to "unsupported" PEBS and BTS MSRs

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Date:   Mon, 20 Jun 2022 16:29:38 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Like Xu <likexu@...cent.com>
Subject: Re: [PATCH 6/7] KVM: x86: Ignore benign host accesses to
 "unsupported" PEBS and BTS MSRs

On 6/13/22 16:09, Sean Christopherson wrote:
> On Sat, Jun 11, 2022, Sean Christopherson wrote:
>> @@ -4122,6 +4132,16 @@ int kvm_get_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>>   		msr_info->data = vcpu->arch.guest_fpu.xfd_err;
>>   		break;
>>   #endif
>> +	case MSR_IA32_PEBS_ENABLE:
>> +	case MSR_IA32_DS_AREA:
>> +	case MSR_PEBS_DATA_CFG:
>> +		if (kvm_pmu_is_valid_msr(vcpu, msr_info->index))
>> +			return kvm_pmu_get_msr(vcpu, msr_info);
>> +		/*
>> +		 * Userspace is allowed to read MSRs that KVM reports as
>> +		 * to-be-saved, even if an MSR isn't fully supported.
>> +		 */
>> +		return !msr_info->host_initiated;
> 
> Gah, this needs to set msr_info->data.

Might also reuse the F15H case:

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 70364d40e3f7..be39968149e6 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3877,9 +3877,16 @@ int kvm_get_msr_common(struct kvm_vcpu *vcpu, 
struct msr_data *msr_info)
  	case MSR_DRAM_ENERGY_STATUS:	/* DRAM controller */
  		msr_info->data = 0;
  		break;
+	case MSR_IA32_PEBS_ENABLE:
+	case MSR_IA32_DS_AREA:
+	case MSR_PEBS_DATA_CFG:
  	case MSR_F15H_PERF_CTL0 ... MSR_F15H_PERF_CTR5:
  		if (kvm_pmu_is_valid_msr(vcpu, msr_info->index))
  			return kvm_pmu_get_msr(vcpu, msr_info);
+		/*
+		 * Userspace is allowed to read MSRs that KVM reports as
+		 * to-be-saved, even if an MSR isn't fully supported.
+		 */
  		if (!msr_info->host_initiated)
  			return 1;
  		msr_info->data = 0;


Paolo