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From:   Jim Mattson <jmattson@google.com>
Date:   Wed, 23 Jun 2021 11:03:39 -0700
Message-ID: <CALMp9eT-KL-xDgV9p31NgnbW2tnwPes7r6GhJbMedim5e9Ab4g@mail.gmail.com>
Subject: Re: [RESEND PATCH v4 03/10] KVM: vmx/pmu: Add MSR_ARCH_LBR_DEPTH
 emulation for Arch LBR
To:     Like Xu <like.xu@linux.intel.com>
Cc:     Paolo Bonzini <pbonzini@redhat.com>,
        Sean Christopherson <seanjc@google.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Joerg Roedel <joro@8bytes.org>,
        Yang Weijiang <weijiang.yang@intel.com>,
        Wei Wang <wei.w.wang@intel.com>,
        kvm list <kvm@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
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On Mon, May 10, 2021 at 1:16 AM Like Xu <like.xu@linux.intel.com> wrote:
>
> The number of Arch LBR entries available for recording operations
> is dictated by the value in MSR_ARCH_LBR_DEPTH.DEPTH. The supported
> LBR depth values can be found in CPUID.(EAX=01CH, ECX=0):EAX[7:0]
> and for each bit "n" set in this field, the MSR_ARCH_LBR_DEPTH.DEPTH
> value of "8*(n+1)" is supported.
>
> On a guest write to MSR_ARCH_LBR_DEPTH, all LBR entries are reset to 0.
> KVM emulates the reset behavior by introducing lbr_desc->arch_lbr_reset.
> KVM writes the guest requested value to the native ARCH_LBR_DEPTH MSR
> (this is safe because the two values will be the same) when the Arch LBR
> records MSRs are pass-through to the guest.
>
> Signed-off-by: Like Xu <like.xu@linux.intel.com>
> ---
>  arch/x86/kvm/vmx/pmu_intel.c | 43 ++++++++++++++++++++++++++++++++++++
>  arch/x86/kvm/vmx/vmx.h       |  3 +++
>  2 files changed, 46 insertions(+)
>
> diff --git a/arch/x86/kvm/vmx/pmu_intel.c b/arch/x86/kvm/vmx/pmu_intel.c
> index 9efc1a6b8693..d9c9cb6c9a4b 100644
> --- a/arch/x86/kvm/vmx/pmu_intel.c
> +++ b/arch/x86/kvm/vmx/pmu_intel.c
> @@ -220,6 +220,9 @@ static bool intel_is_valid_msr(struct kvm_vcpu *vcpu, u32 msr)
>         case MSR_CORE_PERF_GLOBAL_OVF_CTRL:
>                 ret = pmu->version > 1;
>                 break;
> +       case MSR_ARCH_LBR_DEPTH:
> +               ret = guest_cpuid_has(vcpu, X86_FEATURE_ARCH_LBR);
> +               break;

This doesn't seem like a very safe test, since userspace can provide
whatever CPUID tables it likes. You should definitely think about
hardening this code against a malicious userspace.

When you add a new guest MSR, it should be enumerated by
KVM_GET_MSR_INDEX_LIST. Otherwise, userspace will not save/restore the
MSR value on suspend/resume.