Re: [PATCH v4 05/10] KVM/x86: expose MSR_IA32_PERF_CAPABILITIES to the guest

[Jim Mattson]

On Wed, Dec 26, 2018 at 2:01 AM Wei Wang <wei.w.wang@xxxxxxxxx> wrote:
>
> Bits [0, 5] of MSR_IA32_PERF_CAPABILITIES tell about the format of
> the addresses stored in the LBR stack. Expose those bits to the guest
> when the guest lbr feature is enabled.
>
> Signed-off-by: Wei Wang <wei.w.wang@xxxxxxxxx>
> Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> Cc: Andi Kleen <ak@xxxxxxxxxxxxxxx>
> ---
>  arch/x86/include/asm/perf_event.h | 2 ++
>  arch/x86/kvm/cpuid.c              | 2 +-
>  arch/x86/kvm/vmx.c                | 9 +++++++++
>  3 files changed, 12 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/include/asm/perf_event.h b/arch/x86/include/asm/perf_event.h
> index 2f82795..eee09b7 100644
> --- a/arch/x86/include/asm/perf_event.h
> +++ b/arch/x86/include/asm/perf_event.h
> @@ -87,6 +87,8 @@
>  #define ARCH_PERFMON_BRANCH_MISSES_RETIRED             6
>  #define ARCH_PERFMON_EVENTS_COUNT                      7
>
> +#define X86_PERF_CAP_MASK_LBR_FMT                      0x3f
> +
>  /*
>   * Intel "Architectural Performance Monitoring" CPUID
>   * detection/enumeration details:
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 7bcfa61..3b8a57b 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -365,7 +365,7 @@ static inline int __do_cpuid_ent(struct kvm_cpuid_entry2 *entry, u32 function,
>                 F(XMM3) | F(PCLMULQDQ) | 0 /* DTES64, MONITOR */ |
>                 0 /* DS-CPL, VMX, SMX, EST */ |
>                 0 /* TM2 */ | F(SSSE3) | 0 /* CNXT-ID */ | 0 /* Reserved */ |
> -               F(FMA) | F(CX16) | 0 /* xTPR Update, PDCM */ |
> +               F(FMA) | F(CX16) | 0 /* xTPR Update*/ | F(PDCM) |
>                 F(PCID) | 0 /* Reserved, DCA */ | F(XMM4_1) |
>                 F(XMM4_2) | F(X2APIC) | F(MOVBE) | F(POPCNT) |
>                 0 /* Reserved*/ | F(AES) | F(XSAVE) | 0 /* OSXSAVE */ | F(AVX) |
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index 8d5d984..ee02967 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -4161,6 +4161,13 @@ static int vmx_get_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>                         return 1;
>                 msr_info->data = vcpu->arch.ia32_xss;
>                 break;
> +       case MSR_IA32_PERF_CAPABILITIES:
> +               if (!boot_cpu_has(X86_FEATURE_PDCM))
> +                       return 1;
> +               msr_info->data = native_read_msr(MSR_IA32_PERF_CAPABILITIES);

Since this isn't guarded by vcpu->kvm->arch.lbr_in_guest, it breaks
backwards compatibility, doesn't it?

> +               if (vcpu->kvm->arch.lbr_in_guest)
> +                       msr_info->data &= X86_PERF_CAP_MASK_LBR_FMT;
> +               break;
>         case MSR_TSC_AUX:
>                 if (!msr_info->host_initiated &&
>                     !guest_cpuid_has(vcpu, X86_FEATURE_RDTSCP))
> @@ -4343,6 +4350,8 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>                 else
>                         clear_atomic_switch_msr(vmx, MSR_IA32_XSS);
>                 break;
> +       case MSR_IA32_PERF_CAPABILITIES:
> +               return 1; /* RO MSR */
>         case MSR_TSC_AUX:
>                 if (!msr_info->host_initiated &&
>                     !guest_cpuid_has(vcpu, X86_FEATURE_RDTSCP))
> --
> 2.7.4
>