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

[Wei Wang <wei.w.wang@intel.com>]

On 01/03/2019 07:40 AM, Jim Mattson wrote:
> On Wed, Dec 26, 2018 at 2:01 AM Wei Wang <wei.w.wang@intel.com> 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@intel.com>
>> Cc: Paolo Bonzini <pbonzini@redhat.com>
>> Cc: Andi Kleen <ak@linux.intel.com>
>> ---
>>   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?

Right, thanks. Probably better to change it to below:

msr_info->data = 0;
data = native_read_msr(MSR_IA32_PERF_CAPABILITIES);
if (vcpu->kvm->arch.lbr_in_guest)
     msr_info->data |= (data & X86_PERF_CAP_MASK_LBR_FMT);

Best,
Wei