Re: [PATCH v3 1/2] x86: KVM: Advertise FSRS and FSRC on AMD to userspace

Jim Mattson
2024-12-04T16:57:39Z

On Wed, Dec 4, 2024 at 5:43 AM Maksim Davydov
<davydov-max@yandex-team.ru> wrote:
>
> Fast short REP STOSB and fast short CMPSB support on AMD processors are
> provided in other CPUID function in comparison with Intel processors:
> * FSRS: 10 bit in 0x80000021_EAX
> * FSRC: 11 bit in 0x80000021_EAX

I have to wonder why these bits aren't documented in the APM. I assume
you pulled them out of some PPR? I would be hesitant to include CPUID
bit definitions that may be microarchitecture-specific rather than
architectural.

Perhaps someone from AMD should at least ACK this change?

> AMD bit numbers differ from existing definition of FSRC and
> FSRS. So, the new appropriate values have to be added with new names.
>
> It's safe to advertise these features to userspace because they are a part
> of CPU model definition and they can't be disabled (as existing Intel
> features).
>
> Fixes: 2a4209d6a9cb ("KVM: x86: Advertise fast REP string features inherent to the CPU")
> Signed-off-by: Maksim Davydov <davydov-max@yandex-team.ru>
> ---
> arch/x86/include/asm/cpufeatures.h | 2 ++
> arch/x86/kvm/cpuid.c | 4 ++--
> 2 files changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
> index 17b6590748c0..45f87a026bba 100644
> --- a/arch/x86/include/asm/cpufeatures.h
> +++ b/arch/x86/include/asm/cpufeatures.h
> @@ -460,6 +460,8 @@
> #define X86_FEATURE_NULL_SEL_CLR_BASE (20*32+ 6) /* Null Selector Clears Base */
> #define X86_FEATURE_AUTOIBRS (20*32+ 8) /* Automatic IBRS */
> #define X86_FEATURE_NO_SMM_CTL_MSR (20*32+ 9) /* SMM_CTL MSR is not present */
> +#define X86_FEATURE_AMD_FSRS (20*32+10) /* AMD Fast short REP STOSB supported */
> +#define X86_FEATURE_AMD_FSRC (20*32+11) /* AMD Fast short REP CMPSB supported */
>
> #define X86_FEATURE_SBPB (20*32+27) /* Selective Branch Prediction Barrier */
> #define X86_FEATURE_IBPB_BRTYPE (20*32+28) /* MSR_PRED_CMD[IBPB] flushes all branch type predictions */
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 097bdc022d0f..7bc095add8ee 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -799,8 +799,8 @@ void kvm_set_cpu_caps(void)
>
> kvm_cpu_cap_mask(CPUID_8000_0021_EAX,
> F(NO_NESTED_DATA_BP) | F(LFENCE_RDTSC) | 0 /* SmmPgCfgLock */ |
> - F(NULL_SEL_CLR_BASE) | F(AUTOIBRS) | 0 /* PrefetchCtlMsr */ |
> - F(WRMSR_XX_BASE_NS)
> + F(NULL_SEL_CLR_BASE) | F(AUTOIBRS) | F(AMD_FSRS) |
> + F(AMD_FSRC) | 0 /* PrefetchCtlMsr */ | F(WRMSR_XX_BASE_NS)
> );
>
> kvm_cpu_cap_check_and_set(X86_FEATURE_SBPB);
> --
> 2.34.1
>