Re: [PATCH 04/13] x86/cpufeatures: Add LbrExtV2 feature bit

From: Peter Zijlstra
Date: Mon Aug 15 2022 - 07:27:50 EST


On Thu, Aug 11, 2022 at 05:59:52PM +0530, Sandipan Das wrote:
> CPUID leaf 0x80000022 i.e. ExtPerfMonAndDbg advertises some new performance
> monitoring features for AMD processors.
>
> Bit 1 of EAX indicates support for Last Branch Record Extension Version 2
> (LbrExtV2) features. If found to be set during PMU initialization, the EBX
> bits of the same leaf can be used to determine the number of available LBR
> entries.
>
> For better utilization of feature words, LbrExtV2 is added as a scattered
> feature bit.
>
> Signed-off-by: Sandipan Das <sandipan.das@xxxxxxx>
> ---
> arch/x86/include/asm/cpufeatures.h | 2 +-
> arch/x86/kernel/cpu/scattered.c | 1 +
> 2 files changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
> index 393f2bbb5e3a..e3fa476a24b0 100644
> --- a/arch/x86/include/asm/cpufeatures.h
> +++ b/arch/x86/include/asm/cpufeatures.h
> @@ -96,7 +96,7 @@
> #define X86_FEATURE_SYSCALL32 ( 3*32+14) /* "" syscall in IA32 userspace */
> #define X86_FEATURE_SYSENTER32 ( 3*32+15) /* "" sysenter in IA32 userspace */
> #define X86_FEATURE_REP_GOOD ( 3*32+16) /* REP microcode works well */
> -/* FREE! ( 3*32+17) */
> +#define X86_FEATURE_LBREXT_V2 ( 3*32+17) /* AMD Last Branch Record Extension Version 2 */
> #define X86_FEATURE_LFENCE_RDTSC ( 3*32+18) /* "" LFENCE synchronizes RDTSC */
> #define X86_FEATURE_ACC_POWER ( 3*32+19) /* AMD Accumulated Power Mechanism */
> #define X86_FEATURE_NOPL ( 3*32+20) /* The NOPL (0F 1F) instructions */
> diff --git a/arch/x86/kernel/cpu/scattered.c b/arch/x86/kernel/cpu/scattered.c
> index dbaa8326d6f2..6be46dffddbf 100644
> --- a/arch/x86/kernel/cpu/scattered.c
> +++ b/arch/x86/kernel/cpu/scattered.c
> @@ -44,6 +44,7 @@ static const struct cpuid_bit cpuid_bits[] = {
> { X86_FEATURE_PROC_FEEDBACK, CPUID_EDX, 11, 0x80000007, 0 },
> { X86_FEATURE_MBA, CPUID_EBX, 6, 0x80000008, 0 },
> { X86_FEATURE_PERFMON_V2, CPUID_EAX, 0, 0x80000022, 0 },
> + { X86_FEATURE_LBREXT_V2, CPUID_EAX, 1, 0x80000022, 0 },
> { 0, 0, 0, 0, 0 }
> };

Would LBR_V2 work at all? It being a new version already seems to imply
extention, no? Then again, I suppose there's an argument to be had for
avoiding confusion vs the Intel LBR thing.. Couldn't you have called
this BRS_V2 :-)

Oh well...