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

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Date:   Mon, 15 Aug 2022 13:27:28 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Sandipan Das <sandipan.das@....com>
Cc:     linux-kernel@...r.kernel.org, linux-perf-users@...r.kernel.org,
        x86@...nel.org, bp@...en8.de, acme@...nel.org, namhyung@...nel.org,
        jolsa@...nel.org, tglx@...utronix.de, mingo@...hat.com,
        mark.rutland@....com, alexander.shishkin@...ux.intel.com,
        dave.hansen@...ux.intel.com, like.xu.linux@...il.com,
        eranian@...gle.com, ananth.narayan@....com, ravi.bangoria@....com,
        santosh.shukla@....com
Subject: Re: [PATCH 04/13] x86/cpufeatures: Add LbrExtV2 feature bit

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@....com>
> ---
>  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...