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

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Date:   Mon, 22 Aug 2022 18:22:25 +0530
From:   Sandipan Das <sandipan.das@....com>
To:     Peter Zijlstra <peterz@...radead.org>
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,
        Stephane Eranian <eranian@...gle.com>, ananth.narayan@....com,
        ravi.bangoria@....com, santosh.shukla@....com
Subject: Re: [PATCH 04/13] x86/cpufeatures: Add LbrExtV2 feature bit

Hi Peter,

On 8/22/2022 2:35 PM, Peter Zijlstra wrote:
> On Mon, Aug 15, 2022 at 12:42:23PM -0700, Stephane Eranian wrote:
>> Hi,
>>
>> On Mon, Aug 15, 2022 at 4:27 AM Peter Zijlstra <peterz@...radead.org> wrote:
>>>
>>> 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 :-)
>>>
>> I believe it is called v2 because there was already a LBR in previous
>> generations, however it
> 
> That's not the question; It's currently called LBREXT_V2, which is a bit
> of a shit name. Then again LBR_V2 is too because AMD and Intel LBR are
> quite different. So in that respect BRS_V2 would be an ever so much
> better name.

AMD LbrExtV2 is similar to Intel LBR. Unlike BRS, LbrExtV2 does not rely on
interrupt holding. The branch records are "frozen" at the time of counter
overflow.

- Sandipan