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

[Peter Zijlstra]

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@xxxxxxxxxxxxx> 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@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 :-)
> >
> 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.