linux-kernel - Re: [PATCH v4 RESEND 2/5] perf/x86/lbr: Simplify the exposure check for the LBR

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Message-ID: <20210323213854.GD4746@worktop.programming.kicks-ass.net>
Date:   Tue, 23 Mar 2021 22:38:54 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Like Xu <like.xu@...ux.intel.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        Kan Liang <kan.liang@...ux.intel.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH v4 RESEND 2/5] perf/x86/lbr: Simplify the exposure check
 for the LBR_INFO registers

On Mon, Mar 22, 2021 at 02:06:32PM +0800, Like Xu wrote:
> If the platform supports LBR_INFO register, the x86_pmu.lbr_info will
> be assigned in intel_pmu_?_lbr_init_?() and it's safe to expose LBR_INFO

You mean: intel_pmu_lbr_*init*(). '?' is a single character glob and
you've got too many '_'s.

> in the x86_perf_get_lbr() directly, instead of relying on lbr_format check.

But, afaict, not every model calls one of those. CORE_YONAH for example
doesn't.

> Also Architectural LBR has IA32_LBR_x_INFO instead of LBR_FORMAT_INFO_x
> to hold metadata for the operation, including mispredict, TSX, and
> elapsed cycle time information.

Relevance?

Wouldn't it be much simpler to simple say something like:

  "x86_pmu.lbr_info is 0 unless explicitly initialized, so there's no
  point checking lbr_fmt"

> Signed-off-by: Like Xu <like.xu@...ux.intel.com>
> Reviewed-by: Kan Liang <kan.liang@...ux.intel.com>
> Reviewed-by: Andi Kleen <ak@...ux.intel.com>
> ---
>  arch/x86/events/intel/lbr.c | 4 +---
>  1 file changed, 1 insertion(+), 3 deletions(-)
> 
> diff --git a/arch/x86/events/intel/lbr.c b/arch/x86/events/intel/lbr.c
> index 21890dacfcfe..355ea70f1879 100644
> --- a/arch/x86/events/intel/lbr.c
> +++ b/arch/x86/events/intel/lbr.c
> @@ -1832,12 +1832,10 @@ void __init intel_pmu_arch_lbr_init(void)
>   */
>  int x86_perf_get_lbr(struct x86_pmu_lbr *lbr)
>  {
> -	int lbr_fmt = x86_pmu.intel_cap.lbr_format;
> -
>  	lbr->nr = x86_pmu.lbr_nr;
>  	lbr->from = x86_pmu.lbr_from;
>  	lbr->to = x86_pmu.lbr_to;
> -	lbr->info = (lbr_fmt == LBR_FORMAT_INFO) ? x86_pmu.lbr_info : 0;
> +	lbr->info = x86_pmu.lbr_info;
>  
>  	return 0;
>  }
> -- 
> 2.29.2
>