linux-kernel - Re: [PATCH V2 08/23] perf/x86: Expose CPUID enumeration bits for arch LBR

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Date:   Tue, 30 Jun 2020 17:01:54 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     kan.liang@...ux.intel.com
Cc:     mingo@...hat.com, acme@...nel.org, tglx@...utronix.de,
        bp@...en8.de, x86@...nel.org, linux-kernel@...r.kernel.org,
        mark.rutland@....com, alexander.shishkin@...ux.intel.com,
        jolsa@...hat.com, namhyung@...nel.org, dave.hansen@...el.com,
        yu-cheng.yu@...el.com, bigeasy@...utronix.de, gorcunov@...il.com,
        hpa@...or.com, alexey.budankov@...ux.intel.com, eranian@...gle.com,
        ak@...ux.intel.com, like.xu@...ux.intel.com,
        yao.jin@...ux.intel.com, wei.w.wang@...el.com
Subject: Re: [PATCH V2 08/23] perf/x86: Expose CPUID enumeration bits for
 arch LBR

On Fri, Jun 26, 2020 at 11:20:05AM -0700, kan.liang@...ux.intel.com wrote:
> From: Kan Liang <kan.liang@...ux.intel.com>
> 
> The LBR capabilities of Architecture LBR are retrieved from the CPUID
> enumeration once at boot time. The capabilities have to be saved for
> future usage.
> 
> Several new fields are added into structure x86_pmu to indicate the
> capabilities. The fields will be used in the following patches.
> 
> Signed-off-by: Kan Liang <kan.liang@...ux.intel.com>
> ---
>  arch/x86/events/perf_event.h      |  5 +++++
>  arch/x86/include/asm/perf_event.h | 40 +++++++++++++++++++++++++++++++++++++++
>  2 files changed, 45 insertions(+)
> 
> diff --git a/arch/x86/events/perf_event.h b/arch/x86/events/perf_event.h
> index d04818b..9b0e533 100644
> --- a/arch/x86/events/perf_event.h
> +++ b/arch/x86/events/perf_event.h
> @@ -690,6 +690,11 @@ struct x86_pmu {
>  	const int	*lbr_sel_map;		   /* lbr_select mappings */
>  	bool		lbr_double_abort;	   /* duplicated lbr aborts */
>  	bool		lbr_pt_coexist;		   /* (LBR|BTS) may coexist with PT */
> +	bool		arch_lbr;		   /* Arch LBR supported */
> +
> +	union cpuid28_eax	lbr_eax;
> +	union cpuid28_ebx	lbr_ebx;
> +	union cpuid28_ecx	lbr_ecx;

Why keep the full CPUID leaf here? What's wrong with something like:

	unsigned int	lbr_depth_mask:8;
	unsigned int	lbr_deep_c_reset:1;
	unsigned int	lbr_lip:1;
	unsigned int	lbr_cpl:1;
	unsigned int	lbr_filter:1;
	unsigned int	lbr_call_stack:1;
	unsigned int	lbr_mispred:1;
	unsigned int	lbr_timed_lbr:1;
	unsigned int	lbr_br_type:1;

That's only 2 bytes, instead of 24.

>  	void		(*lbr_reset)(void);
>  	void		(*lbr_read)(struct cpu_hw_events *cpuc);