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 11:36:29 -0400
From:   "Liang, Kan" <kan.liang@...ux.intel.com>
To:     Peter Zijlstra <peterz@...radead.org>
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 6/30/2020 11:01 AM, Peter Zijlstra wrote:
> 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.

Yes, it can save the space.
I will add some codes in patch 14 to initialize these items in 
intel_pmu_arch_lbr_init().

Thanks,
Kan

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