linux-kernel - Re: [PATCH V2 14/23] perf/x86/intel/lbr: Support Architectural LBR

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Date:   Tue, 30 Jun 2020 12:17:57 -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 14/23] perf/x86/intel/lbr: Support Architectural LBR



On 6/30/2020 11:49 AM, Peter Zijlstra wrote:
> On Fri, Jun 26, 2020 at 11:20:11AM -0700, kan.liang@...ux.intel.com wrote:
> 
>> +	if (boot_cpu_has(X86_FEATURE_ARCH_LBR))
>> +		intel_pmu_arch_lbr_init();
> 
>> +static inline bool is_lbr_call_stack_bit_set(u64 config)
>> +{
>> +	if (x86_pmu.arch_lbr)
>> +		return !!(config & ARCH_LBR_CALL_STACK);
>> +
>> +	return !!(config & LBR_CALL_STACK);
>> +}
> 
>> +	if (!x86_pmu.arch_lbr && !pmi && cpuc->lbr_sel)
>>   		wrmsrl(MSR_LBR_SELECT, lbr_select);
> 
>> +	if (!x86_pmu.arch_lbr)
>> +		debugctl |= DEBUGCTLMSR_LBR;
> 
>> +	if (x86_pmu.arch_lbr)
>> +		wrmsrl(MSR_ARCH_LBR_CTL, lbr_select | ARCH_LBR_CTL_LBREN);
>>   }
> 
> etc...
> 
>> +void __init intel_pmu_arch_lbr_init(void)
>> +{
>> +	unsigned int unused_edx;
>> +	u64 lbr_nr;
>> +
>> +	/* Arch LBR Capabilities */
>> +	cpuid(28, &x86_pmu.lbr_eax.full, &x86_pmu.lbr_ebx.full,
>> +		  &x86_pmu.lbr_ecx.full, &unused_edx);
>> +
>> +	lbr_nr = x86_pmu_get_max_arch_lbr_nr();
>> +	if (!lbr_nr)
>> +		return;
>> +
>> +	/* Apply the max depth of Arch LBR */
>> +	if (wrmsrl_safe(MSR_ARCH_LBR_DEPTH, lbr_nr))
>> +		return;
>> +
>> +	x86_pmu.lbr_nr = lbr_nr;
>> +	x86_get_pmu()->task_ctx_size = sizeof(struct x86_perf_task_context_arch_lbr) +
>> +				       lbr_nr * sizeof(struct lbr_entry);
>> +
>> +	x86_pmu.lbr_from = MSR_ARCH_LBR_FROM_0;
>> +	x86_pmu.lbr_to = MSR_ARCH_LBR_TO_0;
>> +	x86_pmu.lbr_info = MSR_ARCH_LBR_INFO_0;
>> +
>> +	/* LBR callstack requires both CPL and Branch Filtering support */
>> +	if (!x86_pmu.lbr_ebx.split.lbr_cpl ||
>> +	    !x86_pmu.lbr_ebx.split.lbr_filter ||
>> +	    !x86_pmu.lbr_ebx.split.lbr_call_stack)
>> +		arch_lbr_ctl_map[PERF_SAMPLE_BRANCH_CALL_STACK_SHIFT] = LBR_NOT_SUPP;
>> +
>> +	if (!x86_pmu.lbr_ebx.split.lbr_cpl) {
>> +		arch_lbr_ctl_map[PERF_SAMPLE_BRANCH_USER_SHIFT] = LBR_NOT_SUPP;
>> +		arch_lbr_ctl_map[PERF_SAMPLE_BRANCH_KERNEL_SHIFT] = LBR_NOT_SUPP;
>> +	} else if (!x86_pmu.lbr_ebx.split.lbr_filter) {
>> +		arch_lbr_ctl_map[PERF_SAMPLE_BRANCH_ANY_SHIFT] = LBR_NOT_SUPP;
>> +		arch_lbr_ctl_map[PERF_SAMPLE_BRANCH_ANY_RETURN_SHIFT] = LBR_NOT_SUPP;
>> +		arch_lbr_ctl_map[PERF_SAMPLE_BRANCH_ANY_CALL_SHIFT] = LBR_NOT_SUPP;
>> +		arch_lbr_ctl_map[PERF_SAMPLE_BRANCH_IND_CALL_SHIFT] = LBR_NOT_SUPP;
>> +		arch_lbr_ctl_map[PERF_SAMPLE_BRANCH_COND_SHIFT] = LBR_NOT_SUPP;
>> +		arch_lbr_ctl_map[PERF_SAMPLE_BRANCH_IND_JUMP_SHIFT] = LBR_NOT_SUPP;
>> +		arch_lbr_ctl_map[PERF_SAMPLE_BRANCH_CALL_SHIFT] = LBR_NOT_SUPP;
>> +	}
>> +
>> +	x86_pmu.lbr_ctl_mask = ARCH_LBR_CTL_MASK;
>> +	x86_pmu.lbr_ctl_map  = arch_lbr_ctl_map;
>> +
>> +	if (!x86_pmu.lbr_ebx.split.lbr_cpl && !x86_pmu.lbr_ebx.split.lbr_filter)
>> +		x86_pmu.lbr_ctl_map = NULL;
>> +
>> +	x86_pmu.lbr_reset = intel_pmu_arch_lbr_reset;
>> +	x86_pmu.lbr_read = intel_pmu_arch_lbr_read;
>> +	x86_pmu.lbr_save = intel_pmu_arch_lbr_save;
>> +	x86_pmu.lbr_restore = intel_pmu_arch_lbr_restore;
>> +
>> +	x86_pmu.arch_lbr = true;
>> +	pr_cont("Architectural LBR, ");
>> +}
> 
> How about we make this here clear FEATURE_ARCH_LBR if it fails and then
> do away with x86_pmu.arch_lbr and use
> static_cpu_has(X86_FEATURE_ARCH_LBR) a lot more?

Yes, it's doable. So we can save a bit for arch_lbr in structure x86_pmu.

I will clear the FEATURE_ARCH_LBR via clear_cpu_cap(&boot_cpu_data, 
X86_FEATURE_ARCH_LBR);, if the check fails.
I will replace x86_pmu.arch_lbr with 
static_cpu_has(X86_FEATURE_ARCH_LBR) everywhere.

Thanks,
Kan