linux-kernel - Re: [PATCH 12/21] perf/x86/intel/lbr: Support Architectural LBR

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Date:   Fri, 19 Jun 2020 21:08:23 +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
Subject: Re: [PATCH 12/21] perf/x86/intel/lbr: Support Architectural LBR

On Fri, Jun 19, 2020 at 07:04:00AM -0700, kan.liang@...ux.intel.com wrote:

> +static void intel_pmu_arch_lbr_enable(bool pmi)
> +{
> +	struct cpu_hw_events *cpuc = this_cpu_ptr(&cpu_hw_events);
> +	u64 debugctl, lbr_ctl = 0, orig_debugctl;
> +
> +	if (pmi)
> +		return;
> +
> +	if (cpuc->lbr_ctl)
> +		lbr_ctl = cpuc->lbr_ctl->config & x86_pmu.lbr_ctl_mask;
> +	/*
> +	 * LBR callstack does not work well with FREEZE_LBRS_ON_PMI.
> +	 * If FREEZE_LBRS_ON_PMI is set, PMI near call/return instructions
> +	 * may be missed, that can lead to confusing results.
> +	 */
> +	rdmsrl(MSR_IA32_DEBUGCTLMSR, debugctl);
> +	orig_debugctl = debugctl;
> +	if (lbr_ctl & ARCH_LBR_CALL_STACK)
> +		debugctl &= ~DEBUGCTLMSR_FREEZE_LBRS_ON_PMI;
> +	else
> +		debugctl |= DEBUGCTLMSR_FREEZE_LBRS_ON_PMI;
> +	if (orig_debugctl != debugctl)
> +		wrmsrl(MSR_IA32_DEBUGCTLMSR, debugctl);
> +
> +	wrmsrl(MSR_ARCH_LBR_CTL, lbr_ctl | ARCH_LBR_CTL_LBREN);
> +}

This is nearly a duplicate of the old one, surely we can do better?

> +static void intel_pmu_arch_lbr_restore(void *ctx)
> +{
> +	struct x86_perf_task_context_arch_lbr *task_ctx = ctx;
> +	struct x86_perf_arch_lbr_entry *entries = task_ctx->entries;
> +	int i;
> +
> +	/* Fast reset the LBRs before restore if the call stack is not full. */
> +	if (!entries[x86_pmu.lbr_nr - 1].lbr_from)
> +		intel_pmu_arch_lbr_reset();
> +
> +	for (i = 0; i < x86_pmu.lbr_nr; i++) {
> +		if (!entries[i].lbr_from)
> +			break;
> +		wrlbr_from(i, entries[i].lbr_from);
> +		wrlbr_to(i, entries[i].lbr_to);
> +		wrmsrl(MSR_ARCH_LBR_INFO_0 + i, entries[i].lbr_info);
> +	}
> +}

This too looks very much like the old one.

> +static void intel_pmu_arch_lbr_save(void *ctx)
> +{
> +	struct x86_perf_task_context_arch_lbr *task_ctx = ctx;
> +	struct x86_perf_arch_lbr_entry *entries = task_ctx->entries;
> +	int i;
> +
> +	for (i = 0; i < x86_pmu.lbr_nr; i++) {
> +		entries[i].lbr_from = rdlbr_from(i);
> +		/* Only save valid branches. */
> +		if (!entries[i].lbr_from)
> +			break;
> +		entries[i].lbr_to = rdlbr_to(i);
> +		rdmsrl(MSR_ARCH_LBR_INFO_0 + i, entries[i].lbr_info);
> +	}
> +
> +	/* LBR call stack is not full. Reset is required in restore. */
> +	if (i < x86_pmu.lbr_nr)
> +		entries[x86_pmu.lbr_nr - 1].lbr_from = 0;
> +}

And again..

> +static void __intel_pmu_arch_lbr_read(struct cpu_hw_events *cpuc, int index,
> +				      u64 from, u64 to, u64 info)
> +{
> +	u64 mis = 0, pred = 0, in_tx = 0, abort = 0, type = 0;
> +	u32 br_type, to_plm;
> +	u16 cycles = 0;
> +
> +	if (x86_pmu.arch_lbr_mispred) {
> +		mis = !!(info & ARCH_LBR_INFO_MISPRED);
> +		pred = !mis;
> +	}
> +	in_tx = !!(info & ARCH_LBR_INFO_IN_TSX);
> +	abort = !!(info & ARCH_LBR_INFO_TSX_ABORT);
> +	if (x86_pmu.arch_lbr_timed_lbr &&
> +	    (info & ARCH_LBR_INFO_CYC_CNT_VALID))
> +		cycles = (info & ARCH_LBR_INFO_CYC_CNT);
> +
> +	/*
> +	 * Parse the branch type recorded in LBR_x_INFO MSR.
> +	 * Doesn't support OTHER_BRANCH decoding for now.
> +	 * OTHER_BRANCH branch type still rely on software decoding.
> +	 */
> +	if (x86_pmu.arch_lbr_br_type) {
> +		br_type = (info & ARCH_LBR_INFO_BR_TYPE) >> ARCH_LBR_INFO_BR_TYPE_OFFSET;
> +
> +		if (br_type <= ARCH_LBR_BR_TYPE_KNOWN_MAX) {
> +			to_plm = kernel_ip(to) ? X86_BR_KERNEL : X86_BR_USER;
> +			type = arch_lbr_br_type_map[br_type] | to_plm;
> +		}
> +	}
> +
> +	cpuc->lbr_entries[index].from		 = from;
> +	cpuc->lbr_entries[index].to		 = to;
> +	cpuc->lbr_entries[index].mispred	 = mis;
> +	cpuc->lbr_entries[index].predicted	 = pred;
> +	cpuc->lbr_entries[index].in_tx		 = in_tx;
> +	cpuc->lbr_entries[index].abort		 = abort;
> +	cpuc->lbr_entries[index].cycles		 = cycles;
> +	cpuc->lbr_entries[index].type		 = type;
> +	cpuc->lbr_entries[index].reserved	 = 0;
> +}
> +
> +static void intel_pmu_arch_lbr_read(struct cpu_hw_events *cpuc)
> +{
> +	u64 from, to, info;
> +	int i;
> +
> +	for (i = 0; i < x86_pmu.lbr_nr; i++) {
> +		from = rdlbr_from(i);
> +		to   = rdlbr_to(i);
> +
> +		/*
> +		 * Read LBR entries until invalid entry (0s) is detected.
> +		 */
> +		if (!from)
> +			break;
> +
> +		rdmsrl(MSR_ARCH_LBR_INFO_0 + i, info);
> +
> +		__intel_pmu_arch_lbr_read(cpuc, i, from, to, info);
> +	}
> +
> +	cpuc->lbr_stack.nr = i;
> +}

> +static void intel_pmu_store_pebs_arch_lbrs(struct pebs_lbr *pebs_lbr,
> +					   struct cpu_hw_events *cpuc)
> +{
> +	struct pebs_lbr_entry *lbr;
> +	int i;
> +
> +	for (i = 0; i < x86_pmu.lbr_nr; i++) {
> +		lbr = &pebs_lbr->lbr[i];
> +
> +		/*
> +		 * Read LBR entries until invalid entry (0s) is detected.
> +		 */
> +		if (!lbr->from)
> +			break;
> +
> +		__intel_pmu_arch_lbr_read(cpuc, i, lbr->from,
> +					  lbr->to, lbr->info);
> +	}
> +
> +	cpuc->lbr_stack.nr = i;
> +	intel_pmu_lbr_filter(cpuc);
> +}

Unless I'm reading cross-eyed again, that too is very similar to what we
already had.