linux-kernel - Re: [PATCH V13 - RESEND 09/10] arm64/perf: Implement branch records save on task sched out

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Date:   Wed,  2 Aug 2023 12:59:31 +0100
From:   Rajnesh Kanwal <rkanwal@...osinc.com>
To:     anshuman.khandual@....com
Cc:     acme@...nel.org, broonie@...nel.org, catalin.marinas@....com,
        james.clark@....com, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, linux-perf-users@...r.kernel.org,
        mark.rutland@....com, maz@...nel.org, mingo@...hat.com,
        peterz@...radead.org, robh@...nel.org, suzuki.poulose@....com,
        will@...nel.org, atishp@...osinc.com
Subject: Re: [PATCH V13 - RESEND 09/10] arm64/perf: Implement branch records save on task sched out

>diff --git a/drivers/perf/arm_brbe.c b/drivers/perf/arm_brbe.c
>index 203cd4f350d5..2177632befa6 100644
>--- a/drivers/perf/arm_brbe.c
>+++ b/drivers/perf/arm_brbe.c
>@@ -165,6 +165,36 @@ static int stitch_stored_live_entries(struct brbe_regset *stored,
> 	return min(nr_live + nr_stored, nr_max);
> }
> 
>+static int brbe_branch_save(struct brbe_regset *live, int nr_hw_entries)
>+{
>+	u64 brbfcr = read_sysreg_s(SYS_BRBFCR_EL1);
>+	int nr_live;
>+
>+	write_sysreg_s(brbfcr | BRBFCR_EL1_PAUSED, SYS_BRBFCR_EL1);
>+	isb();
>+
>+	nr_live = capture_brbe_regset(live, nr_hw_entries);
>+
>+	write_sysreg_s(brbfcr & ~BRBFCR_EL1_PAUSED, SYS_BRBFCR_EL1);
>+	isb();
>+
>+	return nr_live;
>+}
>+
>+void armv8pmu_branch_save(struct arm_pmu *arm_pmu, void *ctx)
>+{
>+	struct arm64_perf_task_context *task_ctx = ctx;
>+	struct brbe_regset live[BRBE_MAX_ENTRIES];
>+	int nr_live, nr_store, nr_hw_entries;
>+
>+	nr_hw_entries = brbe_get_numrec(arm_pmu->reg_brbidr);
>+	nr_live = brbe_branch_save(live, nr_hw_entries);
>+	nr_store = task_ctx->nr_brbe_records;
>+	nr_store = stitch_stored_live_entries(task_ctx->store, live, nr_store,
>+					      nr_live, nr_hw_entries);
>+	task_ctx->nr_brbe_records = nr_store;
>+}

Asking out-of-curiosity. Have you thought about virtualization use
case. Current LBR implementation create an event for the guest
and save/restore happens using the sched_task callback. (Correct me
if I am wrong). Given you are only saving and processing those saved
entries, how do you plan to expose the entries to the guest?

Thanks
Rajnesh

>+
> /*
>  * Generic perf branch filters supported on BRBE
>  *
>diff --git a/drivers/perf/arm_pmuv3.c b/drivers/perf/arm_pmuv3.c
>index 408974d5c57b..aa3c7b3dcdd6 100644
>--- a/drivers/perf/arm_pmuv3.c
>+++ b/drivers/perf/arm_pmuv3.c
>@@ -923,9 +923,19 @@ static int armv8pmu_user_event_idx(struct perf_event *event)
> static void armv8pmu_sched_task(struct perf_event_pmu_context *pmu_ctx, bool sched_in)
> {
> 	struct arm_pmu *armpmu = to_arm_pmu(pmu_ctx->pmu);
>+	void *task_ctx = pmu_ctx ? pmu_ctx->task_ctx_data : NULL;
> 
>-	if (sched_in && armpmu->has_branch_stack)
>-		armv8pmu_branch_reset();
>+	if (armpmu->has_branch_stack) {
>+		/* Save branch records in task_ctx on sched out */
>+		if (task_ctx && !sched_in) {
>+			armv8pmu_branch_save(armpmu, task_ctx);
>+			return;
>+		}
>+
>+		/* Reset branch records on sched in */
>+		if (sched_in)
>+			armv8pmu_branch_reset();
>+	}
> }