Re: [PATCH V11 06/10] arm64/perf: Enable branch stack events via FEAT_BRBE

[Namhyung Kim <namhyung@kernel.org>]

Hello,

On Tue, May 30, 2023 at 9:21 PM Anshuman Khandual
<anshuman.khandual@arm.com> wrote:
>
> This enables branch stack sampling events in ARMV8 PMU, via an architecture
> feature FEAT_BRBE aka branch record buffer extension. This defines required
> branch helper functions pmuv8pmu_branch_XXXXX() and the implementation here
> is wrapped with a new config option CONFIG_ARM64_BRBE.
>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Will Deacon <will@kernel.org>
> Cc: Mark Rutland <mark.rutland@arm.com>
> Cc: linux-arm-kernel@lists.infradead.org
> Cc: linux-kernel@vger.kernel.org
> Tested-by: James Clark <james.clark@arm.com>
> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
> ---

[SNIP]
> +void armv8pmu_branch_read(struct pmu_hw_events *cpuc, struct perf_event *event)
> +{
> +       struct brbe_hw_attr *brbe_attr = (struct brbe_hw_attr *)cpuc->percpu_pmu->private;
> +       u64 brbfcr, brbcr;
> +       int idx, loop1_idx1, loop1_idx2, loop2_idx1, loop2_idx2, count;
> +
> +       brbcr = read_sysreg_s(SYS_BRBCR_EL1);
> +       brbfcr = read_sysreg_s(SYS_BRBFCR_EL1);
> +
> +       /* Ensure pause on PMU interrupt is enabled */
> +       WARN_ON_ONCE(!(brbcr & BRBCR_EL1_FZP));
> +
> +       /* Pause the buffer */
> +       write_sysreg_s(brbfcr | BRBFCR_EL1_PAUSED, SYS_BRBFCR_EL1);
> +       isb();
> +
> +       /* Determine the indices for each loop */
> +       loop1_idx1 = BRBE_BANK0_IDX_MIN;
> +       if (brbe_attr->brbe_nr <= BRBE_BANK_MAX_ENTRIES) {
> +               loop1_idx2 = brbe_attr->brbe_nr - 1;
> +               loop2_idx1 = BRBE_BANK1_IDX_MIN;
> +               loop2_idx2 = BRBE_BANK0_IDX_MAX;

Is this to disable the bank1?  Maybe need a comment.


> +       } else {
> +               loop1_idx2 = BRBE_BANK0_IDX_MAX;
> +               loop2_idx1 = BRBE_BANK1_IDX_MIN;
> +               loop2_idx2 = brbe_attr->brbe_nr - 1;
> +       }

The loop2_idx1 is the same for both cases.  Maybe better
to move it out of the if statement.

Thanks,
Namhyung


> +
> +       /* Loop through bank 0 */
> +       select_brbe_bank(BRBE_BANK_IDX_0);
> +       for (idx = 0, count = loop1_idx1; count <= loop1_idx2; idx++, count++) {
> +               if (!capture_branch_entry(cpuc, event, idx))
> +                       goto skip_bank_1;
> +       }
> +
> +       /* Loop through bank 1 */
> +       select_brbe_bank(BRBE_BANK_IDX_1);
> +       for (count = loop2_idx1; count <= loop2_idx2; idx++, count++) {
> +               if (!capture_branch_entry(cpuc, event, idx))
> +                       break;
> +       }
> +
> +skip_bank_1:
> +       cpuc->branches->branch_stack.nr = idx;
> +       cpuc->branches->branch_stack.hw_idx = -1ULL;
> +       process_branch_aborts(cpuc);
> +
> +       /* Unpause the buffer */
> +       write_sysreg_s(brbfcr & ~BRBFCR_EL1_PAUSED, SYS_BRBFCR_EL1);
> +       isb();
> +       armv8pmu_branch_reset();
> +}

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