Re: [PATCH 4/5] perf arm-spe: Implement find_snapshot callback

[German Gomez <german.gomez@arm.com>]

Hi Leo,

On 17/10/2021 07:13, Leo Yan wrote:
> [...]
>
> I looked into the Arm SPE driver and found it doesn't really support
> free run mode for AUX ring buffer when the driver runs in snapshot
> mode, the pair functions perf_aux_output_end() and
> perf_aux_output_begin() are invoked when every time handle the
> interrupt.  The detailed flow is:
>
>   arm_spe_pmu_irq_handler()
>     `> arm_spe_pmu_buf_get_fault_act()
>          `> arm_spe_perf_aux_output_end()
>               `> set SPE registers
>               `> perf_aux_output_end()
>     `> arm_spe_perf_aux_output_begin()
>          `> perf_aux_output_begin()
>          `> set SPE registers
>
> Seems to me, a possible solution is to add an extra parameter 'int
> in_interrupt' for functions arm_spe_perf_aux_output_end() and
> arm_spe_perf_aux_output_begin(), if this parameter is passed as 1 in
> the interrupt handling, these two functions should skip invoking
> perf_aux_output_end() and perf_aux_output_begin() so can avoid the
> redundant perf event PERF_RECORD_AUX.
>
>   arm_spe_pmu_irq_handler()
>     `> arm_spe_pmu_buf_get_fault_act()
>          `> arm_spe_perf_aux_output_end(..., in_interrupt=1)
>               `> set SPE registers
>     `> arm_spe_perf_aux_output_begin(..., in_interrupt=1)
>          `> set SPE registers

I brought the issue of the redundant AUX events to the team, and we know
of at least one tool in Arm relying on these events in snapshot mode. So
we think that changing this behavior of the driver might not be easy to
do right now.

>
> P.s. I think Intel-PT has supported free run mode for snapshot mode,
> so it should not generate interrupt in this mode.  Thus Intel-PT can
> avoid this issue, please see the code [2].
>
> Thanks,
> Leo
>
> [1] https://people.linaro.org/~leo.yan/spe/snapshot_test/perf.data
> [2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/x86/events/intel/pt.c#n753

Thanks,
German