Re: [PATCH v2 01/18] perf cs-etm: Queue context packets for frontend

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] Incorrect CPU mode attribution in `cs_etm__synth_instruction_sample()` during flush due to relying on `tidq->packet->el` instead of the exception level belonging to the instructions being sampled.
--

commit b9c32b9ed06ee8adb88b0b63943c694b4108e629
Author: James Clark <james.clark@linaro.org>

perf cs-etm: Queue context packets for frontend

This commit adds a CS_ETM_CONTEXT packet to carry thread ID and exception
level to the frontend. It separates memory access functions between frontend
and decoder, and removes prev_packet_thread tracking.

Closes: https://lore.kernel.org/linux-perf-users/20260515021135.1729028-1-aaupov@meta.com/

> diff --git a/tools/perf/util/cs-etm.c b/tools/perf/util/cs-etm.c
> index 40c6ddfa8c8d9..524e2a6feba8e 100644
> --- a/tools/perf/util/cs-etm.c
> +++ b/tools/perf/util/cs-etm.c

[ ... ]

> @@ -1588,15 +1632,15 @@ static int cs_etm__synth_instruction_sample(struct cs_etm_queue *etmq,
>  
>  	perf_sample__init(&sample, /*all=*/true);
>  	event->sample.header.type = PERF_RECORD_SAMPLE;
> -	event->sample.header.misc = cs_etm__cpu_mode(etmq, addr, tidq->el);
> +	event->sample.header.misc = cs_etm__cpu_mode(etmq, addr, tidq->packet->el);

[Severity: Medium]
Does this misattribute guest traces to the host when synthesizing samples
during a flush?

When cs_etm__flush() or cs_etm__end_block() calls
cs_etm__synth_instruction_sample(), it generates samples for the execution
range of tidq->prev_packet. In these scenarios, tidq->packet holds a
CS_ETM_DISCONTINUITY packet (or a stale packet) where the exception level
is ocsd_EL_unknown.

Passing this unknown EL into cs_etm__cpu_mode() causes guest trace execution
from before the discontinuity to be misattributed to the host machine
(PERF_RECORD_MISC_KERNEL or PERF_RECORD_MISC_USER) in the profile statistics.

>  	event->sample.header.size = sizeof(struct perf_event_header);
>  
>  	/* Set time field based on etm auxtrace config. */
>  	sample.time = cs_etm__resolve_sample_time(etmq, tidq);
>  
>  	sample.ip = addr;
> -	sample.pid = thread__pid(tidq->thread);
> -	sample.tid = thread__tid(tidq->thread);

[ ... ]

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260602-james-cs-context-tracking-fix-v2-0-85b5ce6f55c6@linaro.org?part=1