linux-kernel - [PATCH 1/1] perf cs-etm: Output 0 instead of 0xdeadbeef when exception packets are flushed

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Message-Id: <20240722152756.59453-2-james.clark@linaro.org>
Date: Mon, 22 Jul 2024 16:27:56 +0100
From: James Clark <james.clark@...aro.org>
To: coresight@...ts.linaro.org,
	gankulkarni@...amperecomputing.com,
	mike.leach@...aro.org,
	leo.yan@....com,
	suzuki.poulose@....com
Cc: James Clark <james.clark@...aro.org>,
	John Garry <john.g.garry@...cle.com>,
	Will Deacon <will@...nel.org>,
	James Clark <james.clark@....com>,
	Leo Yan <leo.yan@...ux.dev>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Namhyung Kim <namhyung@...nel.org>,
	Mark Rutland <mark.rutland@....com>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Jiri Olsa <jolsa@...nel.org>,
	Ian Rogers <irogers@...gle.com>,
	Adrian Hunter <adrian.hunter@...el.com>,
	"Liang, Kan" <kan.liang@...ux.intel.com>,
	linux-arm-kernel@...ts.infradead.org,
	linux-perf-users@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH 1/1] perf cs-etm: Output 0 instead of 0xdeadbeef when exception packets are flushed

Normally exception packets don't directly output a branch sample, but
if they're the last record in a buffer then they will. Because they
don't have addresses set we'll see the placeholder value
CS_ETM_INVAL_ADDR (0xdeadbeef) in the output.

Since commit 6035b6804bdf ("perf cs-etm: Support dummy address value for
CS_ETM_TRACE_ON packet") we've used 0 as an externally visible "not set"
address value. For consistency reasons and to not make exceptions look
like an error, change them to use 0 too.

This is particularly visible when doing userspace only tracing because
trace is disabled when jumping to the kernel, causing the flush and then
forcing the last exception packet to be emitted as a branch. With kernel
trace included, there is no flush so exception packets don't generate
samples until the next range packet and they'll pick up the correct
address.

Before:

  $ perf record -e cs_etm//u -- stress -i 1 -t 1
  $ perf script -F comm,ip,addr,flags

  stress   syscall                    ffffb7eedbc0 => deadbeefdeadbeef
  stress   syscall                    ffffb7f14a14 => deadbeefdeadbeef
  stress   syscall                    ffffb7eedbc0 => deadbeefdeadbeef

After:

  stress   syscall                    ffffb7eedbc0 =>                0
  stress   syscall                    ffffb7f14a14 =>                0
  stress   syscall                    ffffb7eedbc0 =>                0

Signed-off-by: James Clark <james.clark@...aro.org>
---
 tools/perf/util/cs-etm.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/tools/perf/util/cs-etm.c b/tools/perf/util/cs-etm.c
index 5e9fbcfad7d4..d3e9c64d17d4 100644
--- a/tools/perf/util/cs-etm.c
+++ b/tools/perf/util/cs-etm.c
@@ -1267,8 +1267,12 @@ static inline int cs_etm__t32_instr_size(struct cs_etm_queue *etmq,
 
 static inline u64 cs_etm__first_executed_instr(struct cs_etm_packet *packet)
 {
-	/* Returns 0 for the CS_ETM_DISCONTINUITY packet */
-	if (packet->sample_type == CS_ETM_DISCONTINUITY)
+	/*
+	 * Return 0 for packets that have no addresses so that CS_ETM_INVAL_ADDR doesn't
+	 * appear in samples.
+	 */
+	if (packet->sample_type == CS_ETM_DISCONTINUITY ||
+	    packet->sample_type == CS_ETM_EXCEPTION)
 		return 0;
 
 	return packet->start_addr;
-- 
2.34.1