linux-kernel - Re: [PATCH v1 3/7] perf cs-etm: Calculate per CPU metadata array size

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Message-ID: <20210111150608.GC222747@leoy-ThinkPad-X240s>
Date:   Mon, 11 Jan 2021 23:06:08 +0800
From:   Leo Yan <leo.yan@...aro.org>
To:     Mike Leach <mike.leach@...aro.org>
Cc:     Suzuki K Poulose <suzuki.poulose@....com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mathieu Poirier <mathieu.poirier@...aro.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        John Garry <john.garry@...wei.com>,
        Will Deacon <will@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Mark Rutland <mark.rutland@....com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Daniel Kiss <Daniel.Kiss@....com>,
        Denis Nikitin <denik@...omium.org>,
        Coresight ML <coresight@...ts.linaro.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v1 3/7] perf cs-etm: Calculate per CPU metadata array size

Hi Mike,

On Mon, Jan 11, 2021 at 12:09:12PM +0000, Mike Leach wrote:
> Hi Leo,
> 
> I think there is an issue here in that your modification assumes that
> all cpus in the system are of the same ETM type. The original routine
> allowed for differing ETM types, thus differing cpu ETM field lengths
> between ETMv4 / ETMv3, the field size was used after the relevant
> magic number for the cpu ETM was read.
> 
> You have replaced two different sizes - with a single calculated size.

Thanks for pointing out this.

> Moving forwards we are seeing the newer FEAT_ETE protocol drivers
> appearing on the list, which will ultimately need a new metadata
> structure.
> 
> We have had discussions within ARM regarding the changing of the
> format to be more self describing - which should probably be opened
> out to the CS mailing list.

I think here have two options.  One option is I think we can use
__perf_cs_etmv3_magic/__perf_cs_etmv4_magic as indicator for the
starting of next metadata array; when copy the metadata, always check
the next item in the buffer, if it's __perf_cs_etmv3_magic or
__perf_cs_etmv4_magic, will break loop and start copying metadata
array for next CPU.  The suggested change is pasted in below.

Another option is I drop patches 03,05/07 in the series and leave the
backward compatibility fixing for a saperate patch series with self
describing method.  Especially, if you think the first option will
introduce trouble for enabling self describing later, then I am happy
to drop patches 03,05.

How about you think for this?

Thanks,
Leo

---8<---

diff --git a/tools/perf/util/cs-etm.c b/tools/perf/util/cs-etm.c
index a2a369e2fbb6..edaec57362f0 100644
--- a/tools/perf/util/cs-etm.c
+++ b/tools/perf/util/cs-etm.c
@@ -2558,12 +2558,19 @@ int cs_etm__process_auxtrace_info(union perf_event *event,
 				err = -ENOMEM;
 				goto err_free_metadata;
 			}
-			for (k = 0; k < CS_ETM_PRIV_MAX; k++)
+			for (k = 0; k < CS_ETM_PRIV_MAX; k++) {
 				metadata[j][k] = ptr[i + k];
 
+				if (ptr[i + k + 1] == __perf_cs_etmv3_magic ||
+				    ptr[i + k + 1] == __perf_cs_etmv4_magic) {
+					k++;
+					break;
+				}
+			}
+
 			/* The traceID is our handle */
 			idx = metadata[j][CS_ETM_ETMTRACEIDR];
-			i += CS_ETM_PRIV_MAX;
+			i += k;
 		} else if (ptr[i] == __perf_cs_etmv4_magic) {
 			metadata[j] = zalloc(sizeof(*metadata[j]) *
 					     CS_ETMV4_PRIV_MAX);
@@ -2571,12 +2578,19 @@ int cs_etm__process_auxtrace_info(union perf_event *event,
 				err = -ENOMEM;
 				goto err_free_metadata;
 			}
-			for (k = 0; k < CS_ETMV4_PRIV_MAX; k++)
+			for (k = 0; k < CS_ETMV4_PRIV_MAX; k++) {
 				metadata[j][k] = ptr[i + k];
 
+				if (ptr[i + k + 1] == __perf_cs_etmv3_magic ||
+				    ptr[i + k + 1] == __perf_cs_etmv4_magic) {
+					k++;
+					break;
+				}
+			}
+
 			/* The traceID is our handle */
 			idx = metadata[j][CS_ETMV4_TRCTRACEIDR];
-			i += CS_ETMV4_PRIV_MAX;
+			i += k;
 		}
 
 		/* Get an RB node for this CPU */