perf intel-pt: Do not try to queue auxtrace data on pipe

When it processes AUXTRACE_INFO, it calls to auxtrace_queue_data() to
collect AUXTRACE data first.  That won't work with pipe since it needs
lseek() to read the scattered aux data.

  $ perf record -o- -e intel_pt// true | perf report -i- --itrace=i100
  # To display the perf.data header info, please use --header/--header-only options.
  #
  0x4118 [0xa0]: failed to process type: 70
  Error:
  failed to process sample

For the pipe mode, it can handle the aux data as it gets.  But there's
no guarantee it can get the aux data in time.  So the following warning
will be shown at the beginning:

  WARNING: Intel PT with pipe mode is not recommended.
           The output cannot relied upon.  In particular,
           time stamps and the order of events may be incorrect.

Fixes: dbd13432 ("perf intel-pt: Add support for decoding AUX area samples")
Reviewed-by: Adrian Hunter <adrian.hunter@intel.com>
Reviewed-by: James Clark <james.clark@arm.com>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Leo Yan <leo.yan@linaro.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: https://lore.kernel.org/r/20230131023350.1903992-3-namhyung@kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Documentation/perf-intel-pt.txt

@@ -1821,6 +1821,36 @@ Can be compiled and traced:
  $
 
 
+Pipe mode
+---------
+Pipe mode is a problem for Intel PT and possibly other auxtrace users.
+It's not recommended to use a pipe as data output with Intel PT because
+of the following reason.
+
+Essentially the auxtrace buffers do not behave like the regular perf
+event buffers.  That is because the head and tail are updated by
+software, but in the auxtrace case the data is written by hardware.
+So the head and tail do not get updated as data is written.
+
+In the Intel PT case, the head and tail are updated only when the trace
+is disabled by software, for example:
+    - full-trace, system wide : when buffer passes watermark
+    - full-trace, not system-wide : when buffer passes watermark or
+                                    context switches
+    - snapshot mode : as above but also when a snapshot is made
+    - sample mode : as above but also when a sample is made
+
+That means finished-round ordering doesn't work.  An auxtrace buffer
+can turn up that has data that extends back in time, possibly to the
+very beginning of tracing.
+
+For a perf.data file, that problem is solved by going through the trace
+and queuing up the auxtrace buffers in advance.
+
+For pipe mode, the order of events and timestamps can presumably
+be messed up.
+
+
 EXAMPLE
 -------
 

tools/perf/util/auxtrace.c

@@ -1133,6 +1133,9 @@ int auxtrace_queue_data(struct perf_session *session, bool samples, bool events)
 	if (auxtrace__dont_decode(session))
 		return 0;
 
+	if (perf_data__is_pipe(session->data))
+		return 0;
+
 	if (!session->auxtrace || !session->auxtrace->queue_data)
 		return -EINVAL;
 

tools/perf/util/intel-pt.c

@@ -4379,6 +4379,12 @@ int intel_pt_process_auxtrace_info(union perf_event *event,
 
 	intel_pt_setup_pebs_events(pt);
 
+	if (perf_data__is_pipe(session->data)) {
+		pr_warning("WARNING: Intel PT with pipe mode is not recommended.\n"
+			   "         The output cannot relied upon.  In particular,\n"
+			   "         timestamps and the order of events may be incorrect.\n");
+	}
+
 	if (pt->sampling_mode || list_empty(&session->auxtrace_index))
 		err = auxtrace_queue_data(session, true, true);
 	else