linux-kernel - [PATCH 2/3] perf intel-pt: Fix sample instruction bytes

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Date:   Wed, 19 May 2021 10:45:14 +0300
From:   Adrian Hunter <adrian.hunter@...el.com>
To:     Arnaldo Carvalho de Melo <acme@...nel.org>,
        Jiri Olsa <jolsa@...hat.com>, Andi Kleen <ak@...ux.intel.com>
Cc:     linux-kernel@...r.kernel.org
Subject: [PATCH 2/3] perf intel-pt: Fix sample instruction bytes

The decoder reports the current instruction if it was decoded. In some
cases the current instruction is not decoded, in which case the instruction
bytes length must be set to zero. Ensure that is always done.

Note perf script can anyway get the instruction bytes for any samples where
they are not present.

Also note, that there is a redundant "ptq->insn_len = 0" statement which is
not removed until a subsequent patch in order to make this patch apply
cleanly to stable branches.

Example:

A machne that supports TSX is required. It will have flag "rtm". Kernel
parameter tsx=on may be required.

 # for w in `cat /proc/cpuinfo | grep -m1 flags `;do echo $w | grep rtm ; done
 rtm

Test program:

 #include <stdio.h>
 #include <immintrin.h>

 int main()
 {
        int x = 0;

        if (_xbegin() == _XBEGIN_STARTED) {
                x = 1;
                _xabort(1);
        } else {
                printf("x = %d\n", x);
        }
        return 0;
 }

Compile with -mrtm i.e.

 gcc -Wall -Wextra -mrtm xabort.c -o xabort

Record:

 perf record -e intel_pt/cyc/u --filter 'filter main @ ./xabort' ./xabort

Before:

 # perf script --itrace=xe -F+flags,+insn,-period --xed --ns
          xabort  1478 [007] 92161.431348581:   transactions:   x                              400b81 main+0x14 (/root/xabort)          mov $0xffffffff, %eax
          xabort  1478 [007] 92161.431348624:   transactions:   tx abrt                        400b93 main+0x26 (/root/xabort)          mov $0xffffffff, %eax

After:

 # perf script --itrace=xe -F+flags,+insn,-period --xed --ns
          xabort  1478 [007] 92161.431348581:   transactions:   x                              400b81 main+0x14 (/root/xabort)          xbegin 0x6
          xabort  1478 [007] 92161.431348624:   transactions:   tx abrt                        400b93 main+0x26 (/root/xabort)          xabort $0x1

Fixes: faaa87680b25d ("perf intel-pt/bts: Report instruction bytes and length in sample")
Signed-off-by: Adrian Hunter <adrian.hunter@...el.com>
Cc: stable@...r.kernel.org
---
 tools/perf/util/intel-pt.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/tools/perf/util/intel-pt.c b/tools/perf/util/intel-pt.c
index 2a5fe1514e65..4428dba24aa7 100644
--- a/tools/perf/util/intel-pt.c
+++ b/tools/perf/util/intel-pt.c
@@ -778,8 +778,10 @@ static int intel_pt_walk_next_insn(struct intel_pt_insn *intel_pt_insn,
 
 			*ip += intel_pt_insn->length;
 
-			if (to_ip && *ip == to_ip)
+			if (to_ip && *ip == to_ip) {
+				intel_pt_insn->length = 0;
 				goto out_no_cache;
+			}
 
 			if (*ip >= al.map->end)
 				break;
@@ -1301,6 +1303,7 @@ static void intel_pt_set_pid_tid_cpu(struct intel_pt *pt,
 
 static void intel_pt_sample_flags(struct intel_pt_queue *ptq)
 {
+	ptq->insn_len = 0;
 	if (ptq->state->flags & INTEL_PT_ABORT_TX) {
 		ptq->flags = PERF_IP_FLAG_BRANCH | PERF_IP_FLAG_TX_ABORT;
 	} else if (ptq->state->flags & INTEL_PT_ASYNC) {
-- 
2.17.1