[PATCH v2 5/6] tools/virtio: use canonical ftrace path

[Ross Zwisler <zwisler@chromium.org>]

The canonical location for the tracefs filesystem is at /sys/kernel/tracing.

But, from Documentation/trace/ftrace.rst:

  Before 4.1, all ftrace tracing control files were within the debugfs
  file system, which is typically located at /sys/kernel/debug/tracing.
  For backward compatibility, when mounting the debugfs file system,
  the tracefs file system will be automatically mounted at:

  /sys/kernel/debug/tracing

A few spots in tools/virtio still refer to this older debugfs
path, so let's update them to avoid confusion.

Signed-off-by: Ross Zwisler <zwisler@google.com>
---
 tools/virtio/virtio-trace/README        |  2 +-
 tools/virtio/virtio-trace/trace-agent.c | 12 ++++++++----
 2 files changed, 9 insertions(+), 5 deletions(-)

diff --git a/tools/virtio/virtio-trace/README b/tools/virtio/virtio-trace/README
index b64845b823ab..cea29a2a4c0a 100644
--- a/tools/virtio/virtio-trace/README
+++ b/tools/virtio/virtio-trace/README
@@ -95,7 +95,7 @@ Run
 
 1) Enable ftrace in the guest
  <Example>
-	# echo 1 > /sys/kernel/debug/tracing/events/sched/enable
+	# echo 1 > /sys/kernel/tracing/events/sched/enable
 
 2) Run trace agent in the guest
  This agent must be operated as root.
diff --git a/tools/virtio/virtio-trace/trace-agent.c b/tools/virtio/virtio-trace/trace-agent.c
index cdfe77c2b4c8..7e2d9bbf0b84 100644
--- a/tools/virtio/virtio-trace/trace-agent.c
+++ b/tools/virtio/virtio-trace/trace-agent.c
@@ -18,8 +18,9 @@
 #define PIPE_DEF_BUFS		16
 #define PIPE_MIN_SIZE		(PAGE_SIZE*PIPE_DEF_BUFS)
 #define PIPE_MAX_SIZE		(1024*1024)
-#define READ_PATH_FMT	\
-		"/sys/kernel/debug/tracing/per_cpu/cpu%d/trace_pipe_raw"
+#define TRACEFS 		"/sys/kernel/tracing"
+#define DEBUGFS 		"/sys/kernel/debug/tracing"
+#define READ_PATH_FMT		"%s/per_cpu/cpu%d/trace_pipe_raw"
 #define WRITE_PATH_FMT		"/dev/virtio-ports/trace-path-cpu%d"
 #define CTL_PATH		"/dev/virtio-ports/agent-ctl-path"
 
@@ -120,9 +121,12 @@ static const char *make_path(int cpu_num, bool this_is_write_path)
 	if (this_is_write_path)
 		/* write(output) path */
 		ret = snprintf(buf, PATH_MAX, WRITE_PATH_FMT, cpu_num);
-	else
+	else {
 		/* read(input) path */
-		ret = snprintf(buf, PATH_MAX, READ_PATH_FMT, cpu_num);
+		ret = snprintf(buf, PATH_MAX, READ_PATH_FMT, TRACEFS, cpu_num);
+		if (ret > 0 && access(buf, F_OK) != 0)
+			ret = snprintf(buf, PATH_MAX, READ_PATH_FMT, DEBUGFS, cpu_num);
+	}
 
 	if (ret <= 0) {
 		pr_err("Failed to generate %s path(CPU#%d):%d\n",
-- 
2.39.1.637.g21b0678d19-goog