Re: [PATCH 7/9] tools/virtio: use canonical ftrace path

On Mon, Jan 30, 2023 at 02:26:43PM -0500, Michael S. Tsirkin wrote:

On Mon, Jan 30, 2023 at 11:19:13AM -0700, Ross Zwisler wrote:

quoted

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 <redacted>
---
 tools/virtio/virtio-trace/README        | 2 +-
 tools/virtio/virtio-trace/trace-agent.c | 2 +-
 2 files changed, 2 insertions(+), 2 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..805942d02e9f 100644
--- a/tools/virtio/virtio-trace/trace-agent.c
+++ b/tools/virtio/virtio-trace/trace-agent.c

@@ -19,7 +19,7 @@
 #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"
+		"/sys/kernel/tracing/per_cpu/cpu%d/trace_pipe_raw"

Should we try both old and new path then?

Sure, I think it makes sense to have all the comments & documentation point
only to the preferred location, but I think for tools it makes sense to check
both places so that we don't see field regressions.  This is what the BPF code
does:

https://elixir.bootlin.com/linux/latest/source/tools/bpf/bpftool/tracelog.c#L98

I'll add a check and fallback to this tool & send out v2.  Thanks for the
review.

quoted

 #define WRITE_PATH_FMT		"/dev/virtio-ports/trace-path-cpu%d"
 #define CTL_PATH		"/dev/virtio-ports/agent-ctl-path"
 
-- 
2.39.1.456.gfc5497dd1b-goog