Documentation: tracing: Add a note about argument and retval access

Add a note about the argument and return value accecss will be best
effort. Depending on the type, it will be passed via stack or a
pair of the registers, but $argN and $retval only support the
single register access.

Link: https://lore.kernel.org/all/169556269377.146934.14829235476649685954.stgit@devnote2/

Suggested-by: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>

1 files changed, 6 insertions, 2 deletions

diff --git a/Documentation/trace/fprobetrace.rst b/Documentation/trace/fprobetrace.rst
index 8e9bebcf0a2e..e35e6b18df40 100644
--- a/Documentation/trace/fprobetrace.rst
+++ b/Documentation/trace/fprobetrace.rst
@@ -59,8 +59,12 @@ Synopsis of fprobe-events
                   and bitfield are supported.
 
   (\*1) This is available only when BTF is enabled.
-  (\*2) only for the probe on function entry (offs == 0).
-  (\*3) only for return probe.
+  (\*2) only for the probe on function entry (offs == 0). Note, this argument access
+        is best effort, because depending on the argument type, it may be passed on
+        the stack. But this only support the arguments via registers.
+  (\*3) only for return probe. Note that this is also best effort. Depending on the
+        return value type, it might be passed via a pair of registers. But this only
+        accesses one register.
   (\*4) this is useful for fetching a field of data structures.
   (\*5) "u" means user-space dereference.