tracing/user_events: Document multi-format flag

User programs can now ask user_events to handle the synchronization of
multiple different formats for an event with the same name via the new
USER_EVENT_REG_MULTI_FORMAT flag.

Add a section for USER_EVENT_REG_MULTI_FORMAT that explains the intended
purpose and caveats of using it. Explain how deletion works in these
cases and how to use /sys/kernel/tracing/dynamic_events for per-version
deletion.

Link: https://lore.kernel.org/linux-trace-kernel/20240222001807.1463-5-beaub@linux.microsoft.com

Signed-off-by: Beau Belgrave <beaub@linux.microsoft.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

Documentation/trace/user_events.rst

@@ -92,6 +92,24 @@ The following flags are currently supported.
   process closes or unregisters the event. Requires CAP_PERFMON otherwise
   -EPERM is returned.
 
++ USER_EVENT_REG_MULTI_FORMAT: The event can contain multiple formats. This
+  allows programs to prevent themselves from being blocked when their event
+  format changes and they wish to use the same name. When this flag is used the
+  tracepoint name will be in the new format of "name.unique_id" vs the older
+  format of "name". A tracepoint will be created for each unique pair of name
+  and format. This means if several processes use the same name and format,
+  they will use the same tracepoint. If yet another process uses the same name,
+  but a different format than the other processes, it will use a different
+  tracepoint with a new unique id. Recording programs need to scan tracefs for
+  the various different formats of the event name they are interested in
+  recording. The system name of the tracepoint will also use "user_events_multi"
+  instead of "user_events". This prevents single-format event names conflicting
+  with any multi-format event names within tracefs. The unique_id is output as
+  a hex string. Recording programs should ensure the tracepoint name starts with
+  the event name they registered and has a suffix that starts with . and only
+  has hex characters. For example to find all versions of the event "test" you
+  can use the regex "^test\.[0-9a-fA-F]+$".
+
 Upon successful registration the following is set.
 
 + write_index: The index to use for this file descriptor that represents this
@@ -106,6 +124,9 @@ or perf record -e user_events:[name] when attaching/recording.
 **NOTE:** The event subsystem name by default is "user_events". Callers should
 not assume it will always be "user_events". Operators reserve the right in the
 future to change the subsystem name per-process to accommodate event isolation.
+In addition if the USER_EVENT_REG_MULTI_FORMAT flag is used the tracepoint name
+will have a unique id appended to it and the system name will be
+"user_events_multi" as described above.
 
 Command Format
 ^^^^^^^^^^^^^^
@@ -156,7 +177,11 @@ to request deletes than the one used for registration due to this.
 to the event. If programs do not want auto-delete, they must use the
 USER_EVENT_REG_PERSIST flag when registering the event. Once that flag is used
 the event exists until DIAG_IOCSDEL is invoked. Both register and delete of an
-event that persists requires CAP_PERFMON, otherwise -EPERM is returned.
+event that persists requires CAP_PERFMON, otherwise -EPERM is returned. When
+there are multiple formats of the same event name, all events with the same
+name will be attempted to be deleted. If only a specific version is wanted to
+be deleted then the /sys/kernel/tracing/dynamic_events file should be used for
+that specific format of the event.
 
 Unregistering
 -------------