trace: emit name <-> ID mapping in simpletrace header

Currently simpletrace assumes that events are given IDs
starting from 0, based on the order in which they appear
in the trace-events file, with no gaps. When the
trace-events file is split up, this assumption becomes
problematic.

To deal with this, extend the simpletrace format so that
it outputs a table of event name <-> ID mappings. That
will allow QEMU to assign arbitrary IDs to events without
breaking simpletrace parsing.

The v3 simple trace format was

  FILE HEADER
  EVENT TRACE RECORD 0
  EVENT TRACE RECORD 1
  ...
  EVENT TRACE RECORD N

The v4 simple trace format is now

  FILE HEADER
  EVENT MAPPING RECORD 0
  EVENT MAPPING RECORD 1
  ...
  EVENT MAPPING RECORD M
  EVENT TRACE RECORD RECORD 0
  EVENT TRACE RECORD RECORD 1
  ...
  EVENT TRACE RECORD N

Although this shows all the mapping records being emitted
upfront, this is not required by the format. While the main
simpletrace backend will emit all mappings at startup,
the systemtap simpletrace.stp script will emit the mappings
at first use. eg

  FILE HEADER
  ...
  EVENT MAPPING RECORD 0
  EVENT TRACE RECORD RECORD 0
  EVENT TRACE RECORD RECORD 1
  EVENT MAPPING RECORD 1
  EVENT TRACE RECORD RECORD 2
  ...
  EVENT TRACE RECORD N

This is more space efficient given that most trace records
only include a subset of events.

In modifying the systemtap simpletrace code, a 'begin' probe
was added to emit the trace event header, so you no longer
need to add '--no-header' when running simpletrace.py for
systemtap generated trace files.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Message-id: 1475588159-30598-12-git-send-email-berrange@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

1 files changed, 22 insertions, 2 deletions

diff --git a/scripts/tracetool/format/simpletrace_stap.py b/scripts/tracetool/format/simpletrace_stap.py
index 7e44bc1811..ac3580fbcc 100644
--- a/scripts/tracetool/format/simpletrace_stap.py
+++ b/scripts/tracetool/format/simpletrace_stap.py
@@ -21,6 +21,25 @@ from tracetool.format.stap import stap_escape
 
 def generate(events, backend):
     out('/* This file is autogenerated by tracetool, do not edit. */',
+        '',
+        'global event_name_to_id_map',
+        'global event_next_id',
+        'function simple_trace_map_event(name)',
+        '',
+        '{',
+        '    if (!([name] in event_name_to_id_map)) {',
+        '        event_name_to_id_map[name] = event_next_id',
+        '        name_len = strlen(name)',
+        '        printf("%%8b%%8b%%4b%%.*s", 0, ',
+        '               event_next_id, name_len, name_len, name)',
+        '        event_next_id = event_next_id + 1',
+        '    }',
+        '    return event_name_to_id_map[name]',
+        '}',
+        'probe begin',
+        '{',
+        '    printf("%%8b%%8b%%8b", 0xffffffffffffffff, 0xf2b177cb0aa429b4, 4)',
+        '}',
         '')
 
     for event_id, e in enumerate(events):
@@ -29,6 +48,7 @@ def generate(events, backend):
 
         out('probe %(probeprefix)s.simpletrace.%(name)s = %(probeprefix)s.%(name)s ?',
             '{',
+            '    id = simple_trace_map_event("%(name)s")',
             probeprefix=probeprefix(),
             name=e.name)
 
@@ -48,7 +68,7 @@ def generate(events, backend):
         sizestr = ' + '.join(sizes)
 
         # Generate format string and value pairs for record header and arguments
-        fields = [('8b', str(event_id)),
+        fields = [('8b', 'id'),
                   ('8b', 'gettimeofday_ns()'),
                   ('4b', sizestr),
                   ('4b', 'pid()')]
@@ -63,7 +83,7 @@ def generate(events, backend):
         # Emit the entire record in a single SystemTap printf()
         fmt_str = '%'.join(fmt for fmt, _ in fields)
         arg_str = ', '.join(arg for _, arg in fields)
-        out('    printf("%%%(fmt_str)s", %(arg_str)s)',
+        out('    printf("%%8b%%%(fmt_str)s", 1, %(arg_str)s)',
             fmt_str=fmt_str, arg_str=arg_str)
 
         out('}')