Re: [Qemu-devel] [PATCH for-2.8 v1 00/60] Modular build of trace event files

[Lluís Vilanova]

Daniel P Berrange writes:

> I previously split the global trace-events file up into one file
> per-subdirectory to avoid merge conflict hell.
[...]

Sorry, I could not find the message where the infrastructure is modified to
provide this. But I think there's a more efficient way to provide modular
auto-generated tracing code without the hierarchical indexing you proposed.

What about using global variables? Instead of the dstate array, each event could
have this on the "public" header:

  /* define for static state */
  #define TRACE_EVENTNAME_ENABLED 1
  /* pointer to event descriptor */
  extern TraceEvent *TRACE_EVENTNAME;
  /* variable with dynamic state */
  extern bool ___TRACE_EVENTNAME_DSTATE;

  void trace_eventname(...) {
      if (trace_event_get_stateTRACE_EVENTNAME_ENABLED && 
___trace_eventname_dstate) {
          /* ... */
      }
  }

The use of event IDs on generic code can be adapted like this:

  #define trace_event_get_state_dynamic_by_id(id) \
      (unlikely(trace_events_enabled_count) && \
       (___ ## id ## _DSTATE))

And then we can concatenate all "trace-events" files to generate the .c files:

  struct TraceEvent {
      /* ... */
      bool *dstate;
  };

  bool ___TRACE_EVENTNAME_DSTATE;

  struct TraceEvent ___trace_events[] = {
      {
          .name = "eventname",
          .sstate = 1,
          .dstate = ___trace_eventname_dstate;
      }
  }

  TraceEvent *TRACE_EVENTNAME = &___trace_events[...];

So updating a single "trace-events" file does not force a recompile of the whole
QEMU, but we retain the performance of the flat dstate array (now a per-event
pointer) and the simpler flat structure for iteration based on event names.


Cheers,
  Lluis