trace: add TRACE_<event>_BACKEND_DSTATE()

QEMU keeps track of trace event enabled/disabled state and provides
monitor commands to inspect and modify the "dstate".  SystemTap and
LTTng UST maintain independent enabled/disabled states for each trace
event, the other backends rely on QEMU dstate.

Introduce a new per-event macro that combines backend-specific dstate
like this:

  #define TRACE_MY_EVENT_BACKEND_DSTATE() ( \
      QEMU_MY_EVENT_ENABLED() || /* SystemTap */ \
      tracepoint_enabled(qemu, my_event) /* LTTng UST */ || \
      false)

This will be used to extend trace_event_get_state() in the next patch.

[Daniel Berrange pointed out that QEMU_MY_EVENT_ENABLED() must be true
by default, not false.  This way events will fire even if the DTrace
implementation does not implement the SystemTap semaphores feature.

Ubuntu Precise uses lttng-ust-dev 2.0.2 which does not have
tracepoint_enabled(), so we need a compatibility wrapper to keep Travis
builds passing.
--Stefan]

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 20170731140718.22010-2-stefanha@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

fixup! trace: add TRACE_<event>_BACKEND_DSTATE()

scripts/tracetool/backend/ust.py

@@ -27,6 +27,11 @@ def generate_h_begin(events, group):
 
     out('#include <lttng/tracepoint.h>',
         '#include "%s"' % header,
+        '',
+        '/* tracepoint_enabled() was introduced in LTTng UST 2.7 */',
+        '#ifndef tracepoint_enabled',
+        '#define tracepoint_enabled(a, b) true',
+        '#endif',
         '')
 
 
@@ -38,3 +43,8 @@ def generate_h(event, group):
     out('    tracepoint(qemu, %(name)s%(tp_args)s);',
         name=event.name,
         tp_args=argnames)
+
+
+def generate_h_backend_dstate(event, group):
+    out('    tracepoint_enabled(qemu, %(name)s) || \\',
+        name=event.name)