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()

1 files changed, 5 insertions, 0 deletions

diff --git a/scripts/tracetool/backend/syslog.py b/scripts/tracetool/backend/syslog.py
index 1ce627f0fc..668fb73fee 100644
--- a/scripts/tracetool/backend/syslog.py
+++ b/scripts/tracetool/backend/syslog.py
@@ -42,3 +42,8 @@ def generate_h(event, group):
         name=event.name,
         fmt=event.fmt.rstrip("\n"),
         argnames=argnames)
+
+
+def generate_h_backend_dstate(event, group):
+    out('    trace_event_get_state_dynamic_by_id(%(event_id)s) || \\',
+        event_id="TRACE_" + event.name.upper())