trace: Add trace-events file for declaring trace events

This patch introduces the trace-events file where trace events can be
declared like so:

qemu_malloc(size_t size) "size %zu"
qemu_free(void *ptr) "ptr %p"

These trace event declarations are processed by a new tool called
tracetool to generate code for the trace events.  Trace event
declarations are independent of the backend tracing system (LTTng User
Space Tracing, ftrace markers, DTrace).

The default "nop" backend generates empty trace event functions.
Therefore trace events are disabled by default.

The trace-events file serves two purposes:

1. Adding trace events is easy.  It is not necessary to understand the
   details of a backend tracing system.  The trace-events file is a
   single location where trace events can be declared without code
   duplication.

2. QEMU is not tightly coupled to one particular backend tracing system.
   In order to support tracing across QEMU host platforms and to
   anticipate new backend tracing systems that are currently maturing,
   it is important to be flexible and not tied to one system.

This commit includes fixes from Prerna Saxena
<prerna@linux.vnet.ibm.com> and Blue Swirl <blauwirbel@gmail.com>.

Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>

1 files changed, 24 insertions, 0 deletions

diff --git a/trace-events b/trace-events
new file mode 100644
index 0000000000..a37d3cc6e3
--- /dev/null
+++ b/trace-events
@@ -0,0 +1,24 @@
+# Trace events for debugging and performance instrumentation
+#
+# This file is processed by the tracetool script during the build.
+#
+# To add a new trace event:
+#
+# 1. Choose a name for the trace event.  Declare its arguments and format
+#    string.
+#
+# 2. Call the trace event from code using trace_##name, e.g. multiwrite_cb() ->
+#    trace_multiwrite_cb().  The source file must #include "trace.h".
+#
+# Format of a trace event:
+#
+# <name>(<type1> <arg1>[, <type2> <arg2>] ...) "<format-string>"
+#
+# Example: qemu_malloc(size_t size) "size %zu"
+#
+# The <name> must be a valid as a C function name.
+#
+# Types should be standard C types.  Use void * for pointers because the trace
+# system may not have the necessary headers included.
+#
+# The <format-string> should be a sprintf()-compatible format string.