trace: recommend "log" backend for getting started with tracing

The "simple" backend is actually more complicated to use than the "log"
backend. Update the quickstart documentation to feature the "log"
backend instead of the "simple" backend.

Suggested-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20201216160923.722894-4-stefanha@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

diff --git a/docs/devel/tracing.rst b/docs/devel/tracing.rst
index f7e589f67c3b5d93c274c74f9dbe6f0bf54d17e9..4ebf8e38ea164319b72a594708c922eba2dd57b2 100644 (file)
--- a/docs/devel/tracing.rst
+++ b/docs/devel/tracing.rst
@@ -11,22 +11,22 @@ for debugging, profiling, and observing execution.
 Quickstart
 ==========
 
-1. Build with the 'simple' trace backend::
-
-    ./configure --enable-trace-backends=simple
-    make
-
-2. Create a file with the events you want to trace::
-
-    echo memory_region_ops_read >/tmp/events
+Enable tracing of ``memory_region_ops_read`` and ``memory_region_ops_write``
+events::
 
-3. Run the virtual machine to produce a trace file::
+    $ qemu --trace "memory_region_ops_*" ...
+    ...
+    719585@1608130130.441188:memory_region_ops_read cpu 0 mr 0x562fdfbb3820 addr 0x3cc value 0x67 size 1
+    719585@1608130130.441190:memory_region_ops_write cpu 0 mr 0x562fdfbd2f00 addr 0x3d4 value 0x70e size 2
 
-    qemu --trace events=/tmp/events ... # your normal QEMU invocation
+This output comes from the "log" trace backend that is enabled by default when
+``./configure --enable-trace-backends=BACKENDS`` was not explicitly specified.
 
-4. Pretty-print the binary trace file::
+More than one trace event pattern can be specified by providing a file
+instead::
 
-    ./scripts/simpletrace.py trace-events-all trace-* # Override * with QEMU <pid>
+    $ echo "memory_region_ops_*" >/tmp/events
+    $ qemu --trace events=/tmp/events ...
 
 Trace events
 ============
@@ -195,7 +195,7 @@ script.
 
 The trace backends are chosen at configure time::
 
-    ./configure --enable-trace-backends=simple
+    ./configure --enable-trace-backends=simple,dtrace
 
 For a list of supported trace backends, try ./configure --help or see below.
 If multiple backends are enabled, the trace is sent to them all.
@@ -227,10 +227,11 @@ uses DPRINTF().
 Simpletrace
 -----------
 
-The "simple" backend supports common use cases and comes as part of the QEMU
-source tree.  It may not be as powerful as platform-specific or third-party
-trace backends but it is portable.  This is the recommended trace backend
-unless you have specific needs for more advanced backends.
+The "simple" backend writes binary trace logs to a file from a thread, making
+it lower overhead than the "log" backend. A Python API is available for writing
+offline trace file analysis scripts. It may not be as powerful as
+platform-specific or third-party trace backends but it is portable and has no
+special library dependencies.
 
 Monitor commands
 ~~~~~~~~~~~~~~~~