docs: Improve our gdbstub documentation
The documentation of our -s and -gdb options is quite old; in
particular it still claims that it will cause QEMU to stop and wait
for the gdb connection, when this has not been true for some time:
you also need to pass -S if you want to make QEMU not launch the
guest on startup.
Improve the documentation to mention this requirement in the
executable's --help output, the documentation of the -gdb option in
the manual, and in the "GDB usage" chapter.
Includes some minor tweaks to these paragraphs of documentation
since I was editing them anyway (such as dropping the description
of our gdb support as "primitive").
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 20200403094014.9589-1-peter.maydell@linaro.org
diff --git a/docs/system/gdb.rst b/docs/system/gdb.rst
index 639f814..a40145f 100644
--- a/docs/system/gdb.rst
+++ b/docs/system/gdb.rst
@@ -3,17 +3,25 @@
GDB usage
---------
-QEMU has a primitive support to work with gdb, so that you can do
-'Ctrl-C' while the virtual machine is running and inspect its state.
+QEMU supports working with gdb via gdb's remote-connection facility
+(the "gdbstub"). This allows you to debug guest code in the same
+way that you might with a low-level debug facility like JTAG
+on real hardware. You can stop and start the virtual machine,
+examine state like registers and memory, and set breakpoints and
+watchpoints.
-In order to use gdb, launch QEMU with the '-s' option. It will wait for
-a gdb connection:
+In order to use gdb, launch QEMU with the ``-s`` and ``-S`` options.
+The ``-s`` option will make QEMU listen for an incoming connection
+from gdb on TCP port 1234, and ``-S`` will make QEMU not start the
+guest until you tell it to from gdb. (If you want to specify which
+TCP port to use or to use something other than TCP for the gdbstub
+connection, use the ``-gdb dev`` option instead of ``-s``.)
.. parsed-literal::
- |qemu_system| -s -kernel bzImage -hda rootdisk.img -append "root=/dev/hda"
- Connected to host network interface: tun0
- Waiting gdb connection on port 1234
+ |qemu_system| -s -S -kernel bzImage -hda rootdisk.img -append "root=/dev/hda"
+
+QEMU will launch but will silently wait for gdb to connect.
Then launch gdb on the 'vmlinux' executable::
diff --git a/qemu-options.hx b/qemu-options.hx
index 16debd0..292d4e7 100644
--- a/qemu-options.hx
+++ b/qemu-options.hx
@@ -3680,14 +3680,26 @@
ERST
DEF("gdb", HAS_ARG, QEMU_OPTION_gdb, \
- "-gdb dev wait for gdb connection on 'dev'\n", QEMU_ARCH_ALL)
+ "-gdb dev accept gdb connection on 'dev'. (QEMU defaults to starting\n"
+ " the guest without waiting for gdb to connect; use -S too\n"
+ " if you want it to not start execution.)\n",
+ QEMU_ARCH_ALL)
SRST
``-gdb dev``
- Wait for gdb connection on device dev (see
- :ref:`gdb_005fusage`). Typical connections will likely be
- TCP-based, but also UDP, pseudo TTY, or even stdio are reasonable
- use case. The latter is allowing to start QEMU from within gdb and
- establish the connection via a pipe:
+ Accept a gdb connection on device dev (see
+ :ref:`gdb_005fusage`). Note that this option does not pause QEMU
+ execution -- if you want QEMU to not start the guest until you
+ connect with gdb and issue a ``continue`` command, you will need to
+ also pass the ``-S`` option to QEMU.
+
+ The most usual configuration is to listen on a local TCP socket::
+
+ -gdb tcp::3117
+
+ but you can specify other backends; UDP, pseudo TTY, or even stdio
+ are all reasonable use cases. For example, a stdio connection
+ allows you to start QEMU from within gdb and establish the
+ connection via a pipe:
.. parsed-literal::
