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

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.
-
-In order to use gdb, launch QEMU with the '-s' option. It will wait for
-a gdb connection:
+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`` 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::
 

qemu-options.hx

@@ -3680,14 +3680,26 @@ SRST
 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::