diff --git a/docs/devel/testing.rst b/docs/devel/testing.rst
index 8359f2ae37..01e1919873 100644
--- a/docs/devel/testing.rst
+++ b/docs/devel/testing.rst
@@ -229,6 +229,17 @@ Debugging a test case
 The following options to the ``check`` script can be useful when debugging
 a failing test:
 
+* ``-gdb`` wraps every QEMU invocation in a ``gdbserver``, which waits for a
+  connection from a gdb client.  The options given to ``gdbserver`` (e.g. the
+  address on which to listen for connections) are taken from the ``$GDB_OPTIONS``
+  environment variable.  By default (if ``$GDB_OPTIONS`` is empty), it listens on
+  ``localhost:12345``.
+  It is possible to connect to it for example with
+  ``gdb -iex "target remote $addr"``, where ``$addr`` is the address
+  ``gdbserver`` listens on.
+  If the ``-gdb`` option is not used, ``$GDB_OPTIONS`` is ignored,
+  regardless of whether it is set or not.
+
 * ``-d`` (debug) just increases the logging verbosity, showing
   for example the QMP commands and answers.