[PATCH 2/2] qemu-options.hx: describe multiplexing of several backend devices

[Roman Penyaev <r.peniaev@gmail.com>]

This adds a few lines describing multiplexer configuration for
multiplexing several backend devices with a single frontend
device.

Signed-off-by: Roman Penyaev <r.peniaev@gmail.com>
Cc: "Marc-André Lureau" <marcandre.lureau@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: qemu-devel@nongnu.org
---
 qemu-options.hx | 44 ++++++++++++++++++++++++++++++++++++++++----
 1 file changed, 40 insertions(+), 4 deletions(-)

diff --git a/qemu-options.hx b/qemu-options.hx
index d94e2cbbaeb1..de1d540b85cd 100644
--- a/qemu-options.hx
+++ b/qemu-options.hx
@@ -3782,7 +3782,7 @@ SRST
 The general form of a character device option is:
 
 ``-chardev backend,id=id[,mux=on|off][,options]``
-    Backend is one of: ``null``, ``socket``, ``udp``, ``msmouse``,
+    Backend is one of: ``null``, ``socket``, ``udp``, ``msmouse``, ``mux``,
     ``vc``, ``ringbuf``, ``file``, ``pipe``, ``console``, ``serial``,
     ``pty``, ``stdio``, ``braille``, ``parallel``,
     ``spicevmc``, ``spiceport``. The specific backend will determine the
@@ -3839,9 +3839,9 @@ The general form of a character device option is:
     the QEMU monitor, and ``-nographic`` also multiplexes the console
     and the monitor to stdio.
 
-    There is currently no support for multiplexing in the other
-    direction (where a single QEMU front end takes input and output from
-    multiple chardevs).
+    If you need to multiplex in the opposite direction (where one QEMU
+    interface receives input and output from multiple chardev devices),
+    please refer to the paragraph below regarding chardev mux configuration.
 
     Every backend supports the ``logfile`` option, which supplies the
     path to a file to record all data transmitted via the backend. The
@@ -3941,6 +3941,42 @@ The available backends are:
     Forward QEMU's emulated msmouse events to the guest. ``msmouse``
     does not take any options.
 
+``-chardev mux,id=id,chardev=chardev-id[,,chardev-idN]``
+    Explicitly create chardev multiplexer with possibility to multiplex
+    in the opposite direction, where one QEMU interface (frontend device)
+    receives input and output from multiple chardev backend devices.
+
+    For example the following is a use case of 2 backend devices: text
+    virtual console ``vc0`` and a socket ``sock0`` connected
+    to a single virtio hvc console frontend device with multiplexer
+    ``mux0`` help. Virtual console renders text to an image, which
+    can be shared over the VNC protocol, in turn socket backend provides
+    biderectional communication to the virtio hvc console over socket.
+    The example configuration can be the following:
+
+    ::
+
+        -chardev socket,path=/tmp/sock,server=on,wait=off,id=sock0 \
+        -chardev vc,id=vc0 \
+        -chardev mux,id=mux0,chardev=vc0,,sock0 \
+        -device virtconsole,chardev=mux0 \
+        -vnc 0.0.0.0:0
+
+    Once QEMU starts VNC client and any TTY emulator can be used to
+    control a single hvc console:
+
+    ::
+
+       # VNC client
+       vncviewer :0
+
+       # TTY emulator
+       socat unix:connect:/tmp/sock pty,link=/tmp/pty & \
+       tio /tmp/pty
+
+    Multiplexing of several backend devices with serveral frontend devices
+    is not supported.
+
 ``-chardev vc,id=id[[,width=width][,height=height]][[,cols=cols][,rows=rows]]``
     Connect to a QEMU text console. ``vc`` may optionally be given a
     specific size.
-- 
2.34.1