qemu-options.hx: describe hub chardev and aggregation of several backends

This adds a few lines describing `hub` aggregator configuration
for aggregation of several backend devices with a single frontend
device.

Signed-off-by: Roman Penyaev <r.peniaev@gmail.com>
Reviewed-by: "Marc-André Lureau" <marcandre.lureau@redhat.com>
Cc: qemu-devel@nongnu.org
Message-ID: <20250123085327.965501-5-r.peniaev@gmail.com>

qemu-options.hx

@@ -3733,7 +3733,7 @@ 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``, ``hub``,
     ``vc``, ``ringbuf``, ``file``, ``pipe``, ``console``, ``serial``,
     ``pty``, ``stdio``, ``braille``, ``parallel``,
     ``spicevmc``, ``spiceport``. The specific backend will determine the
@@ -3790,9 +3790,10 @@ 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 aggregate data in the opposite direction (where one
+    QEMU frontend interface receives input and output from multiple
+    backend chardev devices), please refer to the paragraph below
+    regarding chardev ``hub`` aggregator device configuration.
 
     Every backend supports the ``logfile`` option, which supplies the
     path to a file to record all data transmitted via the backend. The
@@ -3892,6 +3893,46 @@ The available backends are:
     Forward QEMU's emulated msmouse events to the guest. ``msmouse``
     does not take any options.
 
+``-chardev hub,id=id,chardevs.0=id[,chardevs.N=id]``
+    Explicitly create chardev backend hub device with the possibility
+    to aggregate input from multiple backend devices and forward it to
+    a single frontend device. Additionally, ``hub`` device takes the
+    output from the frontend device and sends it back to all the
+    connected backend devices. This allows for seamless interaction
+    between different backend devices and a single frontend
+    interface. Aggregation supported for up to 4 chardev
+    devices. (Since 10.0)
+
+    For example, the following is a use case of 2 backend devices:
+    virtual console ``vc0`` and a pseudo TTY ``pty0`` connected to
+    a single virtio hvc console frontend device with a hub ``hub0``
+    help. Virtual console renders text to an image, which can be
+    shared over the VNC protocol. In turn, pty backend provides
+    bidirectional communication to the virtio hvc console over the
+    pseudo TTY file. The example configuration can be as follows:
+
+    ::
+
+       -chardev pty,path=/tmp/pty,id=pty0 \
+       -chardev vc,id=vc0 \
+       -chardev hub,id=hub0,chardevs.0=pty0,chardevs.1=vc0 \
+       -device virtconsole,chardev=hub0 \
+       -vnc 0.0.0.0:0
+
+    Once QEMU starts VNC client and any TTY emulator can be used to
+    control a single hvc console:
+
+    ::
+
+       # Start TTY emulator
+       tio /tmp/pty
+
+       # Start VNC client and switch to virtual console Ctrl-Alt-2
+       vncviewer :0
+
+    Several frontend devices is not supported. Stacking of multiplexers
+    and hub devices is not supported as well.
+
 ``-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.