Re: [PATCH v4 8/8] qemu-options.hx: describe multiplexing of several backend devices

["Marc-André Lureau" <marcandre.lureau@redhat.com>]

Hi

On Wed, Oct 16, 2024 at 2:28 PM Roman Penyaev <r.peniaev@gmail.com> wrote:
>
> This adds a few lines describing `mux-be` 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: qemu-devel@nongnu.org
> ---
>  qemu-options.hx | 78 ++++++++++++++++++++++++++++++++++++-------------
>  1 file changed, 58 insertions(+), 20 deletions(-)
>
> diff --git a/qemu-options.hx b/qemu-options.hx
> index daae49414740..dd5dfe8596f0 100644
> --- a/qemu-options.hx
> +++ b/qemu-options.hx
> @@ -3677,37 +3677,37 @@ DEFHEADING(Character device options:)
>
>  DEF("chardev", HAS_ARG, QEMU_OPTION_chardev,
>      "-chardev help\n"
> -    "-chardev null,id=id[,mux=on|off][,logfile=PATH][,logappend=on|off]\n"
> +    "-chardev null,id=id[,mux=on|off][,mux-be-id=id][,logfile=PATH][,logappend=on|off]\n"
>      "-chardev socket,id=id[,host=host],port=port[,to=to][,ipv4=on|off][,ipv6=on|off][,nodelay=on|off]\n"
> -    "         [,server=on|off][,wait=on|off][,telnet=on|off][,websocket=on|off][,reconnect=seconds][,mux=on|off]\n"
> +    "         [,server=on|off][,wait=on|off][,telnet=on|off][,websocket=on|off][,reconnect=seconds][,mux=on|off][,mux-be-id=id]\n"
>      "         [,logfile=PATH][,logappend=on|off][,tls-creds=ID][,tls-authz=ID] (tcp)\n"
>      "-chardev socket,id=id,path=path[,server=on|off][,wait=on|off][,telnet=on|off][,websocket=on|off][,reconnect=seconds]\n"
> -    "         [,mux=on|off][,logfile=PATH][,logappend=on|off][,abstract=on|off][,tight=on|off] (unix)\n"
> +    "         [,mux=on|off][,mux-be-id=id][,logfile=PATH][,logappend=on|off][,abstract=on|off][,tight=on|off] (unix)\n"
>      "-chardev udp,id=id[,host=host],port=port[,localaddr=localaddr]\n"
> -    "         [,localport=localport][,ipv4=on|off][,ipv6=on|off][,mux=on|off]\n"
> +    "         [,localport=localport][,ipv4=on|off][,ipv6=on|off][,mux=on|off][,mux-be-id=id]\n"
>      "         [,logfile=PATH][,logappend=on|off]\n"
> -    "-chardev msmouse,id=id[,mux=on|off][,logfile=PATH][,logappend=on|off]\n"
> +    "-chardev msmouse,id=id[,mux=on|off][,mux-be-id=id][,logfile=PATH][,logappend=on|off]\n"
>      "-chardev vc,id=id[[,width=width][,height=height]][[,cols=cols][,rows=rows]]\n"
> -    "         [,mux=on|off][,logfile=PATH][,logappend=on|off]\n"
> +    "         [,mux=on|off][,mux-be-id=id][,logfile=PATH][,logappend=on|off]\n"
>      "-chardev ringbuf,id=id[,size=size][,logfile=PATH][,logappend=on|off]\n"
> -    "-chardev file,id=id,path=path[,input-path=input-file][,mux=on|off][,logfile=PATH][,logappend=on|off]\n"
> -    "-chardev pipe,id=id,path=path[,mux=on|off][,logfile=PATH][,logappend=on|off]\n"
> +    "-chardev file,id=id,path=path[,input-path=input-file][,mux=on|off][,mux-be-id=id][,logfile=PATH][,logappend=on|off]\n"
> +    "-chardev pipe,id=id,path=path[,mux=on|off][,mux-be-id=id][,logfile=PATH][,logappend=on|off]\n"
>  #ifdef _WIN32
> -    "-chardev console,id=id[,mux=on|off][,logfile=PATH][,logappend=on|off]\n"
> -    "-chardev serial,id=id,path=path[,mux=on|off][,logfile=PATH][,logappend=on|off]\n"
> +    "-chardev console,id=id[,mux=on|off][,mux-be-id=id][,logfile=PATH][,logappend=on|off]\n"
> +    "-chardev serial,id=id,path=path[,mux=on|off][,mux-be-id=id][,logfile=PATH][,logappend=on|off]\n"
>  #else
> -    "-chardev pty,id=id[,path=path][,mux=on|off][,logfile=PATH][,logappend=on|off]\n"
> -    "-chardev stdio,id=id[,mux=on|off][,signal=on|off][,logfile=PATH][,logappend=on|off]\n"
> +    "-chardev pty,id=id[,path=path][,mux=on|off][,mux-be-id=id][,logfile=PATH][,logappend=on|off]\n"
> +    "-chardev stdio,id=id[,mux=on|off][,mux-be-id=id][,signal=on|off][,logfile=PATH][,logappend=on|off]\n"
>  #endif
>  #ifdef CONFIG_BRLAPI
> -    "-chardev braille,id=id[,mux=on|off][,logfile=PATH][,logappend=on|off]\n"
> +    "-chardev braille,id=id[,mux=on|off][,mux-be-id=id][,logfile=PATH][,logappend=on|off]\n"
>  #endif
>  #if defined(__linux__) || defined(__sun__) || defined(__FreeBSD__) \
>          || defined(__NetBSD__) || defined(__OpenBSD__) || defined(__DragonFly__)
> -    "-chardev serial,id=id,path=path[,mux=on|off][,logfile=PATH][,logappend=on|off]\n"
> +    "-chardev serial,id=id,path=path[,mux=on|off][,mux-be-id=id][,logfile=PATH][,logappend=on|off]\n"
>  #endif
>  #if defined(__linux__) || defined(__FreeBSD__) || defined(__DragonFly__)
> -    "-chardev parallel,id=id,path=path[,mux=on|off][,logfile=PATH][,logappend=on|off]\n"
> +    "-chardev parallel,id=id,path=path[,mux=on|off][,mux-be-id=id][,logfile=PATH][,logappend=on|off]\n"
>  #endif
>  #if defined(CONFIG_SPICE)
>      "-chardev spicevmc,id=id,name=name[,debug=debug][,logfile=PATH][,logappend=on|off]\n"
> @@ -3719,8 +3719,8 @@ DEF("chardev", HAS_ARG, QEMU_OPTION_chardev,
>  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``,
> +``-chardev backend,id=id[,mux=on|off][,mux-be-id=id][,options]``
> +    Backend is one of: ``null``, ``socket``, ``udp``, ``msmouse``, ``mux-be``,
>      ``vc``, ``ringbuf``, ``file``, ``pipe``, ``console``, ``serial``,
>      ``pty``, ``stdio``, ``braille``, ``parallel``,
>      ``spicevmc``, ``spiceport``. The specific backend will determine the
> @@ -3777,9 +3777,10 @@ 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 multiplex in the opposite direction (where one QEMU
> +    interface receives input and output from multiple chardev devices),
> +    each character device needs ``mux-be-id=id`` option. Please refer
> +    to the paragraph below regarding chardev ``mux-be`` configuration.
>
>      Every backend supports the ``logfile`` option, which supplies the
>      path to a file to record all data transmitted via the backend. The
> @@ -3879,6 +3880,43 @@ The available backends are:
>      Forward QEMU's emulated msmouse events to the guest. ``msmouse``
>      does not take any options.
>
> +``-chardev mux-be,id=id``
> +    Explicitly create chardev backend multiplexer with possibility to
> +    multiplex in the opposite direction, where one QEMU interface
> +    (frontend device) receives input and output from multiple chardev
> +    backend devices.

We are not very good at documenting the version, but you could add (Since 9.2)

> +
> +    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.

bidirectional

> +    The example configuration can be the following:
> +
> +    ::
> +
> +       -chardev mux-be,id=mux0 \
> +       -chardev socket,path=/tmp/sock,server=on,wait=off,id=sock0,mux-be-id=mux0 \
> +       -chardev vc,id=vc0,mux-be-id=mux0 \
> +       -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

several

> +    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
>