Re: [Qemu-devel] [PATCH] qemu-options.hx: Improve documentation of chardev multiplexing mode

[Laszlo Ersek <lersek@redhat.com>]

On 02/16/16 17:03, Peter Maydell wrote:
> The current documentation of chardev mux=on is rather brief and opaque;
> expand it to hopefully be a bit more helpful.
> 
> Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
> ---
> There was some discussion on #qemu yesterday evening about multiplexing,
> and "make the docs a bit less confusing" was one suggestion...
> ---
>  qemu-options.hx | 20 +++++++++++++++++++-
>  1 file changed, 19 insertions(+), 1 deletion(-)
> 
> diff --git a/qemu-options.hx b/qemu-options.hx
> index 2f0465e..6b46dd4 100644
> --- a/qemu-options.hx
> +++ b/qemu-options.hx
> @@ -2162,8 +2162,26 @@ All devices must have an id, which can be any string up to 127 characters long.
>  It is used to uniquely identify this device in other command line directives.
>  
>  A character device may be used in multiplexing mode by multiple front-ends.
> +Specify @option{mux=on} to enable this mode.
> +A multiplexer is a "1:N" device, and here the "1" end is your specified chardev
> +backend, and the "N" end is the various parts of QEMU that can talk to a chardev.
> +If you create a chardev with @option{id=myid} and @option{mux=on}, QEMU will
> +create a multiplexer with your specified ID, and you can then configure multiple
> +front ends to use that chardev ID for their input/output. Up to four different
> +front ends can be connected to a single multiplexed chardev. (Without
> +multiplexing enabled, a chardev can only be used by a single front end.)
> +For instance you could use this to allow a single stdio chardev to be used by
> +two serial ports and the QEMU monitor.
> +
>  The key sequence of @key{Control-a} and @key{c} will rotate the input focus
> -between attached front-ends. Specify @option{mux=on} to enable this mode.
> +of a multiplexed character device between attached front-ends.
> +
> +You can have more than one multiplexer in a system configuration; for instance
> +you could have a TCP port multiplexed between UART 0 and UART 1, and stdio
> +multiplexed between the QEMU monitor and a parallel port.
> +
> +There is currently no support for multiplexing in the other direction
> +(where a single QEMU front end takes input and output from multiple chardevs).
>  
>  Every backend supports the @option{logfile} option, which supplies the path
>  to a file to record all data transmitted via the backend. The @option{logappend}
> 

I'm a big fan of muxing the stdio backend between the (human) monitor
and the guest's serial console:

  -chardev stdio,signal=off,mux=on,id=char0 \
  -mon chardev=char0,mode=readline,default \
  -serial chardev:char0 \

I think if you could mention the following in the above, that would be
even more helpful:
- how to enter a real Control-A character (relevant when typing at the
  guest's shell prompt, over serial)
- not strictly related, but a warning about using this in "screen"
  (I use "screen" all the time to capture serial IO) -- Control-A is
  also piggy-backed by "screen"
- how the initially active frontend gets selected
- the fact that frontends use different syntaxes to refer to the
  backend. You have two examples in the text being added ("for
  instance, ..."), so I think it would be really helpful to provide
  actual command line options for those.

Just my two cents. The patch looks great.

Thanks
Laszlo