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Re: [Qemu-trivial] [Qemu-devel] [PATCH] qemu-options.hx: Improve documen
From: |
Laszlo Ersek |
Subject: |
Re: [Qemu-trivial] [Qemu-devel] [PATCH] qemu-options.hx: Improve documentation of chardev multiplexing mode |
Date: |
Tue, 16 Feb 2016 17:32:24 +0100 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.5.1 |
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 <address@hidden>
> ---
> 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