Re: [Qemu-devel] [PATCH] qemu-char: fix terminal crash when using "-monitor stdio -nographic"

[Gerd Hoffmann <kraxel@redhat.com>]

On Fr, 2014-09-05 at 11:04 +0200, Markus Armbruster wrote:
> Li Liu <john.liuli@huawei.com> writes:
> 
> > Ping, any more comments? Thanks.
> 
> I'd like to hear Gerd's opinion (cc'ed).
> 
> >>> But is having multiple character devices use the same terminal valid?

No (guess we should catch that case in stdio init).

Beside the tty initialization and cleanup you also have the problem that
both users are racing for input.  Well, maybe not in the qemu case as it
is the same process and it very well might be that it polls the two
chardevs in a well defined order, so one of them gets all input and the
other gets nothing.  With two processes reading from the terminal (try
'cat | less') it is actually random though.

> >> I'm not sure. But I have found such comments in vl.c
> >> "According to documentation and historically, -nographic redirects
> >> serial port, parallel port and monitor to stdio"

In that case mux chardev is used (that is the piece which handles the
input switching between serial and monitor via 'Ctrl-A c').  There is
one stdio instance, and one mux instance, the mux is chained to stdio,
and mux allows multiple backends to connect.

You can construct it on the command line this way:

qemu -nographic -nodefaults \
   -chardev stdio,mux=on,id=terminal \
   -serial chardev:terminal \
   -monitor chardev:terminal

[ serial is default, so no output here, unless you boot a guest
  with serial console configured ]

[ Hit 'Ctrl-A h' now ]

C-a h    print this help
C-a x    exit emulator
C-a s    save disk data back to file (if -snapshot)
C-a t    toggle console timestamps
C-a b    send break (magic sysrq)
C-a c    switch between console and monitor
C-a C-a  sends C-a

[ Hit 'Ctrl-A c' now ]

QEMU 2.1.50 monitor - type 'help' for more information
(qemu) info chardev
terminal: filename=mux
terminal-base: filename=stdio
(qemu) 

HTH,
  Gerd