Re: [Qemu-devel] [PATCH 0/2] Tunnel character device data over VNC (v1)

["Daniel P. Berrange" <berrange@redhat.com>]

On Wed, Jul 01, 2009 at 02:59:23PM -0500, Anthony Liguori wrote:
> Gerd Hoffmann wrote:
> >Monitor is a special case.  Multiple connections to the same session 
> >are not very useful there.  Multiple sessions are a different (albeit 
> >related) problem.
> 
> Serial is the same.  Imagine a bash shell running on the serial port 
> with two VNC client connected and stdio connected.  Utter chaos.  You 
> really want to use one or the other, never both at the same time.  If 
> that's the case, then just switch the character device on the fly.  
> Don't pretend that we can arbitrate multiple clients at once.

This isn't really any different to have 2 VNC clients connected at the
same time and using the same Xorg/gnome-terminal instance concurrently.
There's no QEMU level technical problem with mirroring updates to multiple 
clients. For it to be useful though, it does require that the people/thing 
using each client is 'intelligent', otherwise they'll just end up fighting
with each other. So I think this is more of a policy decision - for both
VNC clients on the graphical console, and multiple char device clients -
for each device you could designate 'exclusive' (only one backend at a time),
"exclusive read/write+shared readonly (one backend can read+write, others 
are forced read only), or 'shared read/write' (every backend can read+write).
This kind of policy is done by many traditional VNC servers, and we could
add it to QEMU without too much trouble I reckon.

For a backend that's intended to be used by some program/service (eg RPC
message bus),  then exlusive would clearly be required. For a backend 
that's intended to be used interactively by a real person, then  any of
the policies could be practical.

Regards,
Daniel
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