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

[Anthony Liguori]

Gerd Hoffmann wrote:
> On 07/01/09 18:50, Daniel P. Berrange wrote:
> > On Wed, Jul 01, 2009 at 06:42:31PM +0200, Gerd Hoffmann wrote:
> > >    Hi,
> > >
> > > > The key requirement here is that it should not
> > > > be neccessary to specifically configure each character device to make
> > > > it available via VNC.  The admin should be able to configure the char
> > > > devices with all current available backends (file, pty, null, tcp, 
> > > > udp,
> > > > unix, etc), and regardless of this config be able to snoop on data 
> > > > from
> > > > any active VNC client on demand.
> > > Hmm.  What is the reason to handle vnc different from
> > > unix/tcp/pty/whatever?  I would expect something like '-serial
> > > vnc,name=foo' here ...
> >
> > As I tried to explain, I want to be able to configure the serial devices
> > using the existing backends *AND* also have the ability to occassionally
> > access the data using a VNC client.
>
> That doesn't answer the question why vnc should be different.
>
> I *do* see the point of being able to have more than one way to access 
> a chardev at the same time.  But when building up that infrastructure: 
> Why stop half-way through and make it a special hack for vnc?  I think 
> it would be *far* more useful to make chardevs in qemu work that way 
> all the time.
>
> Each serial-type driver gets a name tag (either user-specified or 
> autogenerated).  chardev backends connect by name.  Advantages:

Dynamically changing the character devices backends should be a not too 
terrible thing to do today and I agree this is the right approach.

Allowing multiple readers/writers for character devices is much more 
complex to get right though.  The current chardev API does not have good 
semantics for just handling one client that connects and disconnects.  
It certainly wouldn't gracefully handle multiple client states at once.

You could do something like a mirrored mode but things get really ugly 
when dealing with multiple read-write clients.  Think of how that would 
behave with the monitor for instance.

I agree that for something like -monitor tcp::1025,server,nowait, you 
want to be able to connect multiple times and get multiple monitor 
sessions.  We would need a serious overhaul of the character device 
infrastructure for that (and that's certainly long overdue).  So IMHO, 
this sort of refactoring shouldn't be a requirement for this patch series.

That doesn't mean we want to take a VNC specific hack though.

Regards,

Anthony Liguori