Re: [Qemu-devel] [PATCH] opengl rendering in the sdl window

[Anthony Liguori <anthony@codemonkey.ws>]

Daniel P. Berrange wrote:
> On Mon, Sep 08, 2008 at 04:08:01PM +0100, Jamie Lokier wrote:
>   
>> Gerd Hoffmann wrote:
>>     
>>> Beside that I still think it would be a good idea to separate qemu and
>>> the gui into two separate processes, so you can close the GUI window and
>>> keep the VM running.  It also solves the dependency issue for distros as
>>> the gtk frontend with all the dependencies can just go into a separate
>>> sub-package.
>>>       
>> Extending VNC with a "shared memory" extension, similar to Xlib's
>> MIT-SHM extension, and implementing it in QEMU and Gtk-VNC would be a
>> nice way to do this.
>>     
>
> Funny you should mention that. Anthony had code todo exactly that against
> for both QEMU and GTK-VNC a while back. We had it in the GTK-VNC for a 
> short while but removed it due to some race conditions in its impl, and
> it interracted badly with our OpenGL impl. We could easily revisit this
> idea if it were thought to be important / useful.
>   

The difficulty is deleting the shared memory segment reliably.

Normally, what you want to do is:

shmget()
shmat()
shmctl(IPC_RMID)

...

shmdt()

And the first sequence has to be as reliable and quick as possible 
because if you exit before the shmctl(), you'll leak the shared memory.

Unfortunately, you have to shmget() in the client and shmat() in the 
server and client, so that means:

shmget()
shmat()

...

notify server of key
wait for server to confirm shmat()

...

shmctl(IPC_RMID)

..

shmdt()

Which leaves a huge window open where bad things can happen.  The client 
can exit, the client can crash.  This is particularly troublesome in a 
library because it's really not nice to try and register an atexit() 
handler or something like that from a library.

It would be nice to come up with a solution to this.

Regards,

Anthony Liguori

> Daniel
>