Re: [Qemu-devel] [PATCH] vnc: shared buffer: skip some optimizations.

[Gerd Hoffmann]

Stefano Stabellini wrote:
> Gerd Hoffmann wrote:
> 
>> The trouble spot is here:  The guest might have updated the screen
>> between (b) and (c).  This will cause the vnc clients view of the screen
>> content become inconsistent with old_data.  On the next update round (b)
>> will stop working correctly due to that.
> 
> 
> Even if old_data becomes inconsistent with the client's screen, the
> worst that can happen is that (b) will set as 'to send' framebuffer
> areas that have already been sent. Is that so bad?

(b) doesn't set the 'to send' bit.  It clears the 'to send' bit for
screen areas which are not modified.

You have two possible effects:
 * screen update is sent twice.
 * screen update is *not* sent.

The second causes persistent screen corruptions, it is not fixed up by
the next refresh.

Wanna have a screen shot?

cheers,
  Gerd