[Qemu-devel] Re: [PATCH] update server bits on vnc_update

[Gerd Hoffmann]

On 07/18/09 06:47, Glauber Costa wrote:
> Since the server/guest split in vnc architecture, we
> no longer update the server bits on large updates. Result
> is screen gets garbled, specially on scroll actions.
>
> I must admit I don't fully understand our vnc code, but after
> a careful reading, it seemed to me the proposed patch would fix
> it, and it indeed, works.

NAK.

VNC screen update workflow is this:

  (1) vnc_update() is called to mark a region as dirty,
      the guest bitmap is updated accordingly.  May happen
      multiple times.
  (2) vnc_update_client() is called by timer.  It walks
      the guest bitmap.  For dirty regions it checks whenever
      guest and server bitmap are *really* different, if so
      it does a guest->server copy and updates the server
      dirty bitmap.
  (3) vnc_update_client() walks the server dirty map and sends
      over the changes to the client.

Your patch kills the optimization in (2) and papers over the real bug.

Some extra care is needed in vnc_dpy_copy() which can send a bitblit 
command to the guest (VNC_ENCODING_COPYRECT).

And while looking at the code I think I've spotted the actual bug:  When 
sending a bitblit to the client we do *not* update the local server 
surface.  Result is the vnc clients and the vnc servers idea of the 
screen content are not in sync any more.  Which in turn will break the 
optimization in (2) and can easily result in a corrupted screen ...

cheers,
  Gerd