[Qemu-devel] [PATCH][VNC] Fix fragments due to incomplete dirty tracking in CGA mode

[Anthony Liguori]

Dirty tracking is done in the VNC server by splitting the framebuffer 
into 16-pixel blocks and then updating a bitmap when a dpy->update() 
occurs.  If dpy->update() is called with an x st (x % 16) != 0, then the 
very last block dirtied by the update could potentially not be tracked.

This is really only visible when in CGA mode or when using the VMware 
VGA driver since they are the only ones that generate updates where x != 
0.  The attached patch addresses this.

Reproducing the fragments isn't 100% reliable but I was able to find a 
somewhat reliable way to reproduce (holding in backspace when editting a 
grub entry) and confirmed that this patch fixes the problem.

Regards,

Anthony Liguori

Index: qemu/vnc.c
===================================================================
--- qemu.orig/vnc.c     2007-12-10 11:39:04.000000000 -0600
+++ qemu/vnc.c  2007-12-10 11:42:57.000000000 -0600
@@ -258,6 +258,13 @@
 
     h += y;
 
+    /* round x down to ensure the loop only spans one 16-pixel block per,
+       iteration.  otherwise, if (x % 16) != 0, the last iteration may span
+       two 16-pixel blocks but we only mark the first as dirty
+    */
+    w += (x % 16);
+    x -= (x % 16);
+
     for (; y < h; y++)
        for (i = 0; i < w; i += 16)
            vnc_set_bit(vs->dirty_row[y], (x + i) / 16);