vga: catch depth 0

depth == 0 is used to indicate 256 color modes.  Our region calculation
goes wrong in that case.  So detect that and just take the safe code
path we already have for the wraparound case.

While being at it also catch depth == 15 (where our region size
calculation goes wrong too).  And make the comment more verbose,
explaining what is going on here.

Without this windows guest install might trigger an assert due to trying
to check dirty bitmap outside the snapshot region.

Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1575541
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Message-id: 20180514103117.21059-1-kraxel@redhat.com

hw/display/vga.c

@@ -1480,13 +1480,28 @@ static void vga_draw_graphic(VGACommonState *s, int full_update)
 
     s->get_resolution(s, &width, &height);
     disp_width = width;
+    depth = s->get_bpp(s);
 
     region_start = (s->start_addr * 4);
     region_end = region_start + (ram_addr_t)s->line_offset * height;
-    region_end += width * s->get_bpp(s) / 8; /* scanline length */
+    region_end += width * depth / 8; /* scanline length */
     region_end -= s->line_offset;
-    if (region_end > s->vbe_size) {
-        /* wraps around (can happen with cirrus vbe modes) */
+    if (region_end > s->vbe_size || depth == 0 || depth == 15) {
+        /*
+         * We land here on:
+         *  - wraps around (can happen with cirrus vbe modes)
+         *  - depth == 0 (256 color palette video mode)
+         *  - depth == 15
+         *
+         * Take the safe and slow route:
+         *   - create a dirty bitmap snapshot for all vga memory.
+         *   - force shadowing (so all vga memory access goes
+         *     through vga_read_*() helpers).
+         *
+         * Given this affects only vga features which are pretty much
+         * unused by modern guests there should be no performance
+         * impact.
+         */
         region_start = 0;
         region_end = s->vbe_size;
         force_shadow = true;
@@ -1520,8 +1535,6 @@ static void vga_draw_graphic(VGACommonState *s, int full_update)
         }
     }
 
-    depth = s->get_bpp(s);
-
     /*
      * Check whether we can share the surface with the backend
      * or whether we need a shadow surface. We share native