x86: Allow bitop functions to be applied only to fields of at least 4

bytes. Otherwise the 'longword' processor instructions used will
overlap with adjacent fields with unpredictable consequences.

This change requires some code fixup and just a few casts (mainly when
operating on guest-shared fields which cannot be changed, and which by
observation are clearly safe).

Based on ideas from Jan Beulich <jbeulich@novell.com>

Signed-off-by: Keir Fraser <keir.fraser@citrix.com>

1 files changed, 1 insertions, 1 deletions

diff --git a/xen/drivers/video/vesa.c b/xen/drivers/video/vesa.c
index d18476421c..e36e358789 100644
--- a/xen/drivers/video/vesa.c
+++ b/xen/drivers/video/vesa.c
@@ -219,7 +219,7 @@ static void vesa_show_line(
                      ((font->width + 7) >> 3));
             for ( b = font->width; b--; )
             {
-                pixel = test_bit(b, bits) ? pixel_on : 0;
+                pixel = (*bits & (1u<<b)) ? pixel_on : 0;
                 memcpy(ptr, &pixel, bpp);
                 ptr += bpp;
             }