[XEN] Shadow: check the _PAGE_PRESENT bit in fast-path MMIO shadow ptes

otherwise the fast-path code is not safe against some kinds of
concurrent shadow updates.
Signed-off-by: Tim Deegan <Tim.Deegan@xensource.com>

diff --git a/xen/arch/x86/mm/shadow/types.h b/xen/arch/x86/mm/shadow/types.h
index 400514f2412d86290c146382de53eddff4bbc61b..8397e7ae2c06a4780cf15aa9c03f934d2b2a9d47 100644 (file)
--- a/xen/arch/x86/mm/shadow/types.h
+++ b/xen/arch/x86/mm/shadow/types.h
@@ -597,7 +597,7 @@ accumulate_guest_flags(struct vcpu *v, walk_t *gw)
  * have reserved bits that we can use for this.
  */
 
-#define SH_L1E_MAGIC 0xffffffff00000000ULL
+#define SH_L1E_MAGIC 0xffffffff00000001ULL
 static inline int sh_l1e_is_magic(shadow_l1e_t sl1e)
 {
     return ((sl1e.l1 & SH_L1E_MAGIC) == SH_L1E_MAGIC);