x86: add memory clobber to save/loadsegment

Add "memory" clobbers to savesegment and loadsegment, since they can
affect memory accesses and we never want the compiler to reorder them
with respect to memory references.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Cc: xen-devel <xen-devel@lists.xensource.com>
Cc: Stephen Tweedie <sct@redhat.com>
Cc: Eduardo Habkost <ehabkost@redhat.com>
Cc: Mark McLoughlin <markmc@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

include/asm-x86/system.h

@@ -153,14 +153,14 @@ extern void load_gs_index(unsigned);
 		     "jmp 2b\n"			\
 		     ".previous\n"		\
 		     _ASM_EXTABLE(1b,3b)	\
-		     : :"r" (value), "r" (0))
+		     : :"r" (value), "r" (0) : "memory")
 
 
 /*
  * Save a segment register away
  */
 #define savesegment(seg, value)				\
-	asm volatile("mov %%" #seg ",%0":"=rm" (value))
+	asm("mov %%" #seg ",%0":"=rm" (value) : : "memory")
 
 static inline unsigned long get_limit(unsigned long segment)
 {