x86/asm: add cpu_relax()

This will be useful to be put inside loops.
Suggested-by: Andrew Jones <drjones@redhat.com>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

lib/x86/asm/barrier.h

@@ -13,4 +13,15 @@
 #define smp_rmb()	barrier()
 #define smp_wmb()	barrier()
 
+/* REP NOP (PAUSE) is a good thing to insert into busy-wait loops. */
+static inline void rep_nop(void)
+{
+	asm volatile("rep; nop" ::: "memory");
+}
+
+static inline void cpu_relax(void)
+{
+	rep_nop();
+}
+
 #endif