xen: arm: weaken SMP barriers to inner shareable.

Since all processors are in the inner-shareable domain and we map everything
that way this is sufficient.

The non-SMP barriers remain full system. Although in principle they could
become outer shareable barriers for some hardware this would require us to
know which class a given device is. Given the small number of device drivers
in Xen itself its probably not worth worrying over, although maybe someone
will benchmark at some point.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Tim Deegan <tim@xen.org>

xen/include/asm-arm/system.h

@@ -20,9 +20,9 @@
 #define rmb()           dsb(sy)
 #define wmb()           dsb(sy)
 
-#define smp_mb()        dmb(sy)
-#define smp_rmb()       dmb(sy)
-#define smp_wmb()       dmb(sy)
+#define smp_mb()        dmb(ish)
+#define smp_rmb()       dmb(ish)
+#define smp_wmb()       dmb(ish)
 
 #define xchg(ptr,x) \
         ((__typeof__(*(ptr)))__xchg((unsigned long)(x),(ptr),sizeof(*(ptr))))