s390: add optimized array_index_mask_nospec

BugLink: http://bugs.launchpad.net/bugs/1768474

[ Upstream commit e2dd8333 ]

Add an optimized version of the array_index_mask_nospec function for
s390 based on a compare and a subtract with borrow.
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

arch/s390/include/asm/barrier.h

@@ -63,4 +63,28 @@ do {									\
 	___p1;								\
 })
 
+/**
+ * array_index_mask_nospec - generate a mask for array_idx() that is
+ * ~0UL when the bounds check succeeds and 0 otherwise
+ * @index: array element index
+ * @size: number of elements in array
+ */
+#define array_index_mask_nospec array_index_mask_nospec
+static inline unsigned long array_index_mask_nospec(unsigned long index,
+						    unsigned long size)
+{
+	unsigned long mask;
+
+	if (__builtin_constant_p(size) && size > 0) {
+		asm("	clgr	%2,%1\n"
+		    "	slbgr	%0,%0\n"
+		    :"=d" (mask) : "d" (size-1), "d" (index) :"cc");
+		return mask;
+	}
+	asm("	clgr	%1,%2\n"
+	    "	slbgr	%0,%0\n"
+	    :"=d" (mask) : "d" (size), "d" (index) :"cc");
+	return ~mask;
+}
+
 #endif /* __ASM_BARRIER_H */