[s390x] Add optimised TCP/IP checksumming

Add an S/390 assembly language implementation of TCP/IP checksumming,
using the hardware "cksm" instruction to first calculate the 32-bit
one's complement checksum and then folding down to 16 bits.

Use an inline function since the whole checksum calculation (including
folding) requires only six instructions.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/s390x/include/bits/tcpip.h

@@ -0,0 +1,41 @@
+#ifndef _BITS_TCPIP_H
+#define _BITS_TCPIP_H
+
+/** @file
+ *
+ * Transport-network layer interface
+ *
+ */
+
+FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
+
+/**
+ * Calculate continued TCP/IP checkum
+ *
+ * @v partial		Checksum of already-summed data, in network byte order
+ * @v data		Data buffer
+ * @v len		Length of data buffer
+ * @ret cksum		Updated checksum, in network byte order
+ */
+static inline __attribute__ (( always_inline )) uint16_t
+tcpip_continue_chksum ( uint16_t partial, const void *data, size_t len ) {
+	struct s390x_pointer_pair pair = { data, len };
+	unsigned int cksum;
+
+	__asm__ ( /* Calculate 32-bit checksum */
+		  "\n1:\n\t"
+		  "cksm %0, %1\n\t"
+		  "jo 1b\n\t"
+		  /* Fold down to 16 bits */
+		  "risbgz %1, %0, 32, 47, 16\n\t"
+		  "alr %0, %1\n\t"
+		  "srl %0, 16\n\t"
+		  "alcr %0, %N1\n\t"
+		  : "=&r" ( cksum ),
+		    "+r" ( pair )
+		  : "0" ( ~partial ) );
+
+	return ~cksum;
+}
+
+#endif /* _BITS_TCPIP_H */