[PATCH] arm64: Implement optimised IP checksum helpers

[Robin Murphy]

AArch64 is capable of 128-bit memory accesses without alignment
restrictions, which makes it both possible and highly practical to slurp
up a typical 20-byte IP header in just 2 loads. Implement our own
version of ip_fast_checksum() to take advantage of that, resulting in
considerably fewer instructions and memory accesses than the generic
version. We can also get more optimal code generation for csum_fold() by
defining it a slightly different way round from the generic version, so
throw that into the mix too.

Suggested-by: Luke Starrett <luke.starrett at broadcom.com>
Signed-off-by: Robin Murphy <robin.murphy at arm.com>
---

Having spent an evening poring over disassembler output, it turns out
that there's really no need to drop into asm for this, hurrah! I've
taken it for a spin on both big and little-endian, with no ill effects.

 arch/arm64/include/asm/checksum.h | 49 +++++++++++++++++++++++++++++++++++++++
 1 file changed, 49 insertions(+)
 create mode 100644 arch/arm64/include/asm/checksum.h

diff --git a/arch/arm64/include/asm/checksum.h b/arch/arm64/include/asm/checksum.h
new file mode 100644
index 000000000000..7e975ffce837
--- /dev/null
+++ b/arch/arm64/include/asm/checksum.h
@@ -0,0 +1,49 @@
+/*
+ * Copyright (C) 2016 ARM Ltd.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+#ifndef __ASM_CHECKSUM_H
+#define __ASM_CHECKSUM_H
+
+static inline __sum16 csum_fold(__wsum csum)
+{
+	u32 sum = (__force u32)csum;
+	sum += (sum >> 16) | (sum << 16);
+	return ~(__force __sum16)(sum >> 16);
+}
+#define csum_fold csum_fold
+
+static inline __sum16 ip_fast_csum(const void *iph, unsigned int ihl)
+{
+	__uint128_t tmp;
+	u64 sum;
+
+	tmp = *(const __uint128_t *)iph;
+	iph += 16;
+	ihl -= 4;
+	tmp += ((tmp >> 64) | (tmp << 64));
+	sum = tmp >> 64;
+	do {
+		sum += *(const u32 *)iph;
+		iph += 4;
+	} while (--ihl);
+
+	sum += ((sum >> 32) | (sum << 32));
+	return csum_fold(sum >> 32);
+}
+#define ip_fast_csum ip_fast_csum
+
+#include <asm-generic/checksum.h>
+
+#endif	/* __ASM_CHECKSUM_H */
-- 
2.8.1.dirty