kernel: backport improved checksum function for ARM64

Improves network performance in some cases when checksum offload is not
available

Signed-off-by: Felix Fietkau <nbd@nbd.name>

1 files changed, 28 insertions, 0 deletions

diff --git a/target/linux/generic/backport-5.4/042-v5.5-arm64-csum-Fix-pathological-zero-length-calls.patch b/target/linux/generic/backport-5.4/042-v5.5-arm64-csum-Fix-pathological-zero-length-calls.patch
new file mode 100644
index 0000000000..50b210e14f
--- /dev/null
+++ b/target/linux/generic/backport-5.4/042-v5.5-arm64-csum-Fix-pathological-zero-length-calls.patch
@@ -0,0 +1,28 @@
+From: Robin Murphy <robin.murphy@arm.com>
+Date: Fri, 17 Jan 2020 15:48:39 +0000
+Subject: [PATCH] arm64: csum: Fix pathological zero-length calls
+
+In validating the checksumming results of the new routine, I sadly
+neglected to test its not-checksumming results. Thus it slipped through
+that the one case where @buff is already dword-aligned and @len = 0
+manages to defeat the tail-masking logic and behave as if @len = 8.
+For a zero length it doesn't make much sense to deference @buff anyway,
+so just add an early return (which has essentially zero impact on
+performance).
+
+Signed-off-by: Robin Murphy <robin.murphy@arm.com>
+Signed-off-by: Will Deacon <will@kernel.org>
+---
+
+--- a/arch/arm64/lib/csum.c
++++ b/arch/arm64/lib/csum.c
+@@ -20,6 +20,9 @@ unsigned int do_csum(const unsigned char
+ 	const u64 *ptr;
+ 	u64 data, sum64 = 0;
+ 
++	if (unlikely(len == 0))
++		return 0;
++
+ 	offset = (unsigned long)buff & 7;
+ 	/*
+ 	 * This is to all intents and purposes safe, since rounding down cannot