[tip: x86/core] x86/csum: Fix initial seed for odd buffers

[tip-bot2 for Eric Dumazet]

The following commit has been merged into the x86/core branch of tip:

Commit-ID:     2a144bcd661c4f0a503e03f9280e88854ac0bb37
Gitweb:        https://git.kernel.org/tip/2a144bcd661c4f0a503e03f9280e88854ac0bb37
Author:        Eric Dumazet <edumazet@xxxxxxxxxx>
AuthorDate:    Thu, 25 Nov 2021 06:18:17 -08:00
Committer:     Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
CommitterDate: Tue, 30 Nov 2021 16:26:03 -08:00

x86/csum: Fix initial seed for odd buffers

When I folded do_csum() into csum_partial(), I missed that we
had to swap odd/even bytes from @sum argument.

This is because this swap will happen again at the end of the function.

[A, B, C, D] -> [B, A, D, C]

As far as Internet checksums (rfc 1071) are concerned, we can instead
rotate the whole 32bit value by 8 (or 24)

-> [D, A, B, C]

Note that I played with the idea of replacing this final swapping:

    result = from32to16(result);
    result = ((result >> 8) & 0xff) | ((result & 0xff) << 8);

With:

    result = ror32(result, 8);

But while the generated code was definitely better for the odd case,
run time cost for the more likely even case was not better for gcc.

gcc is replacing a well predicted conditional branch
with a cmov instruction after a ror instruction which adds
a cost canceling the cmov gain.

Many thanks to Noah Goldstein for reporting this issue.

[ dhansen: * spelling: swaping => swapping
	   * updated Fixes commit  ]

Cc: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Fixes: d31c3c683ee6 ("x86/csum: Rewrite/optimize csum_partial()")
Reported-by: Noah Goldstein <goldstein.w.n@xxxxxxxxx>
Signed-off-by: Eric Dumazet <edumazet@xxxxxxxxxx>
Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Link: https://lkml.kernel.org/r/20211125141817.3541501-1-eric.dumazet@xxxxxxxxx
---
 arch/x86/lib/csum-partial_64.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/x86/lib/csum-partial_64.c b/arch/x86/lib/csum-partial_64.c
index 1eb8f2d..40b527b 100644
--- a/arch/x86/lib/csum-partial_64.c
+++ b/arch/x86/lib/csum-partial_64.c
@@ -41,6 +41,7 @@ __wsum csum_partial(const void *buff, int len, __wsum sum)
 	if (unlikely(odd)) {
 		if (unlikely(len == 0))
 			return sum;
+		temp64 = ror32((__force u32)sum, 8);
 		temp64 += (*(unsigned char *)buff << 8);
 		len--;
 		buff++;