Messages in this thread Patch in this message | | | Date | Wed, 01 Dec 2021 00:33:02 -0000 | From | "tip-bot2 for Eric Dumazet" <> | Subject | [tip: x86/core] x86/csum: Fix initial seed for odd buffers |
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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@google.com> AuthorDate: Thu, 25 Nov 2021 06:18:17 -08:00 Committer: Dave Hansen <dave.hansen@linux.intel.com> 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@infradead.org> Fixes: d31c3c683ee6 ("x86/csum: Rewrite/optimize csum_partial()") Reported-by: Noah Goldstein <goldstein.w.n@gmail.com> Signed-off-by: Eric Dumazet <edumazet@google.com> Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com> Link: https://lkml.kernel.org/r/20211125141817.3541501-1-eric.dumazet@gmail.com --- 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++;
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