Messages in this thread Patch in this message | | | From | David Laight <> | Subject | [PATCH v2] x86/lib: Remove the special case for odd-aligned buffers in csum-partial_64.c | Date | Thu, 6 Jan 2022 14:45:41 +0000 |
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There is no need to special case the very unusual odd-aligned buffers. They are no worse than 4n+2 aligned buffers.
Signed-off-by: David Laight <david.laight@aculab.com> Acked-by: Eric Dumazet ---
resend - v1 seems to have got lost :-)
v2: Also delete from32to16() Add acked-by from Eric (he sent one at some point) Fix possible whitespace error in the last hunk.
The penalty for any misaligned access seems to be minimal. On an i7-7700 misaligned buffers add 2 or 3 clocks (in 115) to a 512 byte checksum. That is less than 1 clock for each cache line! That is just measuring the main loop with an lfence prior to rdpmc to read PERF_COUNT_HW_CPU_CYCLES.
arch/x86/lib/csum-partial_64.c | 28 ++-------------------------- 1 file changed, 2 insertions(+), 26 deletions(-)
diff --git a/arch/x86/lib/csum-partial_64.c b/arch/x86/lib/csum-partial_64.c index 1f8a8f895173..061b1ed74d6a 100644 --- a/arch/x86/lib/csum-partial_64.c +++ b/arch/x86/lib/csum-partial_64.c @@ -11,16 +11,6 @@ #include <asm/checksum.h> #include <asm/word-at-a-time.h> -static inline unsigned short from32to16(unsigned a) -{ - unsigned short b = a >> 16; - asm("addw %w2,%w0\n\t" - "adcw $0,%w0\n" - : "=r" (b) - : "0" (b), "r" (a)); - return b; -} - /* * Do a checksum on an arbitrary memory area. * Returns a 32bit checksum. @@ -30,22 +20,12 @@ static inline unsigned short from32to16(unsigned a) * * Still, with CHECKSUM_COMPLETE this is called to compute * checksums on IPv6 headers (40 bytes) and other small parts. - * it's best to have buff aligned on a 64-bit boundary + * The penalty for misaligned buff is negligable. */ __wsum csum_partial(const void *buff, int len, __wsum sum) { u64 temp64 = (__force u64)sum; - unsigned odd, result; - - odd = 1 & (unsigned long) buff; - if (unlikely(odd)) { - if (unlikely(len == 0)) - return sum; - temp64 = ror32((__force u32)sum, 8); - temp64 += (*(unsigned char *)buff << 8); - len--; - buff++; - } + unsigned result; while (unlikely(len >= 64)) { asm("addq 0*8(%[src]),%[res]\n\t" @@ -130,10 +110,6 @@ __wsum csum_partial(const void *buff, int len, __wsum sum) #endif } result = add32_with_carry(temp64 >> 32, temp64 & 0xffffffff); - if (unlikely(odd)) { - result = from32to16(result); - result = ((result >> 8) & 0xff) | ((result & 0xff) << 8); - } return (__force __wsum)result; } EXPORT_SYMBOL(csum_partial); -- 2.17.1 - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
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