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Subject[PATCH v2] x86/lib: Remove the special case for odd-aligned buffers in csum-partial_64.c
Date
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
-
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