RE: [PATCH] lib/x86: Optimise csum_partial of buffers that are not multiples of 8 bytes.

-----Original Message-----
From: David Laight <redacted>
Sent: Monday, December 13, 2021 10:01 AM
To: 'Noah Goldstein' <redacted>; 'Eric Dumazet'
[off-list ref]
Cc: 'tglx@linutronix.de' <redacted>; 'mingo@redhat.com'
[off-list ref]; 'Borislav Petkov' [off-list ref];
'dave.hansen@linux.intel.com' [off-list ref]; 'X86 ML'
[off-list ref]; 'hpa@zytor.com' [off-list ref];
'peterz@infradead.org' [off-list ref]; Alexander Duyck
[off-list ref]; 'open list' [off-list ref];
'netdev' [off-list ref]
Subject: [PATCH] lib/x86: Optimise csum_partial of buffers that are not
multiples of 8 bytes.


Add in the trailing bytes first so that there is no need to worry about the sum
exceeding 64 bits.

Signed-off-by: David Laight <redacted>
---

This ought to be faster - because of all the removed 'adc $0'.
Guessing how fast x86 code will run is hard!
There are other ways of handing buffers that are shorter than 8 bytes, but I'd
rather hope they don't happen in any hot paths.

Note - I've not even compile tested it.
(But have tested an equivalent change before.)

 arch/x86/lib/csum-partial_64.c | 55 ++++++++++++----------------------
 1 file changed, 19 insertions(+), 36 deletions(-)

diff --git a/arch/x86/lib/csum-partial_64.c b/arch/x86/lib/csum-partial_64.c
index abf819dd8525..fbcc073fc2b5 100644
--- a/arch/x86/lib/csum-partial_64.c
+++ b/arch/x86/lib/csum-partial_64.c

@@ -37,6 +37,24 @@ __wsum csum_partial(const void *buff, int len,

__wsum sum)
 	u64 temp64 = (__force u64)sum;
 	unsigned result;

+	if (len & 7) {
+		if (unlikely(len < 8)) {
+			/* Avoid falling off the start of the buffer */
+			if (len & 4) {
+				temp64 += *(u32 *)buff;
+				buff += 4;
+			}
+			if (len & 2) {
+				temp64 += *(u16 *)buff;
+				buff += 2;
+			}
+			if (len & 1)
+				temp64 += *(u8 *)buff;
+			goto reduce_to32;
+		}
+		temp64 += *(u64 *)(buff + len - 8) << (8 - (len & 7)) * 8;
+	}
+

 	while (unlikely(len >= 64)) {
 		asm("addq 0*8(%[src]),%[res]\n\t"
 		    "adcq 1*8(%[src]),%[res]\n\t"

@@ -82,43 +100,8 @@ __wsum csum_partial(const void *buff, int len,

__wsum sum)
 			: "memory");
 		buff += 8;
 	}
-	if (len & 7) {
-#ifdef CONFIG_DCACHE_WORD_ACCESS
-		unsigned int shift = (8 - (len & 7)) * 8;
-		unsigned long trail;
-
-		trail = (load_unaligned_zeropad(buff) << shift) >> shift;