x86/csum: Rewrite/optimize csum_partial()

With more NICs supporting CHECKSUM_COMPLETE, and IPv6 being widely
used csum_partial() is heavily used with small amount of bytes, and is
consuming many cycles.

IPv6 header size, for instance, is 40 bytes.

Another thing to consider is that NET_IP_ALIGN is 0 on x86, meaning
that network headers are not word-aligned, unless the driver forces
this.

This means that csum_partial() fetches one u16 to 'align the buffer',
then performs three u64 additions with carry in a loop, then a
remaining u32, then a remaining u16.

With this new version, it performs a loop only for the 64 bytes blocks,
then the remaining is bisected.

Testing on various CPUs, all of them show a big reduction in
csum_partial() cost (by 50 to 80 %)

Before:
	4.16%  [kernel]       [k] csum_partial
After:
	0.83%  [kernel]       [k] csum_partial

If run in a loop 1,000,000 times:

Before:
	26,922,913      cycles                    # 3846130.429 GHz
	80,302,961      instructions              #    2.98  insn per cycle
	21,059,816      branches                  # 3008545142.857 M/sec
	     2,896      branch-misses             #    0.01% of all branches
After:
	17,960,709      cycles                    # 3592141.800 GHz
	41,292,805      instructions              #    2.30  insn per cycle
	11,058,119      branches                  # 2211623800.000 M/sec
	     2,997      branch-misses             #    0.03% of all branches

 [ bp: Massage, merge in subsequent fixes into a single patch:
   - um compilation error due to missing load_unaligned_zeropad():
	- Reported-by: kernel test robot <lkp@intel.com>
	- Link: https://lkml.kernel.org/r/20211118175239.1525650-1-eric.dumazet@gmail.com
   - Fix initial seed for odd buffers
	- Reported-by: Noah Goldstein <goldstein.w.n@gmail.com>
	- Link: https://lkml.kernel.org/r/20211125141817.3541501-1-eric.dumazet@gmail.com
  ]
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Alexander Duyck <alexanderduyck@fb.com>
Link: https://lore.kernel.org/r/20211112161950.528886-1-eric.dumazet@gmail.com

arch/x86/lib/csum-partial_64.c

@@ -9,6 +9,7 @@
 #include <linux/compiler.h>
 #include <linux/export.h>
 #include <asm/checksum.h>
+#include <asm/word-at-a-time.h>
 
 static inline unsigned short from32to16(unsigned a) 
 {
@@ -21,54 +22,32 @@ static inline unsigned short from32to16(unsigned a)
 }
 
 /*
- * Do a 64-bit checksum on an arbitrary memory area.
+ * Do a checksum on an arbitrary memory area.
  * Returns a 32bit checksum.
  *
  * This isn't as time critical as it used to be because many NICs
  * do hardware checksumming these days.
  *
- * Things tried and found to not make it faster:
- * Manual Prefetching
- * Unrolling to an 128 bytes inner loop.
- * Using interleaving with more registers to break the carry chains.
+ * 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
  */
-static unsigned do_csum(const unsigned char *buff, unsigned len)
+__wsum csum_partial(const void *buff, int len, __wsum sum)
 {
-	unsigned odd, count;
-	unsigned long result = 0;
+	u64 temp64 = (__force u64)sum;
+	unsigned odd, result;
 
-	if (unlikely(len == 0))
-		return result; 
 	odd = 1 & (unsigned long) buff;
 	if (unlikely(odd)) {
-		result = *buff << 8;
+		if (unlikely(len == 0))
+			return sum;
+		temp64 = ror32((__force u32)sum, 8);
+		temp64 += (*(unsigned char *)buff << 8);
 		len--;
 		buff++;
 	}
-	count = len >> 1;		/* nr of 16-bit words.. */
-	if (count) {
-		if (2 & (unsigned long) buff) {
-			result += *(unsigned short *)buff;
-			count--;
-			len -= 2;
-			buff += 2;
-		}
-		count >>= 1;		/* nr of 32-bit words.. */
-		if (count) {
-			unsigned long zero;
-			unsigned count64;
-			if (4 & (unsigned long) buff) {
-				result += *(unsigned int *) buff;
-				count--;
-				len -= 4;
-				buff += 4;
-			}
-			count >>= 1;	/* nr of 64-bit words.. */
 
-			/* main loop using 64byte blocks */
-			zero = 0;
-			count64 = count >> 3;
-			while (count64) { 
+	while (unlikely(len >= 64)) {
 		asm("addq 0*8(%[src]),%[res]\n\t"
 		    "adcq 1*8(%[src]),%[res]\n\t"
 		    "adcq 2*8(%[src]),%[res]\n\t"
@@ -77,64 +56,85 @@ static unsigned do_csum(const unsigned char *buff, unsigned len)
 		    "adcq 5*8(%[src]),%[res]\n\t"
 		    "adcq 6*8(%[src]),%[res]\n\t"
 		    "adcq 7*8(%[src]),%[res]\n\t"
-				    "adcq %[zero],%[res]"
-				    : [res] "=r" (result)
-				    : [src] "r" (buff), [zero] "r" (zero),
-				    "[res]" (result));
+		    "adcq $0,%[res]"
+		    : [res] "+r" (temp64)
+		    : [src] "r" (buff)
+		    : "memory");
 		buff += 64;
-				count64--;
+		len -= 64;
 	}
 
-			/* last up to 7 8byte blocks */
-			count %= 8; 
-			while (count) { 
-				asm("addq %1,%0\n\t"
-				    "adcq %2,%0\n" 
-					    : "=r" (result)
-				    : "m" (*(unsigned long *)buff), 
-				    "r" (zero),  "0" (result));
-				--count; 
+	if (len & 32) {
+		asm("addq 0*8(%[src]),%[res]\n\t"
+		    "adcq 1*8(%[src]),%[res]\n\t"
+		    "adcq 2*8(%[src]),%[res]\n\t"
+		    "adcq 3*8(%[src]),%[res]\n\t"
+		    "adcq $0,%[res]"
+			: [res] "+r" (temp64)
+			: [src] "r" (buff)
+			: "memory");
+		buff += 32;
+	}
+	if (len & 16) {
+		asm("addq 0*8(%[src]),%[res]\n\t"
+		    "adcq 1*8(%[src]),%[res]\n\t"
+		    "adcq $0,%[res]"
+			: [res] "+r" (temp64)
+			: [src] "r" (buff)
+			: "memory");
+		buff += 16;
+	}
+	if (len & 8) {
+		asm("addq 0*8(%[src]),%[res]\n\t"
+		    "adcq $0,%[res]"
+			: [res] "+r" (temp64)
+			: [src] "r" (buff)
+			: "memory");
 		buff += 8;
 	}
-			result = add32_with_carry(result>>32,
-						  result&0xffffffff); 
+	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;
 
+		asm("addq %[trail],%[res]\n\t"
+		    "adcq $0,%[res]"
+			: [res] "+r" (temp64)
+			: [trail] "r" (trail));
+#else
 		if (len & 4) {
-				result += *(unsigned int *) buff;
+			asm("addq %[val],%[res]\n\t"
+			    "adcq $0,%[res]"
+				: [res] "+r" (temp64)
+				: [val] "r" ((u64)*(u32 *)buff)
+				: "memory");
 			buff += 4;
 		}
-		}
 		if (len & 2) {
-			result += *(unsigned short *) buff;
+			asm("addq %[val],%[res]\n\t"
+			    "adcq $0,%[res]"
+				: [res] "+r" (temp64)
+				: [val] "r" ((u64)*(u16 *)buff)
+				: "memory");
 			buff += 2;
 		}
+		if (len & 1) {
+			asm("addq %[val],%[res]\n\t"
+			    "adcq $0,%[res]"
+				: [res] "+r" (temp64)
+				: [val] "r" ((u64)*(u8 *)buff)
+				: "memory");
+		}
+#endif
 	}
-	if (len & 1)
-		result += *buff;
-	result = add32_with_carry(result>>32, result & 0xffffffff); 
+	result = add32_with_carry(temp64 >> 32, temp64 & 0xffffffff);
 	if (unlikely(odd)) {
 		result = from32to16(result);
 		result = ((result >> 8) & 0xff) | ((result & 0xff) << 8);
 	}
-	return result;
-}
-
-/*
- * computes the checksum of a memory block at buff, length len,
- * and adds in "sum" (32-bit)
- *
- * returns a 32-bit number suitable for feeding into itself
- * or csum_tcpudp_magic
- *
- * this function must be called with even lengths, except
- * for the last fragment, which may be odd
- *
- * it's best to have buff aligned on a 64-bit boundary
- */
-__wsum csum_partial(const void *buff, int len, __wsum sum)
-{
-	return (__force __wsum)add32_with_carry(do_csum(buff, len),
-						(__force u32)sum);
+	return (__force __wsum)result;
 }
 EXPORT_SYMBOL(csum_partial);
 
@@ -147,4 +147,3 @@ __sum16 ip_compute_csum(const void *buff, int len)
 	return csum_fold(csum_partial(buff,len,0));
 }
 EXPORT_SYMBOL(ip_compute_csum);
-