arch/arm64/lib/csum.c

   1 // SPDX-License-Identifier: GPL-2.0-only
   2 // Copyright (C) 2019-2020 Arm Ltd.
   3
   4 #include <linux/compiler.h>
   5 #include <linux/kasan-checks.h>
   6 #include <linux/kernel.h>
   7
   8 #include <net/checksum.h>
   9
  10 /* Looks dumb, but generates nice-ish code */
  11 static u64 accumulate(u64 sum, u64 data)
  12 {
  13         __uint128_t tmp = (__uint128_t)sum + data;
  14         return tmp + (tmp >> 64);
  15 }
  16
  17 unsigned int do_csum(const unsigned char *buff, int len)
  18 {
  19         unsigned int offset, shift, sum;
  20         const u64 *ptr;
  21         u64 data, sum64 = 0;
  22
  23         if (unlikely(len == 0))
  24                 return 0;
  25
  26         offset = (unsigned long)buff & 7;
  27         /*
  28          * This is to all intents and purposes safe, since rounding down cannot
  29          * result in a different page or cache line being accessed, and @buff
  30          * should absolutely not be pointing to anything read-sensitive. We do,
  31          * however, have to be careful not to piss off KASAN, which means using
  32          * unchecked reads to accommodate the head and tail, for which we'll
  33          * compensate with an explicit check up-front.
  34          */
  35         kasan_check_read(buff, len);
  36         ptr = (u64 *)(buff - offset);
  37         len = len + offset - 8;
  38
  39         /*
  40          * Head: zero out any excess leading bytes. Shifting back by the same
  41          * amount should be at least as fast as any other way of handling the
  42          * odd/even alignment, and means we can ignore it until the very end.
  43          */
  44         shift = offset * 8;
  45         data = READ_ONCE_NOCHECK(*ptr++);
  46 #ifdef __LITTLE_ENDIAN
  47         data = (data >> shift) << shift;
  48 #else
  49         data = (data << shift) >> shift;
  50 #endif
  51
  52         /*
  53          * Body: straightforward aligned loads from here on (the paired loads
  54          * underlying the quadword type still only need dword alignment). The
  55          * main loop strictly excludes the tail, so the second loop will always
  56          * run at least once.
  57          */
  58         while (unlikely(len > 64)) {
  59                 __uint128_t tmp1, tmp2, tmp3, tmp4;
  60
  61                 tmp1 = READ_ONCE_NOCHECK(*(__uint128_t *)ptr);
  62                 tmp2 = READ_ONCE_NOCHECK(*(__uint128_t *)(ptr + 2));
  63                 tmp3 = READ_ONCE_NOCHECK(*(__uint128_t *)(ptr + 4));
  64                 tmp4 = READ_ONCE_NOCHECK(*(__uint128_t *)(ptr + 6));
  65
  66                 len -= 64;
  67                 ptr += 8;
  68
  69                 /* This is the "don't dump the carry flag into a GPR" idiom */
  70                 tmp1 += (tmp1 >> 64) | (tmp1 << 64);
  71                 tmp2 += (tmp2 >> 64) | (tmp2 << 64);
  72                 tmp3 += (tmp3 >> 64) | (tmp3 << 64);
  73                 tmp4 += (tmp4 >> 64) | (tmp4 << 64);
  74                 tmp1 = ((tmp1 >> 64) << 64) | (tmp2 >> 64);
  75                 tmp1 += (tmp1 >> 64) | (tmp1 << 64);
  76                 tmp3 = ((tmp3 >> 64) << 64) | (tmp4 >> 64);
  77                 tmp3 += (tmp3 >> 64) | (tmp3 << 64);
  78                 tmp1 = ((tmp1 >> 64) << 64) | (tmp3 >> 64);
  79                 tmp1 += (tmp1 >> 64) | (tmp1 << 64);
  80                 tmp1 = ((tmp1 >> 64) << 64) | sum64;
  81                 tmp1 += (tmp1 >> 64) | (tmp1 << 64);
  82                 sum64 = tmp1 >> 64;
  83         }
  84         while (len > 8) {
  85                 __uint128_t tmp;
  86
  87                 sum64 = accumulate(sum64, data);
  88                 tmp = READ_ONCE_NOCHECK(*(__uint128_t *)ptr);
  89
  90                 len -= 16;
  91                 ptr += 2;
  92
  93 #ifdef __LITTLE_ENDIAN
  94                 data = tmp >> 64;
  95                 sum64 = accumulate(sum64, tmp);
  96 #else
  97                 data = tmp;
  98                 sum64 = accumulate(sum64, tmp >> 64);
  99 #endif
 100         }
 101         if (len > 0) {
 102                 sum64 = accumulate(sum64, data);
 103                 data = READ_ONCE_NOCHECK(*ptr);
 104                 len -= 8;
 105         }
 106         /*
 107          * Tail: zero any over-read bytes similarly to the head, again
 108          * preserving odd/even alignment.
 109          */
 110         shift = len * -8;
 111 #ifdef __LITTLE_ENDIAN
 112         data = (data << shift) >> shift;
 113 #else
 114         data = (data >> shift) << shift;
 115 #endif
 116         sum64 = accumulate(sum64, data);
 117
 118         /* Finally, folding */
 119         sum64 += (sum64 >> 32) | (sum64 << 32);
 120         sum = sum64 >> 32;
 121         sum += (sum >> 16) | (sum << 16);
 122         if (offset & 1)
 123                 return (u16)swab32(sum);
 124
 125         return sum >> 16;
 126 }