libxfs/crc32defs.h

   1 /*
   2  * Use slice-by-8, which is the fastest variant.
   3  *
   4  * Calculate checksum 8 bytes at a time with a clever slicing algorithm.
   5  * This is the fastest algorithm, but comes with a 8KiB lookup table.
   6  * Most modern processors have enough cache to hold this table without
   7  * thrashing the cache.
   8  *
   9  * The Linux kernel uses this as the default implementation "unless you
  10  * have a good reason not to".  The reason why Kconfig urges you to pick
  11  * SLICEBY8 is because people challenged the assertion that we should
  12  * always use slice by 8, so Darrick wrote a crc microbenchmark utility
  13  * and ran it on as many machines as he could get his hands on to show
  14  * that sb8 was the fastest.
  15  *
  16  * Every 64-bit machine (and most of the 32-bit ones too) saw the best
  17  * results with sb8.  Any machine with more than 4K of cache saw better
  18  * results.  The spreadsheet still exists today[1]; note that
  19  * 'crc32-kern-le' corresponds to the slice by 4 algorithm which is the
  20  * default unless CRC_LE_BITS is defined explicitly.
  21  *
  22  * FWIW, there are a handful of board defconfigs in the kernel that
  23  * don't pick sliceby8.  These are all embedded 32-bit mips/ppc systems
  24  * with very small cache sizes which experience cache thrashing with the
  25  * slice by 8 algorithm, and therefore chose to pick defaults that are
  26  * saner for their particular board configuration.  For nearly all of
  27  * XFS' perceived userbase (which we assume are 32 and 64-bit machines
  28  * with sufficiently large CPU cache and largeish storage devices) slice
  29  * by 8 is the right choice.
  30  *
  31  * [1] https://goo.gl/0LSzsG ("crc32c_bench")
  32  */
  33 #define CRC_LE_BITS 64
  34
  35 /*
  36  * There are multiple 16-bit CRC polynomials in common use, but this is
  37  * *the* standard CRC-32 polynomial, first popularized by Ethernet.
  38  * x^32+x^26+x^23+x^22+x^16+x^12+x^11+x^10+x^8+x^7+x^5+x^4+x^2+x^1+x^0
  39  */
  40 #define CRCPOLY_LE 0xedb88320
  41 #define CRCPOLY_BE 0x04c11db7
  42
  43 /*
  44  * This is the CRC32c polynomial, as outlined by Castagnoli.
  45  * x^32+x^28+x^27+x^26+x^25+x^23+x^22+x^20+x^19+x^18+x^14+x^13+x^11+x^10+x^9+
  46  * x^8+x^6+x^0
  47  */
  48 #define CRC32C_POLY_LE 0x82F63B78
  49
  50 /* Try to choose an implementation variant via Kconfig */
  51 #ifdef CONFIG_CRC32_SLICEBY8
  52 # define CRC_LE_BITS 64
  53 # define CRC_BE_BITS 64
  54 #endif
  55 #ifdef CONFIG_CRC32_SLICEBY4
  56 # define CRC_LE_BITS 32
  57 # define CRC_BE_BITS 32
  58 #endif
  59 #ifdef CONFIG_CRC32_SARWATE
  60 # define CRC_LE_BITS 8
  61 # define CRC_BE_BITS 8
  62 #endif
  63 #ifdef CONFIG_CRC32_BIT
  64 # define CRC_LE_BITS 1
  65 # define CRC_BE_BITS 1
  66 #endif
  67
  68 /*
  69  * How many bits at a time to use.  Valid values are 1, 2, 4, 8, 32 and 64.
  70  * For less performance-sensitive, use 4 or 8 to save table size.
  71  * For larger systems choose same as CPU architecture as default.
  72  * This works well on X86_64, SPARC64 systems. This may require some
  73  * elaboration after experiments with other architectures.
  74  */
  75 #ifndef CRC_LE_BITS
  76 #  ifdef CONFIG_64BIT
  77 #  define CRC_LE_BITS 64
  78 #  else
  79 #  define CRC_LE_BITS 32
  80 #  endif
  81 #endif
  82 #ifndef CRC_BE_BITS
  83 #  ifdef CONFIG_64BIT
  84 #  define CRC_BE_BITS 64
  85 #  else
  86 #  define CRC_BE_BITS 32
  87 #  endif
  88 #endif
  89
  90 /*
  91  * Little-endian CRC computation.  Used with serial bit streams sent
  92  * lsbit-first.  Be sure to use cpu_to_le32() to append the computed CRC.
  93  */
  94 #if CRC_LE_BITS > 64 || CRC_LE_BITS < 1 || CRC_LE_BITS == 16 || \
  95         CRC_LE_BITS & CRC_LE_BITS-1
  96 # error "CRC_LE_BITS must be one of {1, 2, 4, 8, 32, 64}"
  97 #endif
  98
  99 /*
 100  * Big-endian CRC computation.  Used with serial bit streams sent
 101  * msbit-first.  Be sure to use cpu_to_be32() to append the computed CRC.
 102  */
 103 #if CRC_BE_BITS > 64 || CRC_BE_BITS < 1 || CRC_BE_BITS == 16 || \
 104         CRC_BE_BITS & CRC_BE_BITS-1
 105 # error "CRC_BE_BITS must be one of {1, 2, 4, 8, 32, 64}"
 106 #endif