libxfs/crc32defs.h

/*
 * Use slice-by-8, which is the fastest variant.
 *
 * Calculate checksum 8 bytes at a time with a clever slicing algorithm.
 * This is the fastest algorithm, but comes with a 8KiB lookup table.
 * Most modern processors have enough cache to hold this table without
 * thrashing the cache.
 *
 * The Linux kernel uses this as the default implementation "unless you
 * have a good reason not to".  The reason why Kconfig urges you to pick
 * SLICEBY8 is because people challenged the assertion that we should
 * always use slice by 8, so Darrick wrote a crc microbenchmark utility
 * and ran it on as many machines as he could get his hands on to show
 * that sb8 was the fastest.
 *
 * Every 64-bit machine (and most of the 32-bit ones too) saw the best
 * results with sb8.  Any machine with more than 4K of cache saw better
 * results.  The spreadsheet still exists today[1]; note that
 * 'crc32-kern-le' corresponds to the slice by 4 algorithm which is the
 * default unless CRC_LE_BITS is defined explicitly.
 *
 * FWIW, there are a handful of board defconfigs in the kernel that
 * don't pick sliceby8.  These are all embedded 32-bit mips/ppc systems
 * with very small cache sizes which experience cache thrashing with the
 * slice by 8 algorithm, and therefore chose to pick defaults that are
 * saner for their particular board configuration.  For nearly all of
 * XFS' perceived userbase (which we assume are 32 and 64-bit machines
 * with sufficiently large CPU cache and largeish storage devices) slice
 * by 8 is the right choice.
 *
 * [1] https://goo.gl/0LSzsG ("crc32c_bench")
 */
#define CRC_LE_BITS 64

/*
 * There are multiple 16-bit CRC polynomials in common use, but this is
 * *the* standard CRC-32 polynomial, first popularized by Ethernet.
 * 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
 */
#define CRCPOLY_LE 0xedb88320
#define CRCPOLY_BE 0x04c11db7

/*
 * This is the CRC32c polynomial, as outlined by Castagnoli.
 * 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+
 * x^8+x^6+x^0
 */
#define CRC32C_POLY_LE 0x82F63B78

/* Try to choose an implementation variant via Kconfig */
#ifdef CONFIG_CRC32_SLICEBY8
# define CRC_LE_BITS 64
# define CRC_BE_BITS 64
#endif
#ifdef CONFIG_CRC32_SLICEBY4
# define CRC_LE_BITS 32
# define CRC_BE_BITS 32
#endif
#ifdef CONFIG_CRC32_SARWATE
# define CRC_LE_BITS 8
# define CRC_BE_BITS 8
#endif
#ifdef CONFIG_CRC32_BIT
# define CRC_LE_BITS 1
# define CRC_BE_BITS 1
#endif

/*
 * How many bits at a time to use.  Valid values are 1, 2, 4, 8, 32 and 64.
 * For less performance-sensitive, use 4 or 8 to save table size.
 * For larger systems choose same as CPU architecture as default.
 * This works well on X86_64, SPARC64 systems. This may require some
 * elaboration after experiments with other architectures.
 */
#ifndef CRC_LE_BITS
#  ifdef CONFIG_64BIT
#  define CRC_LE_BITS 64
#  else
#  define CRC_LE_BITS 32
#  endif
#endif
#ifndef CRC_BE_BITS
#  ifdef CONFIG_64BIT
#  define CRC_BE_BITS 64
#  else
#  define CRC_BE_BITS 32
#  endif
#endif

/*
 * Little-endian CRC computation.  Used with serial bit streams sent
 * lsbit-first.  Be sure to use cpu_to_le32() to append the computed CRC.
 */
#if CRC_LE_BITS > 64 || CRC_LE_BITS < 1 || CRC_LE_BITS == 16 || \
	CRC_LE_BITS & CRC_LE_BITS-1
# error "CRC_LE_BITS must be one of {1, 2, 4, 8, 32, 64}"
#endif

/*
 * Big-endian CRC computation.  Used with serial bit streams sent
 * msbit-first.  Be sure to use cpu_to_be32() to append the computed CRC.
 */
#if CRC_BE_BITS > 64 || CRC_BE_BITS < 1 || CRC_BE_BITS == 16 || \
	CRC_BE_BITS & CRC_BE_BITS-1
# error "CRC_BE_BITS must be one of {1, 2, 4, 8, 32, 64}"
#endif