On 2018/7/17 11:34 AM, Eric Biggers wrote: > Hi Coly, > > On Tue, Jul 17, 2018 at 12:55:05AM +0800, Coly Li wrote: >> This patch adds the re-write crc64 calculation routines for Linux kernel. >> The CRC64 polynomical arithmetic follows ECMA-182 specification, inspired >> by CRC paper of Dr. Ross N. Williams >> (see http://www.ross.net/crc/download/crc_v3.txt) and other public domain >> implementations. >> >> All the changes work in this way, >> - When Linux kernel is built, host program lib/gen_crc64table.c will be >> compiled to lib/gen_crc64table and executed. >> - The output of gen_crc64table execution is an array called as lookup >> table (a.k.a POLY 0x42f0e1eba9ea369) which contain 256 64bits-long >> numbers, this talbe is dumped into header file lib/crc64table.h. >> - Then the header file is included by lib/crc64.c for normal 64bit crc >> calculation. >> - Function declaration of the crc64 calculation routines is placed in >> include/linux/crc64.h >> > [...] >> diff --git a/lib/crc64.c b/lib/crc64.c >> new file mode 100644 >> index 000000000000..03f078303bd3 >> --- /dev/null >> +++ b/lib/crc64.c >> @@ -0,0 +1,71 @@ >> +// SPDX-License-Identifier: GPL-2.0 >> +/* >> + * Normal 64bit CRC calculation. >> + * >> + * This is a basic crc64 implementation following ECMA-182 specification, >> + * which can be found from, >> + * http://www.ecma-international.org/publications/standards/Ecma-182.htm >> + * >> + * Dr. Ross N. Williams has a great document to introduce the idea of CRC >> + * algorithm, here the CRC64 code is also inspired by the table-driven >> + * algorithm and detail example from this paper. This paper can be found >> + * from, >> + * http://www.ross.net/crc/download/crc_v3.txt >> + * >> + * crc64table_le[256] is the lookup table of a table-driver 64bit CRC >> + * calculation, which is generated by gen_crc64table.c in kernel build >> + * time. The polynomial of crc64 arithmetic is from ECMA-182 specification >> + * as well, which is defined as, >> + * >> + * x^64 + x^62 + x^57 + x^55 + x^54 + x^53 + x^52 + x^47 + x^46 + x^45 + >> + * x^40 + x^39 + x^38 + x^37 + x^35 + x^33 + x^32 + x^31 + x^29 + x^27 + >> + * x^24 + x^23 + x^22 + x^21 + x^19 + x^17 + x^13 + x^12 + x^10 + x^9 + >> + * x^7 + x^4 + x + 1 >> + * >> + * Copyright 2018 SUSE Linux. >> + * Author: Coly Li <colyli@xxxxxxx> >> + * >> + */ >> + >> +#include <linux/module.h> >> +#include <uapi/linux/types.h> >> +#include "crc64table.h" >> + >> +MODULE_DESCRIPTION("CRC64 calculations"); >> +MODULE_LICENSE("GPL"); >> + >> +__le64 crc64_le_update(__le64 crc, const void *_p, size_t len) >> +{ >> + size_t i, t; >> + >> + const unsigned char *p = _p; >> + >> + for (i = 0; i < len; i++) { >> + t = ((crc >> 56) ^ (__le64)(*p++)) & 0xFF; >> + crc = crc64table_le[t] ^ (crc << 8); >> + } >> + >> + return crc; >> +} >> +EXPORT_SYMBOL_GPL(crc64_le_update); >> + >> +__le64 crc64_le(const void *p, size_t len) >> +{ >> + __le64 crc = 0x0000000000000000ULL; >> + >> + crc = crc64_le_update(crc, p, len); >> + >> + return crc; >> +} >> +EXPORT_SYMBOL_GPL(crc64_le); >> + >> +/* For checksum calculation in drivers/md/bcache/ */ >> +__le64 crc64_le_bch(const void *p, size_t len) >> +{ >> + __le64 crc = 0xFFFFFFFFFFFFFFFFULL; >> + >> + crc = crc64_le_update(crc, p, len); >> + >> + return (crc ^ 0xFFFFFFFFFFFFFFFFULL); >> +} >> +EXPORT_SYMBOL_GPL(crc64_le_bch); > Hi Eric, > Using __le64 here makes no sense, because that type indicates the endianness of > the *bytes*, whereas with CRC's "little endian" and "big endian" refer to the > order in which the *bits* are mapped to the polynomial coefficients. > > Also as you can see for lib/crc32.c you really only need to provide a function > > u64 __pure crc64_le(u64 crc, unsigned char const *p, size_t len); > > and the callers can invert at the beginning and/or end if needed. Let me explain why I explicit use __le64 here. When crc64 is used as on-disk checksum, the input of crc64 calculation should be in a explicit specific byte order. Currently check sum in bcache code assumes the CPU is in little endian and just feeds in-memory data into crc64 calculation, then the code does not work on big endian machine like s390x. To solve such problem, before calculating CRC the in-memory data should be swapped into a specific byte order (in bcache case it should be little endian). For data storage or transfer, CRC calculation without explicit endian is more easy to introduce bugs. When I declare the type of input and output value as __le64, on big endian machine, I expect a type mismatch warning if the input memory buffer is not swapped into little endian. For u64, there is no such type checking warning. This is the initial version of lib/crc64.c, people may add their crc64 calculation routines when necessary, e.g. crc64_be() or crc64(). I only add crc64_le_update() and crc64_le_bch() because bcache code needs them. Indeed there is no user of crc64_le() for now, but the file is name as lib/crc64.c, I think there should be a crc64 calculation at least, so I add crc64_le(). > > Also your function names make it sound like inverting the bits is the exception > or not recommended, since you called the function which does the inversions > "crc32_le_bch()" so it sounds like a bcache-specific hack, while the one that > doesn't do the inversions is simply called "crc32_le()". But actually it's > normally recommended to do CRC's with the inversions, so that leading and > trailing zeroes affect the resulting CRC. > I notice this, normally there are two crc routines provided, with and without inversion. The reason that there is no inversion version is no-user in Linux kernel. Indeed there is no user of crc64_le() in Linnux kernel so far. For performance reason, I doubt whether there will be more user to do 64bit crc in kernel. I prefer two crc32 calculation for a 64bit value, but meta data checksum by crc64 calculation is used in bcache for years, the consistency has to be kept. >> diff --git a/lib/gen_crc64table.c b/lib/gen_crc64table.c >> new file mode 100644 >> index 000000000000..5f292f287498 >> --- /dev/null >> +++ b/lib/gen_crc64table.c >> @@ -0,0 +1,77 @@ >> +// SPDX-License-Identifier: GPL-2.0 >> +/* >> + * Generate lookup table for the talbe-driven CRC64 calculation. >> + * >> + * gen_crc64table is executed in kernel build time and generates >> + * lib/crc64table.h. This header is included by lib/crc64.c for >> + * the table-driver CRC64 calculation. >> + * >> + * See lib/crc64.c for more information about which specification >> + * and polynomical arithmetic that gen_crc64table.c follows to >> + * generate the lookup table. >> + * >> + * Copyright 2018 SUSE Linux. >> + * Author: Coly Li <colyli@xxxxxxx> >> + * >> + */ >> + >> +#include <inttypes.h> >> +#include <linux/swab.h> >> +#include <stdio.h> >> +#include "../usr/include/asm/byteorder.h" >> + >> +#define CRC64_ECMA182_POLY 0x42F0E1EBA9EA3693ULL > > Okay, that's actually the ECMA-182 polynomial in "big endian" form (highest > order bit is the coefficient of x^63, lowest order bit is the coefficient of > x^0), so you're actually doing a "big endian" CRC. So everything in your patch > series that claims it's a little endian or "le" CRC is incorrect. > >> + >> +#ifdef __LITTLE_ENDIAN >> +# define cpu_to_le64(x) ((__le64)(x)) >> +#else >> +# define cpu_to_le64(x) ((__le64)__swab64(x)) >> +#endif >> + >> +static int64_t crc64_table[256] = {0,}; >> + >> +static void generate_crc64_table(void) >> +{ >> + uint64_t i, j, c, crc; >> + >> + for (i = 0; i < 256; i++) { >> + crc = 0; >> + c = i << 56; >> + >> + for (j = 0; j < 8; j++) { >> + if ((crc ^ c) & 0x8000000000000000ULL) >> + crc = (crc << 1) ^ CRC64_ECMA182_POLY; >> + else >> + crc <<= 1; >> + c <<= 1; > > See here, it's shifting out the most significant bit, which means it's the > coefficient of the x^63 term ("big endian" or "normal" convention), not the x^0 > term ("little endian" or "reversed" convention). I see your point here. I am not expert in coding theory, the knowledge I have is from wikipedia, ECMA-182 and the document from Dr. Ross Williams. From ECMA-182 document, I don't see any word with 'big endian', so I take it as a standard poly and regardless the byte order. And on wikepedia page https://en.wikipedia.org/wiki/Cyclic_redundancy_check , CRC-64-ECMA references the same poly and call "0x42F0E1EBA9EA3693" as normal poly, which one links to polynomial "x^64 + x^62 + x^57 + x^55 + x^54 + ....x^7 + x^4 + x + 1" if I understand correctly. But from your information, it seems the polynomial in generate_crc64_table() is x^64 + x^61 ..... Maybe I misunderstand you, could you please give me more hint ? Thanks. Coly Li -- To unsubscribe from this list: send the line "unsubscribe linux-bcache" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
