On 2018/7/17 3:13 PM, Eric Biggers wrote: > On Tue, Jul 17, 2018 at 02:25:24PM +0800, Coly Li wrote: >> 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. > > No, the implementation never loads multi-byte values, so CPU endianness doesn't > matter for the input. CPU endianness *does* matter when serializing the final If the checksum is generated on big endian machine and checked on little endian machine, non-specific endianness will be problematic. > calculated CRC into a byte array for storing on-disk, so maybe bcache gets that > part wrong, I don't know. Either way, that has nothing to do with how the > polynomial coefficients (bits) are ordered *within bytes*, which is what the > "_be" and "_le" refer to in the CRC-32 implementation. Yes, the naming is > unfortunate as it can easily be confused with the usual "bytewise" endianness, > but you need to understand it. > I see, it seems I misunderstand _le and _be in CRC-32 implementation. OK, I will find a way to fix the naming and data type issues in v3 series. > Again, using __le64 makes absolutely no sense. You're even doing operations > like shifts directly on a "__le64" which sparse will (correctly) complain about. > Sure, you are correct here :-) >> >> 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. > > Well, your response didn't actually address my points. But it raises the > question: if there won't be any other users, then why move CRC-64 to lib/ at > all? > The only motivation I can see is becachefs, which share part of the code base with bcache, including crc64 calculation. And before CPU supports build-in instructors for CRC64, I don't see the reason why people should use 64bit CRC other than 32bit ones. >> >> >>>> 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 ? > > As I said, the "normal" convention is the same as "big endian", and the > "reversed" convention is the same as "little endian" (again, meaning "bitwise" > endianness, not the usual "bytewise" endianness). The polynomial is correct but > you are claiming the polynomial coefficients are mapped to bits in a different > order than they actually are. Copied, thanks for the hint :-) 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
