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:

quoted

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

[...]

quoted

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@suse.de>
+ *
+ */
+
+#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.

quoted

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@suse.de>
+ *
+ */
+
+#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.

quoted

+
+#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