linux-kernel - Re: [PATCH 2/3] x86/hash: swap parameters of crc32

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Message-ID: <530B3BDB.3010908@zytor.com>
Date:	Mon, 24 Feb 2014 04:32:27 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Daniel Borkmann <dborkman@...hat.com>,
	Jan Beulich <JBeulich@...e.com>
CC:	davem@...emloft.net, mingo@...e.hu, tglx@...utronix.de,
	ffusco@...hat.com, tgraf@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] x86/hash: swap parameters of crc32_u32()

On 02/24/2014 03:46 AM, Daniel Borkmann wrote:
>>>>>>
>>>>>> --- 3.14-rc3-x86-hash-crc32.orig/arch/x86/lib/hash.c
>>>>>> +++ 3.14-rc3-x86-hash-crc32/arch/x86/lib/hash.c
>>>>>> @@ -37,7 +37,7 @@
>>>>>>     #include <asm/cpufeature.h>
>>>>>>     #include <asm/hash.h>
>>>>>>
>>>>>> -static inline u32 crc32_u32(u32 crc, u32 val)
>>>>>> +static inline u32 crc32_u32(u32 val, u32 crc)
>>>>>>     {
>>>>>>     #ifdef CONFIG_AS_CRC32
>>>>>>         asm ("crc32l %1,%0\n" : "+r" (crc) : "rm" (val));
>>>>>

OK, this whole tread is really confusing, but the change proposed seems 
actively wrong.

First of all:

static inline uint32_t
rte_hash_crc_4byte(uint32_t data, uint32_t init_val)
{
	return _mm_crc32_u32(data, init_val);
}

... from the DPDK code is confusing all by itself, because the 
definition of the _mm_crc32_u32() intrinsic per the Intel SDM is:

unsigned int _mm_crc32_u32(unsigned int crc, unsigned int data);

... where "crc" is the destination operand, i.e. the accumulator if you 
actually would be computing a CRC32C.

So I'm guessing this hash is deliberately using the CRC32 instruction 
"backwards", which would actually make sense: an actual CRC is actually 
a pretty poor hash due to linearity.

This has confused people elsewhere, too:

http://permalink.gmane.org/gmane.comp.networking.dpdk.devel/954

So if this is a bug it is a bug in the upstream code, but I'm guessing 
the operand reversal is intentional.

Therefore, this patch should be actively NAKed.

Nacked-by: H. Peter Anvin <hpa@...or.com>

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