Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752538AbaBXMcz (ORCPT ); Mon, 24 Feb 2014 07:32:55 -0500 Received: from terminus.zytor.com ([198.137.202.10]:34179 "EHLO mail.zytor.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751722AbaBXMcy (ORCPT ); Mon, 24 Feb 2014 07:32:54 -0500 Message-ID: <530B3BDB.3010908@zytor.com> Date: Mon, 24 Feb 2014 04:32:27 -0800 From: "H. Peter Anvin" User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.3.0 MIME-Version: 1.0 To: Daniel Borkmann , Jan Beulich CC: davem@davemloft.net, mingo@elte.hu, tglx@linutronix.de, ffusco@redhat.com, tgraf@redhat.com, linux-kernel@vger.kernel.org Subject: Re: [PATCH 2/3] x86/hash: swap parameters of crc32_u32() References: <53073986020000780011E2E1@nat28.tlf.novell.com> <5308938A.8070506@redhat.com> <530B0AF2020000780011E97B@nat28.tlf.novell.com> <530B1D60.8010602@redhat.com> <530B32A4020000780011EACB@nat28.tlf.novell.com> <530B3102.4050102@redhat.com> In-Reply-To: <530B3102.4050102@redhat.com> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 >>>>>> #include >>>>>> >>>>>> -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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/