From mboxrd@z Thu Jan 1 00:00:00 1970 From: Yerden Zhumabekov Subject: Re: [PATCH 0/2] rewritten rte_hash_crc() call Date: Fri, 14 Nov 2014 22:43:39 +0600 Message-ID: <5466313B.6040407@sts.kz> References: <1409724351-23786-1-git-send-email-e_zhumabekov@sts.kz> <2916837.QJI9btJm0N@xps13> <20141114005211.GC14230@localhost.localdomain> <5465AC00.1070602@sts.kz> <20141114113327.GA19147@hmsreliant.think-freely.org> <5465EE3F.2010404@sts.kz> <20141114135308.GD19147@hmsreliant.think-freely.org> Mime-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable Cc: dev-VfR2kkLFssw@public.gmane.org To: Neil Horman Return-path: In-Reply-To: <20141114135308.GD19147-B26myB8xz7F8NnZeBjwnZQMhkBWG/bsMQH7oEaQurus@public.gmane.org> List-Id: patches and discussions about DPDK List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: dev-bounces-VfR2kkLFssw@public.gmane.org Sender: "dev" 14.11.2014 19:53, Neil Horman =D0=BF=D0=B8=D1=88=D0=B5=D1=82: > On Fri, Nov 14, 2014 at 05:57:51PM +0600, Yerden Zhumabekov wrote: >> 14.11.2014 17:33, Neil Horman =D0=BF=D0=B8=D1=88=D0=B5=D1=82: >>> Not really. That covers the case of applications selecting the hash = function >>> using the DEFUALT_HASH_FUNC macro, but doesn't nothing for applicatio= ns using >>> the function directly. Test_hash_perf is an example of this, and os= tensibly >>> because of the behavior without SSE4.2 it defines these huge test tab= les twice >>> based on the availability of SSE4.2. It would be better if we could = allow >>> applications to use rte_hash_crc regardless, and make the code it use= s at run >>> time configurable. >> I see, then we have a problem here :) >> >> Actually, that was one of my concerns when developing these patches. I= >> looked through the source code of libs and examples and I saw the >> '#ifdef..#include..#endif'-like appoach while selecting hash function >> was common. So I organized patches to minimize the impact on API and n= ot >> to contradict this approach. >> > Thats a reasonable approach, but I really hate the idea of continuing t= his need > to select cpu features at compile time if its not nececcesary. > >> If we prefer to change this approach then, I guess, we need to introdu= ce >> broader changes to rte_hash library and change other code which uses i= t. >> If that's what's needed, then it'll take some time for me to rework >> these patches. >> > Well, its possible you'll get lucky. crc is such a common operation, i= ts > entirely possible that the gcc intrinsic emits software based crc compu= tation if > the SSE4.2 instructions aren't enabled. I recommend modifying the test= _hash_crc > function to use rte_hash_crc with SSE4.2 disabled, and see if you get a= crash. > If you don't examine the disassembly of your new function and confirm t= hat > something reasonable that doesn't use SSE4.2 is emitted. If thats the = case, > your patch is fine, and we can focus on how to change the ifdefs in the= existing > code, as use of the rte_hash_crc functions should be safe. > Unfortunately, it seems not to be the case. Trying to force compiling a test program with _mm_crc32_u32 intrinsic on computer with no SSE4.2 support leads to "Illegal instruction error". So it looks like GCC does not fall back to crc32 software implementation. --=20 Sincerely, Yerden Zhumabekov State Technical Service Astana, KZ