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Miller" , Jakub Kicinski , netdev , , Alexander Duyck , Andrew Cooper , References: <20211111065322.1261275-1-eric.dumazet@gmail.com> From: Andrew Cooper Subject: Re: [RFC] x86/csum: rewrite csum_partial() Message-ID: <2b36e547-f080-a1ee-f0f0-a9e5fe1e4693@citrix.com> Date: Thu, 11 Nov 2021 18:17:56 +0000 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.14.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable Content-Language: en-GB X-ClientProxiedBy: LO4P123CA0359.GBRP123.PROD.OUTLOOK.COM (2603:10a6:600:18d::22) To BYAPR03MB3623.namprd03.prod.outlook.com (2603:10b6:a02:aa::12) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-Office365-Filtering-Correlation-Id: a8e968b0-96c4-47ab-3e65-08d9a53f99f7 X-MS-TrafficTypeDiagnostic: BYAPR03MB4741: X-Microsoft-Antispam-PRVS: X-MS-Oob-TLC-OOBClassifiers: OLM:7219; X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0; X-Microsoft-Antispam-Message-Info: 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 X-Forefront-Antispam-Report: CIP:255.255.255.255;CTRY:;LANG:en;SCL:1;SRV:;IPV:NLI;SFV:NSPM;H:BYAPR03MB3623.namprd03.prod.outlook.com;PTR:;CAT:NONE;SFS:(4636009)(366004)(2906002)(5660300002)(508600001)(8676002)(2616005)(186003)(4326008)(66946007)(86362001)(956004)(83380400001)(36756003)(6486002)(8936002)(31686004)(38100700002)(16576012)(53546011)(66476007)(54906003)(316002)(66556008)(6666004)(110136005)(31696002)(26005)(966005)(82960400001)(45980500001)(43740500002);DIR:OUT;SFP:1101; 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(I was doing tests for >> (len & 32), (len & 16) .. to not have to update len in these blocks. >> >> Then, I tried to add an inline version, a la ip_fast_csum() but for IPv6= . >> >> Then I came up with the version I sent, for some reason my .config had >> temporarily disabled CONFIG_RETPOLINE, >> thanks for reminding me this ! >> >> I also missed this warning anyway : >> arch/x86/lib/csum-partial_64.o: warning: objtool: csum_partial()+0x2f: >> unannotated intra-function call >> >> I will spend a bit more time on this before sending a V2, thanks again ! > BTW, I could not understand why : > > result =3D add32_with_carry(result, *(u32 *)buff); > > generates this code : > > 123: 41 8b 09 mov (%r9),%ecx > 126: 89 4d f8 mov %ecx,-0x8(%rbp) > 129: 03 45 f8 add -0x8(%rbp),%eax > 12c: 83 d0 00 adc $0x0,%eax Are you using Clang?=C2=A0 There is a long outstanding code generation bug where an "rm" constraint is converted to "m" internally. https://bugs.llvm.org/show_bug.cgi?id=3D47530 Even a stopgap of pretending "rm" means "r" would result in far better code, 99% of the time. > Apparently add32_with_carry() forces the use of use of a temporary in mem= ory > > While > asm(" addl 0*4(%[src]),%[res]\n" > " adcl $0,%[res]\n" > : [res] "=3Dr" (result) > : [src] "r" (buff), "[res]" (result) > : "memory"); Just as a minor note about the asm constraints here and elsewhere =C2=A0=C2=A0=C2=A0 : [res] "=3Dr" (result) =C2=A0=C2=A0=C2=A0 : "res" (result) ought to be just [res] "+r" (result).=C2=A0 The result variable really is read and written by the asm fragments. ~Andrew