From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 5A07EC433EF for ; Fri, 3 Dec 2021 14:57:12 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1381603AbhLCPAf convert rfc822-to-8bit (ORCPT ); Fri, 3 Dec 2021 10:00:35 -0500 Received: from eu-smtp-delivery-151.mimecast.com ([185.58.86.151]:28019 "EHLO eu-smtp-delivery-151.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1381558AbhLCPAc (ORCPT ); Fri, 3 Dec 2021 10:00:32 -0500 Received: from AcuMS.aculab.com (156.67.243.121 [156.67.243.121]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA384) id uk-mta-87-7C6-g3jKNyKLlUs9HDGzMQ-1; Fri, 03 Dec 2021 14:57:05 +0000 X-MC-Unique: 7C6-g3jKNyKLlUs9HDGzMQ-1 Received: from AcuMS.Aculab.com (fd9f:af1c:a25b:0:994c:f5c2:35d6:9b65) by AcuMS.aculab.com (fd9f:af1c:a25b:0:994c:f5c2:35d6:9b65) with Microsoft SMTP Server (TLS) id 15.0.1497.26; Fri, 3 Dec 2021 14:57:04 +0000 Received: from AcuMS.Aculab.com ([fe80::994c:f5c2:35d6:9b65]) by AcuMS.aculab.com ([fe80::994c:f5c2:35d6:9b65%12]) with mapi id 15.00.1497.026; Fri, 3 Dec 2021 14:57:04 +0000 From: David Laight To: 'Vladimir Oltean' CC: Eric Dumazet , Eric Dumazet , "David S . Miller" , Jakub Kicinski , netdev Subject: RE: [PATCH net-next 2/2] net: optimize skb_postpull_rcsum() Thread-Topic: [PATCH net-next 2/2] net: optimize skb_postpull_rcsum() Thread-Index: AQHX57zbDWxMByH9C0Scwj/3q1CYoawfrjxggAAMpYCAASAbAA== Date: Fri, 3 Dec 2021 14:57:04 +0000 Message-ID: References: <20211124202446.2917972-1-eric.dumazet@gmail.com> <20211124202446.2917972-3-eric.dumazet@gmail.com> <20211202131040.rdxzbfwh2slhftg5@skbuf> <20211202162916.ieb2wn35z5h4aubh@skbuf> <20211202204036.negad3mnrm2gogjd@skbuf> <9eefc224988841c9b1a0b6c6eb3348b8@AcuMS.aculab.com> <20211202214009.5hm3diwom4qsbsjd@skbuf> In-Reply-To: <20211202214009.5hm3diwom4qsbsjd@skbuf> Accept-Language: en-GB, en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: x-ms-exchange-transport-fromentityheader: Hosted x-originating-ip: [10.202.205.107] MIME-Version: 1.0 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=C51A453 smtp.mailfrom=david.laight@aculab.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: aculab.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8BIT Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org From: Vladimir Oltean > Sent: 02 December 2021 21:40 ... > > > > Try replacing both ~ with -. > > So replace: > > skb->csum = ~csum_partial(start, len, ~skb->csum); > > with: > > skb->csum = -csum_partial(start, len, -skb->csum); > > > > That should geneate ~0u instead 0 (if I've got my maths right). > > Indeed, replacing both one's complement operations with two's complement > seems to produce correct results (consistent with old code) in all cases > that I am testing with (ICMP, TCP, UDP). Thanks! You need to generate (or persuade Eric to generate) a patch. I don't have the right source tree. Any code that does ~csum_partial() is 'dubious' unless followed by a check for 0. The two's compliment negate save the conditional - provided the offset of 1 can be added in earlier. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)