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 129BDC433F5 for ; Thu, 2 Dec 2021 15:06:36 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1358701AbhLBPJt convert rfc822-to-8bit (ORCPT ); Thu, 2 Dec 2021 10:09:49 -0500 Received: from eu-smtp-delivery-151.mimecast.com ([185.58.85.151]:31554 "EHLO eu-smtp-delivery-151.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1358403AbhLBPJf (ORCPT ); Thu, 2 Dec 2021 10:09:35 -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-190-L0sbvhYUMzm2UAavlZ4DdQ-1; Thu, 02 Dec 2021 15:06:07 +0000 X-MC-Unique: L0sbvhYUMzm2UAavlZ4DdQ-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; Thu, 2 Dec 2021 15:06:06 +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; Thu, 2 Dec 2021 15:06:06 +0000 From: David Laight To: 'Vladimir Oltean' , Eric Dumazet CC: "David S . Miller" , Jakub Kicinski , netdev , Eric Dumazet 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: AQHX534DDWxMByH9C0Scwj/3q1CYoawfSXcQ Date: Thu, 2 Dec 2021 15:06:06 +0000 Message-ID: <88b82ae31dc54a4c8b2173487f61ffe9@AcuMS.aculab.com> References: <20211124202446.2917972-1-eric.dumazet@gmail.com> <20211124202446.2917972-3-eric.dumazet@gmail.com> <20211202131040.rdxzbfwh2slhftg5@skbuf> In-Reply-To: <20211202131040.rdxzbfwh2slhftg5@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 13:11 ... > > --- a/include/linux/skbuff.h > > +++ b/include/linux/skbuff.h > > @@ -3485,7 +3485,11 @@ __skb_postpull_rcsum(struct sk_buff *skb, const void *start, unsigned int > len, > > static inline void skb_postpull_rcsum(struct sk_buff *skb, > > const void *start, unsigned int len) > > { > > - __skb_postpull_rcsum(skb, start, len, 0); > > + if (skb->ip_summed == CHECKSUM_COMPLETE) > > + skb->csum = ~csum_partial(start, len, ~skb->csum); You can't do that, the domain is 1..0xffff (or maybe 0xffffffff). The invert has to convert ~0 to ~0 not zero. ... > There seems to be a disparity when the skb->csum is calculated by > skb_postpull_rcsum as zero. Before, it was calculated as 0xffff. Which is what that will do for some inputs at least. Maybe: skb->csum = 1 + ~csum_partial(start, len, ~skb->csum + 1); is right. I think that is the same as: skb->csum = -csum_partial(start, len, -skb->csum); Although letting the compiler do that transform probably makes the code easier to read. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)