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 X-Spam-Level: X-Spam-Status: No, score=-3.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id BCD40C56202 for ; Sun, 22 Nov 2020 13:58:37 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 919BC20789 for ; Sun, 22 Nov 2020 13:58:37 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727795AbgKVN6Q convert rfc822-to-8bit (ORCPT ); Sun, 22 Nov 2020 08:58:16 -0500 Received: from eu-smtp-delivery-151.mimecast.com ([185.58.86.151]:52821 "EHLO eu-smtp-delivery-151.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727740AbgKVN6P (ORCPT ); Sun, 22 Nov 2020 08:58:15 -0500 Received: from AcuMS.aculab.com (156.67.243.126 [156.67.243.126]) (Using TLS) by relay.mimecast.com with ESMTP id uk-mta-83-68wCZSYwMYuMi8rKowULxQ-1; Sun, 22 Nov 2020 13:58:11 +0000 X-MC-Unique: 68wCZSYwMYuMi8rKowULxQ-1 Received: from AcuMS.Aculab.com (fd9f:af1c:a25b:0:43c:695e:880f:8750) by AcuMS.aculab.com (fd9f:af1c:a25b:0:43c:695e:880f:8750) with Microsoft SMTP Server (TLS) id 15.0.1347.2; Sun, 22 Nov 2020 13:58:10 +0000 Received: from AcuMS.Aculab.com ([fe80::43c:695e:880f:8750]) by AcuMS.aculab.com ([fe80::43c:695e:880f:8750%12]) with mapi id 15.00.1347.000; Sun, 22 Nov 2020 13:58:10 +0000 From: David Laight To: 'David Howells' , Linus Torvalds CC: Pavel Begunkov , Matthew Wilcox , Jens Axboe , Alexander Viro , linux-fsdevel , linux-block , Linux Kernel Mailing List Subject: RE: [PATCH 01/29] iov_iter: Switch to using a table of operations Thread-Topic: [PATCH 01/29] iov_iter: Switch to using a table of operations Thread-Index: AQHWwNQxMI88twW4zUSLxoYAnZFgBanUKLlw Date: Sun, 22 Nov 2020 13:58:10 +0000 Message-ID: <4890290b302e480fb0d1cc66bd0d6ce9@AcuMS.aculab.com> References: <160596800145.154728.7192318545120181269.stgit@warthog.procyon.org.uk> <160596801020.154728.15935034745159191564.stgit@warthog.procyon.org.uk> <254318.1606051984@warthog.procyon.org.uk> In-Reply-To: <254318.1606051984@warthog.procyon.org.uk> 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: linux-fsdevel@vger.kernel.org From: David Howells > Sent: 22 November 2020 13:33 > > Linus Torvalds wrote: > > > - I worry a bit about the indirect call overhead and spectre v2. > > I don't know enough about how spectre v2 works to say if this would be a > problem for the ops-table approach, but wouldn't it also affect the chain of > conditional branches that we currently use, since it's branch-prediction > based? The advantage of the 'chain of branches' is that it can be converted into a 'tree of branches' because the values are all separate bits. So as well as putting the (expected) common one first; you can do: if (likely((a & (A | B))) { if (a & A) { code for A; } else { code for B; } else ... So get better control over the branch sequence. (Hopefully the compiler doesn't change the logic. I want a dumb compiler that (mostly) compiles what I write!) Part of the difficulty is deciding the common case. There'll always be a benchmark that exercises an uncommon case. Adding an indirect call does let you do things like adding ITER_IOVER_SINGLE and ITER_KVEC_SINGLE that are used in the common case of a single buffer fragment. That might be a measurable gain. It is also possible to optimise the common case to a direct call (or even inline code) and use an indirect call for everything else. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)