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=-5.8 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,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 D2CB4C56202 for ; Sun, 22 Nov 2020 13:34:05 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 83FEC20771 for ; Sun, 22 Nov 2020 13:34:05 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="GWERhRBH" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727874AbgKVNdP (ORCPT ); Sun, 22 Nov 2020 08:33:15 -0500 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:53139 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727817AbgKVNdO (ORCPT ); Sun, 22 Nov 2020 08:33:14 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1606051993; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=lnPf0qqP5JlhbpeDaQweU3dHG38zjWq/RJM2h9qX9nk=; b=GWERhRBH/+dG1OatJBcPmNkGouYC/T7pfwjb2l1LWr6CWIas3uB1uzuD1+h8DRYIw6ob5W E7Vn7hwOWhZ9GsffLhDmIHFNjPxkVdDioZBu+tniYOY3mpdnmHHV7cyP1PWUdiuoKMt4K5 F9RNpa80F1faApglPwcWeD5BOzs5ngE= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-557-Z1FtuahSPYu6b0Cj1_fiog-1; Sun, 22 Nov 2020 08:33:09 -0500 X-MC-Unique: Z1FtuahSPYu6b0Cj1_fiog-1 Received: from smtp.corp.redhat.com (int-mx07.intmail.prod.int.phx2.redhat.com [10.5.11.22]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 63AB0809DC3; Sun, 22 Nov 2020 13:33:07 +0000 (UTC) Received: from warthog.procyon.org.uk (ovpn-112-246.rdu2.redhat.com [10.10.112.246]) by smtp.corp.redhat.com (Postfix) with ESMTP id 5B97710016F7; Sun, 22 Nov 2020 13:33:05 +0000 (UTC) Organization: Red Hat UK Ltd. Registered Address: Red Hat UK Ltd, Amberley Place, 107-111 Peascod Street, Windsor, Berkshire, SI4 1TE, United Kingdom. Registered in England and Wales under Company Registration No. 3798903 From: David Howells In-Reply-To: References: <160596800145.154728.7192318545120181269.stgit@warthog.procyon.org.uk> <160596801020.154728.15935034745159191564.stgit@warthog.procyon.org.uk> To: Linus Torvalds Cc: dhowells@redhat.com, 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 MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-ID: <254317.1606051984.1@warthog.procyon.org.uk> Date: Sun, 22 Nov 2020 13:33:04 +0000 Message-ID: <254318.1606051984@warthog.procyon.org.uk> X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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? David