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=-2.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 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 3251CC433E0 for ; Mon, 6 Jul 2020 16:26:35 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 06D5220723 for ; Mon, 6 Jul 2020 16:26:35 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1594052795; bh=ELC0Qf8GBOcW7d74k7xE/syM0cqkEfG1ZwnEhNH0Xwc=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:List-ID: From; b=q60DY+LPcS8n2cQhGXyaax9sJWsLabV3ts5st9EcnaTa/Q9iFyUeMNzOXLTmRFDPt h94FE0SRcYpGFUKfph4mbVWmgoo8FBeNrSzltN+kS0NLm0Kplem23QbihPP5lzaQf3 qCluMasb7nYHCANsxfuJupLjI0U0SCa66Cf+q3Ag= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729384AbgGFQ0e (ORCPT ); Mon, 6 Jul 2020 12:26:34 -0400 Received: from mail.kernel.org ([198.145.29.99]:59546 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729358AbgGFQ0e (ORCPT ); Mon, 6 Jul 2020 12:26:34 -0400 Received: from paulmck-ThinkPad-P72.home (50-39-111-31.bvtn.or.frontiernet.net [50.39.111.31]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id C63D220702; Mon, 6 Jul 2020 16:26:33 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1594052793; bh=ELC0Qf8GBOcW7d74k7xE/syM0cqkEfG1ZwnEhNH0Xwc=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=NWnqF8yNh1kmeojBtU/hvJnKQi+pZfEw8tRN6xyWsQTMlbnwqrlnw5lTOXvjlFbT7 B5rw2d8haDBAV5oFbA0B0HdNl5tN/Er570PuYfYx1NWaxj3XhsCuq3jsWx3nC14oRK 3IoGxJUeXAP3v3i6Usrl18oSx/B8BE0q7l0vU15Q= Received: by paulmck-ThinkPad-P72.home (Postfix, from userid 1000) id B23463521502; Mon, 6 Jul 2020 09:26:33 -0700 (PDT) Date: Mon, 6 Jul 2020 09:26:33 -0700 From: "Paul E. McKenney" To: Peter Zijlstra Cc: Marco Elver , Nick Desaulniers , Sami Tolvanen , Masahiro Yamada , Will Deacon , Greg Kroah-Hartman , Kees Cook , clang-built-linux , Kernel Hardening , linux-arch , Linux ARM , Linux Kbuild mailing list , LKML , linux-pci@vger.kernel.org, "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" Subject: Re: [PATCH 00/22] add support for Clang LTO Message-ID: <20200706162633.GA13288@paulmck-ThinkPad-P72> Reply-To: paulmck@kernel.org References: <20200630203016.GI9247@paulmck-ThinkPad-P72> <20200701114027.GO4800@hirez.programming.kicks-ass.net> <20200701140654.GL9247@paulmck-ThinkPad-P72> <20200701150512.GH4817@hirez.programming.kicks-ass.net> <20200701160338.GN9247@paulmck-ThinkPad-P72> <20200702082040.GB4781@hirez.programming.kicks-ass.net> <20200702175948.GV9247@paulmck-ThinkPad-P72> <20200703131330.GX4800@hirez.programming.kicks-ass.net> <20200703144228.GF9247@paulmck-ThinkPad-P72> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200703144228.GF9247@paulmck-ThinkPad-P72> User-Agent: Mutt/1.9.4 (2018-02-28) Sender: linux-pci-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-pci@vger.kernel.org On Fri, Jul 03, 2020 at 07:42:28AM -0700, Paul E. McKenney wrote: > On Fri, Jul 03, 2020 at 03:13:30PM +0200, Peter Zijlstra wrote: > > On Thu, Jul 02, 2020 at 10:59:48AM -0700, Paul E. McKenney wrote: > > > On Thu, Jul 02, 2020 at 10:20:40AM +0200, Peter Zijlstra wrote: > > > > On Wed, Jul 01, 2020 at 09:03:38AM -0700, Paul E. McKenney wrote: [ . . . ] > > Also, if C goes and specifies load dependencies, in any form, is then > > not the corrolary that they need to specify control dependencies? How > > else can they exclude the transformation. > > By requiring that any temporaries generated from variables that are > marked _Dependent_ptr also be marked _Dependent_ptr. This is of course > one divergence of _Dependent_ptr from the volatile keyword. > > > And of course, once we're there, can we get explicit support for control > > dependencies too? :-) :-) > > Keep talking like this and I am going to make sure that you attend a > standards committee meeting. If need be, by arranging for you to be > physically dragged there. ;-) > > More seriously, for control dependencies, the variable that would need > to be marked would be the program counter, which might require some > additional syntax. And perhaps more constructively, we do need to prioritize address and data dependencies over control dependencies. For one thing, there are a lot more address/data dependencies in existing code than there are control dependencies, and (sadly, perhaps more importantly) there are a lot more people who are convinced that address/data dependencies are important. For another (admittedly more theoretical) thing, the OOTA scenarios stemming from control dependencies are a lot less annoying than those from address/data dependencies. And address/data dependencies are as far as I know vulnerable to things like conditional-move instructions that can cause problems for control dependencies. Nevertheless, yes, control dependencies also need attention. Thanx, Paul