Received: by 10.213.65.68 with SMTP id h4csp3322269imn; Tue, 3 Apr 2018 02:53:19 -0700 (PDT) X-Google-Smtp-Source: AIpwx4/aQfyclYhxWDbE9/5zBg5+YF/i0bXRI1tOj8uZcP726xK3PDn+m3Tt9/5gFE/75AYCdOuq X-Received: by 10.98.141.20 with SMTP id z20mr9950404pfd.144.1522749199238; Tue, 03 Apr 2018 02:53:19 -0700 (PDT) ARC-Seal: i=1; a=rsa-sha256; t=1522749199; cv=none; d=google.com; s=arc-20160816; b=rxU9DN1k5Vv2jRCwcx3H9uW3WnSAWIopCqD4bRrXi5yHunwsR5MGoNgCsrNlOydnNu Lmjx7HJ38YEdLsBZuDNaicPQi+TzI8oX/WBpAfTzqB9ceu++hMSP53GSETrTeMD9YUa/ 6PiFuBlZcvpKxS9WQJAlN1y+LYUm9QXfXrI9etuLtJS70/vvPTjebwgjTffj3GUCKzNm BOgamnNlxc3ad4of54dP/HDtg8rPYHfKsxM2xr7YYgrDvhBplj6EejMX074wUsuxJpod UGxprueu00cfzyelqBCq6qc1WN07F7aSVnExYEEixRJeBBnAeVI/zS7cBGhRGT6+UvES 26qQ== ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=arc-20160816; h=list-id:precedence:sender:user-agent:in-reply-to :content-disposition:mime-version:references:message-id:subject:cc :to:from:date:dkim-signature:arc-authentication-results; bh=Ja6iKq+Wt4mOhU+RrBF12EgDbbD9Nt8A9CfCaDWw6F0=; b=ut+i59HgYiS957zWUCakkGDiWAUKXCTj2Cc/7jmTAd42jpBG+dZNeuI6TBUIXKxwX3 JCC0Pq4eaVVaSoXOBPcGDCeDsv3UoO5nQdD6wmKcTKckphndy+MLrDk6GSh2jTosTsyf CZHj8/FMSStEKWynmtDq1MYYtjeis7dgp7UN9eiMuFvmkqX2of1OvVAK/od8MOJHkRQ8 mgdc8Te4daln2YpJwMlhJY/+6uH+v7s7G058qx1x6KZorXFlUptY6HbZRrFYo5HD+4hQ yVdvkIzuWQlNbhPAsb5G88Hs8hp2TRJIg68YnJcLwnscwFiBhoPhY6n+UUzEliXMFkwm 3XfA== ARC-Authentication-Results: i=1; mx.google.com; dkim=fail header.i=@gmail.com header.s=20161025 header.b=C4EXsuZ9; spf=pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org Return-Path: Received: from vger.kernel.org (vger.kernel.org. [209.132.180.67]) by mx.google.com with ESMTP id f33-v6si157781plb.585.2018.04.03.02.53.05; Tue, 03 Apr 2018 02:53:19 -0700 (PDT) Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67; Authentication-Results: mx.google.com; dkim=fail header.i=@gmail.com header.s=20161025 header.b=C4EXsuZ9; spf=pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755060AbeDCJvY (ORCPT + 99 others); Tue, 3 Apr 2018 05:51:24 -0400 Received: from mail-wm0-f67.google.com ([74.125.82.67]:38811 "EHLO mail-wm0-f67.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755014AbeDCJvW (ORCPT ); Tue, 3 Apr 2018 05:51:22 -0400 Received: by mail-wm0-f67.google.com with SMTP id i3so9983671wmf.3 for ; Tue, 03 Apr 2018 02:51:22 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=sender:date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to:user-agent; bh=Ja6iKq+Wt4mOhU+RrBF12EgDbbD9Nt8A9CfCaDWw6F0=; b=C4EXsuZ9124jJ0vc/j+FPrhHJjVpjsropKMwdgqM//I5IU2x/ubZBG80uDIkVZ3BWx txhcbGoed3ewc26E8CBq6PAw56TE6jR24X9frzzDbtXE2iaYbLVLJXBUgI4hEvklSlWL 5i8iosB+4oVnQu63uM4fhba2aiVjJn+FVwQKLH/dJQql6iRvT01ACwuwi/jNsDyHGvIy md+YlZasUBkj9cVFOa8vYvzvo/i3HTmWVQVGiIEybO7MuZ7PsVWE1+enMqQyYfgZGF2P 3dv9sWMXUAzG69fjMbAbcEteQfNxDWoTQ+YzKd0o+x2FLXxeTjaxLqOLZGHrVaN2bEfx /pig== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:sender:date:from:to:cc:subject:message-id :references:mime-version:content-disposition:in-reply-to:user-agent; bh=Ja6iKq+Wt4mOhU+RrBF12EgDbbD9Nt8A9CfCaDWw6F0=; b=R6Qw5oQJYGgPkbQGpGBI7WndIk8+iIvYjOuVu+HeQBtkYbiLuBYawVdAEW7fPnwgxL 8AbysP7tiQY1GmnZezYT9ElDnxWs/x6N0AER7/IdPe9qtpSopBBGkuGfiFf+Wm3gwAbj FrUgbsLQBoSurf04OPMYpAmsvmwJdEp4xfhdeAZIM/tOFViCxU5U0dYDpLysUcp3DPWy y+CQ+4Etcl5Nzrw5vp6j3RrRQQfuRj/iHlkZie8tqfb8AOIgo3vXbppSswxSAlP1eG32 pPda1Q0rL64NqmLEvwPms2tGkzHiXn+OhCCnEZRLLn0cvhQEdLyNyghZt44wMxeyi5SA 0K3Q== X-Gm-Message-State: AElRT7Hns+amsi6e9yrpt19Y8Jf2aRFhjbqbIQOl4lLIr2xV7mJPOoFq i+B8vd/Gu3omQm9aZ8ON+b8= X-Received: by 10.28.87.73 with SMTP id l70mr3857818wmb.123.1522749081537; Tue, 03 Apr 2018 02:51:21 -0700 (PDT) Received: from gmail.com (2E8B0CD5.catv.pool.telekom.hu. [46.139.12.213]) by smtp.gmail.com with ESMTPSA id y6sm246038wmy.16.2018.04.03.02.51.20 (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Tue, 03 Apr 2018 02:51:20 -0700 (PDT) Date: Tue, 3 Apr 2018 11:51:18 +0200 From: Ingo Molnar To: Peter Zijlstra Cc: Linus Torvalds , Matthias Kaehlcke , Linux Kernel Mailing List , Thomas Gleixner , Andrew Morton Subject: Re: [GIT PULL] x86/build changes for v4.17 Message-ID: <20180403095118.rpf7tj577dppvx7d@gmail.com> References: <20180402095033.nfzcrmxvpm46dhbl@gmail.com> <20180403085904.GY4082@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180403085904.GY4082@hirez.programming.kicks-ass.net> User-Agent: NeoMutt/20170609 (1.8.3) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * Peter Zijlstra wrote: > On Mon, Apr 02, 2018 at 02:44:48PM -0700, Linus Torvalds wrote: > > On Mon, Apr 2, 2018 at 2:50 AM, Ingo Molnar wrote: > > > > > > The biggest change is the forcing of asm-goto support on x86, which effectively > > > increases the GCC minimum supported version to gcc-4.5 (on x86). > > > > So my biggest worry isn't gcc-4.5 (anybody who hasn't updated deserves > > to be forced, or can stay with old kernels). > > > > No, my biggest worry is clang. What's the status there? > > > > I've pulled this, and honestly, the disaster with > > -fmerge-all-constants makes me think that clang isn't that good a > > compiler choice anyway, but it's sad if this undoes a lot of clang > > work just because of the worries about Spectre and mis-speculated > > branches. > > It's not just spectre, I believe you yourself wanted to use asm-goto > somewhere in the x86 code: > > http://lkml.kernel.org/r/CA+55aFyCp-9Qqjcn9wp=VDp2KO7tfYuUMJxVKC75Xxu0wEB5Cw@mail.gmail.com > > There was some KVM talk of relying on it here: > > http://lkml.kernel.org/r/6a5f2453-cf51-d491-db54-5f239caa29bc@redhat.com > > And there's the comment here: > > https://elixir.bootlin.com/linux/v4.16-rc7/source/arch/x86/kvm/emulate.c#L457 > > As to the suitablility of using clang, there's also this unresolved > issue: > > http://lkml.kernel.org/r/20180321211931.GA111711@google.com > > The fact that even without asm-goto they cannot correctly compile a > kernel and have sat on their hands regarding asm-goto for the past 7 odd > years makes me care very little. > > And since they need to spin a new version of the compiler with all the > various bugs fixed, they might as well include asm-goto in that and be > done with it. So there's really two questions here: - This asm-goto change only impacts x86, is there any production x86 kernel being built with Clang? I think the Pixel kernel is built with Clang, but that's ARM. - Is there anyone on the Clang side _actually_ bending metal and working on asm-goto support, with something like very early alpha test patches available, etc.? Last I saw the communicated Clang POV was still that they wanted to do something "better" (and incompatible to ...) asm-goto. Has this changed? If it's being relied on, or if there's actually something firmly planned, which we could track, then I'd have no problem with reverting this change and waiting one more kernel cycle or so. Thanks, Ingo