From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755233AbeDCI7L (ORCPT ); Tue, 3 Apr 2018 04:59:11 -0400 Received: from bombadil.infradead.org ([198.137.202.133]:51406 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755086AbeDCI7K (ORCPT ); Tue, 3 Apr 2018 04:59:10 -0400 Date: Tue, 3 Apr 2018 10:59:04 +0200 From: Peter Zijlstra To: Linus Torvalds Cc: Ingo Molnar , Matthias Kaehlcke , Linux Kernel Mailing List , Thomas Gleixner , Andrew Morton Subject: Re: [GIT PULL] x86/build changes for v4.17 Message-ID: <20180403085904.GY4082@hirez.programming.kicks-ass.net> References: <20180402095033.nfzcrmxvpm46dhbl@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.9.3 (2018-01-21) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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.