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=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, 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 7973FC33CAA for ; Tue, 21 Jan 2020 08:35:34 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 5527F20661 for ; Tue, 21 Jan 2020 08:35:34 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728916AbgAUIfd (ORCPT ); Tue, 21 Jan 2020 03:35:33 -0500 Received: from mx2.suse.de ([195.135.220.15]:39638 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725789AbgAUIfd (ORCPT ); Tue, 21 Jan 2020 03:35:33 -0500 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx2.suse.de (Postfix) with ESMTP id 8588FAE09; Tue, 21 Jan 2020 08:35:30 +0000 (UTC) Date: Tue, 21 Jan 2020 09:35:28 +0100 (CET) From: Miroslav Benes To: Josh Poimboeuf cc: Peter Zijlstra , Steven Rostedt , Joe Lawrence , Jessica Yu , x86@kernel.org, linux-kernel@vger.kernel.org, mhiramat@kernel.org, bristot@redhat.com, jbaron@akamai.com, torvalds@linux-foundation.org, tglx@linutronix.de, mingo@kernel.org, namit@vmware.com, hpa@zytor.com, luto@kernel.org, ard.biesheuvel@linaro.org, live-patching@vger.kernel.org, Randy Dunlap Subject: Re: [PATCH v3 5/6] x86/ftrace: Use text_poke() In-Reply-To: <20200120165039.6hohicj5o52gdghu@treble> Message-ID: References: <20191011125903.GN2359@hirez.programming.kicks-ass.net> <20191015130739.GA23565@linux-8ccs> <20191015135634.GK2328@hirez.programming.kicks-ass.net> <88bab814-ea24-ece9-2bc0-7a1e10a62f12@redhat.com> <20191015153120.GA21580@linux-8ccs> <7e9c7dd1-809e-f130-26a3-3d3328477437@redhat.com> <20191015182705.1aeec284@gandalf.local.home> <20191016074217.GL2328@hirez.programming.kicks-ass.net> <20191021150549.bitgqifqk2tbd3aj@treble> <20200120165039.6hohicj5o52gdghu@treble> User-Agent: Alpine 2.21 (LSU 202 2017-01-01) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, 20 Jan 2020, Josh Poimboeuf wrote: > On Mon, Oct 21, 2019 at 10:05:49AM -0500, Josh Poimboeuf wrote: > > On Wed, Oct 16, 2019 at 09:42:17AM +0200, Peter Zijlstra wrote: > > > > which are not compatible with livepatching. GCC upstream now has > > > > -flive-patching option, which disables all those interfering optimizations. > > > > > > Which, IIRC, has a significant performance impact and should thus really > > > not be used... > > > > > > If distros ship that crap, I'm going to laugh at them the next time they > > > want a single digit performance improvement because *important*. > > > > I have a crazy plan to try to use objtool to detect function changes at > > a binary level, which would hopefully allow us to drop this flag. > > > > But regardless, I wonder if we enabled this flag prematurely. We still > > don't have a reasonable way to use it for creating source-based live > > patches upstream, and it should really be optional for CONFIG_LIVEPATCH, > > since kpatch-build doesn't need it. > > I also just discovered that -flive-patching is responsible for all those > "unreachable instruction" objtool warnings which Randy has been > dutifully bugging me about over the last several months. For some > reason it subtly breaks GCC implicit noreturn detection for local > functions. Ugh, that is unfortunate. Have you reported it? > At this point, I only see downsides of -flive-patching, at least until > we actually have real upstream code which needs it. Can you explain this? The option makes GCC to avoid optimizations which are difficult to detect and would make live patching unsafe. I consider it useful as it is, so if you shared the other downsides and what you meant by real upstream code, we could discuss it. > If there aren't any objections I'll be posting a patch soon to revert. I think it would be a setback. Regards Miroslav