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=-0.9 required=3.0 tests=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 E49D0C2D0DB for ; Tue, 28 Jan 2020 17:03:15 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id BB55E214AF for ; Tue, 28 Jan 2020 17:03:15 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="FeMKUDyA" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726767AbgA1RDP (ORCPT ); Tue, 28 Jan 2020 12:03:15 -0500 Received: from us-smtp-delivery-1.mimecast.com ([207.211.31.120]:60829 "EHLO us-smtp-1.mimecast.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726540AbgA1RDO (ORCPT ); Tue, 28 Jan 2020 12:03:14 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1580230992; 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=UEzamJ8ELA4VGrlkVRtVzx3OFq6AUZqHXicgISU+c/U=; b=FeMKUDyAjNlyzYZ9U3+ifjZtaVw86icT0G81W6RWIs6Yfrpa624MyZZI0lM240upKbHZQk V9nt8pefZhlNInn86O6r5YV2n2tJEwN+wcfWsp0JsG5rsbVarDejN+0tl9if1KIjyadxs/ MC/9oNo+TpBN/1stG0v67L/ZuMgYAmE= 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-281-q9mkc36-PouBlOJvpY7NaQ-1; Tue, 28 Jan 2020 12:03:09 -0500 X-MC-Unique: q9mkc36-PouBlOJvpY7NaQ-1 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.phx2.redhat.com [10.5.11.11]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id AF580100550E; Tue, 28 Jan 2020 17:03:04 +0000 (UTC) Received: from treble (ovpn-124-151.rdu2.redhat.com [10.10.124.151]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 199CC84BC5; Tue, 28 Jan 2020 17:02:56 +0000 (UTC) Date: Tue, 28 Jan 2020 11:02:54 -0600 From: Josh Poimboeuf To: Petr Mladek Cc: Miroslav Benes , 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() Message-ID: <20200128170254.igb72ib5n7lvn3ds@treble> References: <20191016074217.GL2328@hirez.programming.kicks-ass.net> <20191021150549.bitgqifqk2tbd3aj@treble> <20200120165039.6hohicj5o52gdghu@treble> <20200121161045.dhihqibnpyrk2lsu@treble> <20200122214239.ivnebi7hiabi5tbs@treble> <20200128150014.juaxfgivneiv6lje@treble> <20200128154046.trkpkdaz7qeovhii@pathway.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20200128154046.trkpkdaz7qeovhii@pathway.suse.cz> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Jan 28, 2020 at 04:40:46PM +0100, Petr Mladek wrote: > On Tue 2020-01-28 09:00:14, Josh Poimboeuf wrote: > > On Tue, Jan 28, 2020 at 10:28:07AM +0100, Miroslav Benes wrote: > > > I don't think we have something special at SUSE not generally available... > > > > > > ...and I don't think it is really important to discuss that and replying > > > to the above, because there is a legitimate use case which relies on the > > > flag. We decided to support different use cases right at the beginning. > > > > > > I understand it currently complicates things for objtool, but objtool is > > > sensitive to GCC code generation by definition. "Issues" appear with every > > > new GCC version. I see no difference here and luckily it is not so > > > difficult to fix it. > > > > > > I am happy to help with acting on those objtool warning reports you > > > mentioned in the other email. Just Cc me where appropriate. We will take a > > > look. > > > > As I said, the objtool warnings aren't even the main issue. > > Great. > > Anyway, I think that we might make your life easier with using > the proposed -Wsuggest-attribute=noreturn. Maybe. Though if I understand correctly, this doesn't help for any of the new warnings because they're for static functions, and this only warns about global functions. > Also it might be possible to create the list of global > noreturn functions using some gcc tool. Similar way that we get > the list of functions that need to be livepatched explicitly > because of the problematic optimizations. > > It sounds like a win-win approach. I don't quite get how that could be done in an automated way, but ideas about how to implement it would certainly be welcome. > > There are N users[*] of CONFIG_LIVEPATCH, where N is perhaps dozens. > > For N-1 users, they have to suffer ALL the drawbacks, with NONE of the > > benefits. > > You wrote in the other mail: > > > The problems associated with it: performance, LTO incompatibility, > > clang incompatibility (I think?), the GCC dead code issue. > > SUSE performance team did extensive testing and did not found > any real performance issues. It was discussed when the option > was enabled upstream. > > Are the other problems affecting real life usage, please? > Could you be more specific about them, please? The original commit mentioned 1-3% scheduler degradation. And I'd expect things to worsen over time as interprocedural optimizations improve. Also, LTO is coming whether we like it or not. As is Clang. Those are real-world things which will need to work with livepatching sooner or later. > > And, even if they wanted those benefits, they have no idea how to get > > them because the patch creation process isn't documented. > > I do not understand this. All the sample modules and selftests are > using source based livepatches. We're talking in circles. Have you read the thread? The samples are a (dangerous) joke. With or without -flive-patching. > It is actually the only somehow documented way. Sure, the > documentation might get improved. Patches are welcome. Are you suggesting for *me* to send documentation for how *you* build patches? > The option is not currently needed by the selftests only because there > is no selftest for this type of problems. But the problems are real. > They would actually deserve selftests. Again, patches are welcome. > > My understanding is that the source based livepatches is the future. I think that still remains to be seen. > N-1 users are just waiting until the 1 user develops more helper tools > for this. No. N-1 users have no idea how to make (safe) source-based patches in the first place. And if *you* don't need the tools, why would anyone else? Why not document the process and encourage the existence of other users so they can get involved and help with the tooling? > I would really like to hear about some serious problems > before we do this step back in upstream. Sometimes you need to take 1 step back before you can take 2 steps forward. I regret ACKing the original patch. It was too early. -- Josh