From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751079AbdKHIHg (ORCPT ); Wed, 8 Nov 2017 03:07:36 -0500 Received: from bombadil.infradead.org ([65.50.211.133]:53455 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750720AbdKHIHe (ORCPT ); Wed, 8 Nov 2017 03:07:34 -0500 Date: Wed, 8 Nov 2017 09:07:22 +0100 From: Peter Zijlstra To: Mikulas Patocka Cc: Josh Poimboeuf , Andy Lutomirski , Borislav Petkov , Brian Gerst , Denys Vlasenko , "H. Peter Anvin" , Jiri Slaby , Thomas Gleixner , live-patching@vger.kernel.org, linux-kernel@vger.kernel.org, Ingo Molnar Subject: Re: [PATCH] objtool: fix build of 64-bit kernel with 32-bit userspace Message-ID: <20171108080722.GS3165@worktop.lehotels.local> References: <20171107175539.zkbzaapmok2b7hbb@treble> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.22.1 (2013-10-16) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Nov 07, 2017 at 04:25:10PM -0500, Mikulas Patocka wrote: > > The technical reason for avoiding the guess unwinder is that it's > > sketchy: it gives false positive results. > > I've always used kernels without frame pointer and I don't see any problem > with decoding stack traces with some phantom entries that were left in the > stack - it's easy to find out which functions could call which functions > and discard the phantom entries. > > > Not only for oopses, but for all the other users of the unwinder: > > /proc//stack, perf, lockdep, etc. So it's a correctness issue. > > Experts need these features, but casual users don't. > > > I agree with you that the frame pointer unwinder has drawbacks, but if > > somebody cares about those drawbacks, I would consider that person an > > "expert" ;-) > > The Kconfig entry says that frame pointers degrade performance by 5-10% - > so almost any user would care about it, not just experts. You're running a 32bit kernel.... isn't that the same as not caring about performance in any case? I suppose the solution you're looking for is making ORC work for it; but given hardly anybody still cares about 32bit x86 you'll probably have to do it yourself.