Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751315AbdGMIvW (ORCPT ); Thu, 13 Jul 2017 04:51:22 -0400 Received: from merlin.infradead.org ([205.233.59.134]:42130 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750953AbdGMIvV (ORCPT ); Thu, 13 Jul 2017 04:51:21 -0400 Date: Thu, 13 Jul 2017 10:51:14 +0200 From: Peter Zijlstra To: Josh Poimboeuf Cc: Andres Freund , x86@kernel.org, linux-kernel@vger.kernel.org, live-patching@vger.kernel.org, Linus Torvalds , Andy Lutomirski , Jiri Slaby , Ingo Molnar , "H. Peter Anvin" , Mike Galbraith Subject: Re: [PATCH v3 00/10] x86: ORC unwinder (previously undwarf) Message-ID: <20170713085114.h4vjgg7jjbl6dohb@hirez.programming.kicks-ass.net> References: <20170712214920.5droainfqjmq7sgu@alap3.anarazel.de> <20170712223225.zkq7tdb7pzgb3wy7@treble> <20170713071253.a3slz3j5tcgy3rkk@hirez.programming.kicks-ass.net> <20170713085015.yjjv5ig2znplx5jl@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170713085015.yjjv5ig2znplx5jl@hirez.programming.kicks-ass.net> User-Agent: NeoMutt/20170609 (1.8.3) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1354 Lines: 31 On Thu, Jul 13, 2017 at 10:50:15AM +0200, Peter Zijlstra wrote: > On Thu, Jul 13, 2017 at 09:12:53AM +0200, Peter Zijlstra wrote: > > On Wed, Jul 12, 2017 at 05:32:25PM -0500, Josh Poimboeuf wrote: > > > If you want perf to be able to use ORC instead of DWARF for user space > > > binaries, that's not currently possible, though I don't see any > > > technical blockers for doing so. Perf would need to be taught to read > > > ORC data. > > > > So the problem with userspace stuff is that the unwind data isn't > > readily available from NMI context. > > > > So the kernel unwinder will trigger a fault and abort. > > > > The very best we can hope for is using the EH [*] stuff that all > > binaries actually have _and_ map. The only problem is that most programs > > don't actually use the EH stuff much so while its mapped, its not > > actually paged in, so we're still stuck. > > One gloriously ugly hack would be to delay the userspace unwind to > return-to-userspace, at which point we have a schedulable context and > can take faults. > > Of course, then you have to somehow identify this later unwind sample > with all relevant prior samples and stitch the whole thing back > together, but that should be doable. > > In fact, it would be at all hard to do, just queue a task_work from the +not > NMI and have that do the EH based unwind.