From mboxrd@z Thu Jan 1 00:00:00 1970 From: Josh Poimboeuf Subject: Re: [PATCH v2 1/2] arm64: implement FTRACE_WITH_REGS Date: Fri, 8 Jul 2016 10:48:24 -0500 Message-ID: <20160708154824.ikq45dgk2auigqn7@treble> References: <20160627151500.2247A68D20@newverein.lst.de> <20160627151717.BD25468D26@newverein.lst.de> <20160708145800.GE3556@pathway.suse.cz> <20160708150709.GA8979@lst.de> <20160708152421.GG3556@pathway.suse.cz> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Return-path: Received: from mx1.redhat.com ([209.132.183.28]:56687 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754902AbcGHPs3 (ORCPT ); Fri, 8 Jul 2016 11:48:29 -0400 Content-Disposition: inline In-Reply-To: <20160708152421.GG3556@pathway.suse.cz> Sender: linux-sparse-owner@vger.kernel.org List-Id: linux-sparse@vger.kernel.org To: Petr Mladek Cc: Torsten Duwe , Catalin Marinas , Will Deacon , takahiro.akashi@linaro.org, Jungseok Lee , Arnd Bergmann , Li Bin , Steven Rostedt , Ingo Molnar , Christopher Li , Jiri Kosina , andrew.wafaa@arm.com, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, live-patching@vger.kernel.org, linux-arch@vger.kernel.org, linux-sparse@vger.kernel.org On Fri, Jul 08, 2016 at 05:24:21PM +0200, Petr Mladek wrote: > On Fri 2016-07-08 17:07:09, Torsten Duwe wrote: > > On Fri, Jul 08, 2016 at 04:58:00PM +0200, Petr Mladek wrote: > > > On Mon 2016-06-27 17:17:17, Torsten Duwe wrote: > > > > Once gcc is enhanced to optionally generate NOPs at the beginning > > > > of each function, like the concept proven in > > > > https://gcc.gnu.org/ml/gcc-patches/2016-04/msg01671.html > > > > (sans the "fprintf (... pad_size);", which spoils the data structure > > > > for kernel use), the generated pads can nicely be used to reroute > > > > function calls for tracing/profiling, or live patching. > > > > diff --git a/arch/arm64/kernel/ftrace.c b/arch/arm64/kernel/ftrace.c > > > > index ebecf9a..917065c 100644 > > > > --- a/arch/arm64/kernel/ftrace.c > > > > +++ b/arch/arm64/kernel/ftrace.c > > > > @@ -39,6 +39,12 @@ static int ftrace_modify_code(unsigned long pc, u32 old, u32 new, > > > > if (aarch64_insn_read((void *)pc, &replaced)) > > > > return -EFAULT; > > > > > > > > + /* If we already have what we'll finally want, > > > > + * report success. This is needed on startup. > > > > + */ > > > > + if (replaced == new) > > > > + return 0; > > > > > > This looks strange. I wonder if it actually hides a real bug that we > > > modify the code twice or so. > > > > Not at all. All "profilers" we abused so far generate code that needs to > > be disabled on boot first. prolog-pad generates nops, initially. > > Yeah, but I cannot find this kind of check in other architectures. > I checked arch/x86/kernel/ftrace.c, arch/s390/kernel/ftrace.c, and > arch/powerpc/kernel/ftrace.c. These all support ftrace with > regs and livepatching. My understanding is that other arches don't need this check because they use -mfentry, so they have to modify the "call fentry" instruction to a nop on startup. Here, with -fprolog-pad, it's already a nop, so no change is needed. -- Josh