From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758297AbXISL1S (ORCPT ); Wed, 19 Sep 2007 07:27:18 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1754225AbXISL1I (ORCPT ); Wed, 19 Sep 2007 07:27:08 -0400 Received: from tomts25-srv.bellnexxia.net ([209.226.175.188]:45406 "EHLO tomts25-srv.bellnexxia.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753012AbXISL1H (ORCPT ); Wed, 19 Sep 2007 07:27:07 -0400 Date: Wed, 19 Sep 2007 07:27:04 -0400 From: Mathieu Desnoyers To: Denys Vlasenko Cc: akpm@linux-foundation.org, linux-kernel@vger.kernel.org Subject: Re: [patch 1/7] Immediate Values - Architecture Independent Code Message-ID: <20070919112704.GG15500@Krystal> References: <20070917184224.549435917@polymtl.ca> <200709182101.12284.vda.linux@googlemail.com> <20070918204718.GB12994@Krystal> <200709190957.53300.vda.linux@googlemail.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Content-Disposition: inline In-Reply-To: <200709190957.53300.vda.linux@googlemail.com> X-Editor: vi X-Info: http://krystal.dyndns.org:8080 X-Operating-System: Linux/2.6.21.3-grsec (i686) X-Uptime: 07:25:44 up 51 days, 11:44, 3 users, load average: 2.75, 1.86, 1.53 User-Agent: Mutt/1.5.13 (2006-08-11) Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org * Denys Vlasenko (vda.linux@googlemail.com) wrote: > On Tuesday 18 September 2007 21:47, Mathieu Desnoyers wrote: > > * Denys Vlasenko (vda.linux@googlemail.com) wrote: > > > On Tuesday 18 September 2007 18:59, Mathieu Desnoyers wrote: > > > > * Denys Vlasenko (vda.linux@googlemail.com) wrote: > > > > > On Monday 17 September 2007 19:42, Mathieu Desnoyers wrote: > > > > > > Index: linux-2.6-lttng/include/asm-generic/vmlinux.lds.h > > > > > > =================================================================== > > > > > > --- linux-2.6-lttng.orig/include/asm-generic/vmlinux.lds.h 2007-09-17 13:25:06.000000000 -0400 > > > > > > +++ linux-2.6-lttng/include/asm-generic/vmlinux.lds.h 2007-09-17 13:35:50.000000000 -0400 > > > > > > @@ -122,6 +122,13 @@ > > > > > > VMLINUX_SYMBOL(__stop___kcrctab_gpl_future) = .; \ > > > > > > } \ > > > > > > \ > > > > > > + /* Immediate values: pointers */ \ > > > > > > + __immediate : AT(ADDR(__immediate) - LOAD_OFFSET) { \ > > > > > > + VMLINUX_SYMBOL(__start___immediate) = .; \ > > > > > > + *(__immediate) \ > > > > > > + VMLINUX_SYMBOL(__stop___immediate) = .; \ > > > > > > + } \ > > > > > > + \ > > > > > > > > > > Why do you need an output section for that? IOW: will this work too? > > > > > > > > > > .data : ... { > > > > > ... > > > > > > > > > > VMLINUX_SYMBOL(__start___immediate) = .; \ > > > > > *(__immediate) \ > > > > > VMLINUX_SYMBOL(__stop___immediate) = .; \ > > > > > ... > > > > > } > > > > > > > > > > > > > This last one could cause alignment problems. We either have to use the > > > > proper ALIGN() before the section, or let AT(ADDR(__immediate) - > > > > LOAD_OFFSET) take care of it. I prefer the latter. > > > > > > This adds yet another output section in vmlinux, and there is > > > no tools which need that. We already have 30+ sections there while we need ~20. > > > > > > I am trying to fix the mess. Please don't add to it. > > > > > > Re alignment: (1) do you really realy REALLY need it? Last I checked, > > > i386 was handling unaligned accesses just fine; and > > > (2) this works: > > > > > > . = ALIGN(4) > > > VMLINUX_SYMBOL(__start___immediate) = .; \ > > > *(__immediate) \ > > > VMLINUX_SYMBOL(__stop___immediate) = .; \ > > > > > > > > > > Alignment: I need the __start___immediate and __stop___immediate values > > to be at the same alignment as the *(__immediate) content, or else we > > end up thinking that padding is data. > > > > . = ALIGN(4) works fine as long as the structure within the section is > > not bigger or equal to 32 bytes: gcc has the habit to align 32 bytes > > structure on 32 bytes multiples. The safest way I found to do it is to > > declare the section as I do: it will cause no breakage if anybody append > > data to the structure. > > If your structure will be padded by gcc, then this: > > +#define immediate_read(name) \ > + ({ \ > + __typeof__(name##__immediate) value; \ > + switch (sizeof(value)) { \ > + case 1: \ > + asm ( ".section __immediate, \"a\", @progbits;\n\t" \ > + ".long %1, (0f)+1, 1;\n\t" \ > + ".previous;\n\t" \ > + "0:\n\t" \ > + "mov %2,%0;\n\t" \ > + : "=r" (value) \ > + : "m" (name##__immediate), \ > + "i" (0)); \ > + break; \ > > will produce wrongly-sized "struct __immediate" (truncated one), > since gcc has no idea that you are building struct __immediate there, > and here: > > +void immediate_update_range(const struct __immediate *begin, > + const struct __immediate *end) > +{ > + const struct __immediate *iter; > + int ret; > + > + for (iter = begin; iter < end; iter++) { > + mutex_lock(&immediate_mutex); > + kernel_text_lock(); > + ret = arch_immediate_update(iter); > + kernel_text_unlock(); > + if (ret) > + printk(KERN_WARNING "Invalid immediate value. " > + "Variable at %p, " > + "instruction at %p, size %lu\n", > + (void*)iter->immediate, > + (void*)iter->var, iter->size); > + mutex_unlock(&immediate_mutex); > + } > +} > > iter++ will go off rails. You are right. It's ok here since we are actually smaller than 32 bytes, but I should force the structure alignment so that if the structure grows, the assembly declaration follows. I'll go for the gcc attribute and then we can remove the section declaration. Mathieu > -- > vda -- Mathieu Desnoyers Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68