Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933792AbbFJTgc (ORCPT ); Wed, 10 Jun 2015 15:36:32 -0400 Received: from mx1.redhat.com ([209.132.183.28]:35228 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933734AbbFJTgV (ORCPT ); Wed, 10 Jun 2015 15:36:21 -0400 Date: Wed, 10 Jun 2015 14:36:18 -0500 From: Josh Poimboeuf To: Andi Kleen Cc: Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" , Michal Marek , Peter Zijlstra , Andy Lutomirski , Borislav Petkov , Linus Torvalds , x86@kernel.org, live-patching@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v5 00/10] x86/asm: Compile-time asm code validation Message-ID: <20150610193618.GB1125@treble.redhat.com> References: <20150610130814.GN19417@two.firstfloor.org> <20150610135203.GA19509@treble.redhat.com> <20150610141104.GQ19417@two.firstfloor.org> <20150610143209.GA28843@treble.redhat.com> <20150610150412.GR19417@two.firstfloor.org> <20150610153155.GB29724@treble.redhat.com> <87lhfr5rw9.fsf@tassilo.jf.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <87lhfr5rw9.fsf@tassilo.jf.intel.com> User-Agent: Mutt/1.5.23.1-rc1 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2443 Lines: 67 On Wed, Jun 10, 2015 at 11:40:06AM -0700, Andi Kleen wrote: > Josh Poimboeuf writes: > > > On Wed, Jun 10, 2015 at 05:04:12PM +0200, Andi Kleen wrote: > >> > > > - duplicate the destination code inside the function > >> > > > - convert the jump to a call > >> > > > >> > > That all won't work for a lot of cases. > >> > > >> > Hm, could you give an example? > >> > >> Just a standard *_user exception handler. > > > > I'm afraid I don't follow. Exception handlers don't work via jump > > instructions, but rather via CPU exceptions. > > > > Or are you talking about something else? > > Let's take an example: > > 102: > .section .fixup,"ax" > 103: addl %ecx,%edx /* ecx is zerorest also */ > jmp copy_user_handle_tail > .previous > > _ASM_EXTABLE(100b,103b) > _ASM_EXTABLE(101b,103b) > > The exception handling code is part of the function, but it's out of line. The jump instruction is in the .fixup section, not in the callable function itself. So it doesn't violate the asmvalidate rules. > > Are you suggesting that we implement this gcc optimization in kernel asm > > code? > > It was how Linux traditionally implemented locking code for example. > Have the hot path handle the uncontended fast path, and the slow path > call. > > I don't know if there is much left of it (a lot of it was removed because > it was hard to describe in dwarf3, needs dwarf4). But it seems bad > to completely disallow it. > > But yes eventually gcc generated code should use it again, because it's > great for icache usage if you measure it correctly at run time > (not the broken "size" approach that is unfortunately far too common) This patch set has no relationship to gcc generated code whatsoever. So it doesn't disallow anything there. For kernel asm code, AFAIK, such a mechanism for hot/cold path separation in separate sections doesn't exist today. So it's not "disallowed" there either. It's just apparently not currently done. If somebody were to create such a mechanism, I think we could standardize it in such a way that it could be compatible with asmvalidate. -- Josh -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/