Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754619AbbFJPcJ (ORCPT ); Wed, 10 Jun 2015 11:32:09 -0400 Received: from mx1.redhat.com ([209.132.183.28]:46341 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754267AbbFJPb5 (ORCPT ); Wed, 10 Jun 2015 11:31:57 -0400 Date: Wed, 10 Jun 2015 10:31:55 -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: <20150610153155.GB29724@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> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20150610150412.GR19417@two.firstfloor.org> 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: 1675 Lines: 54 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? > > Well, I don't see how that's really a logical conclusion. > > What's special about assembler code? Ok, I'll bite: - it's human-generated - it's much simpler - it doesn't have stack metadata by default - it has far fewer constraints But I don't see this line of discussion going anywhere without any real examples of why you need external jumps in asm functions... > > But we're > > probably being too vague here... Do you have any examples where you > > really need to jump outside of a callable function? > > It's not needed, but it's an optimization to optimize icache usage. > It is optional (-freorder-blocks-and-partition) > > In this case gcc splits the function into two (hot and cold) > > It's actually a nice optimization and it would be sad from stopping > the kernel from using it. Sorry if I wasn't clear, I was trying to ask for examples in kernel asm code. Are you suggesting that we implement this gcc optimization in kernel asm code? -- 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/