Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933815AbbFJSkN (ORCPT ); Wed, 10 Jun 2015 14:40:13 -0400 Received: from mga14.intel.com ([192.55.52.115]:34967 "EHLO mga14.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754536AbbFJSkH (ORCPT ); Wed, 10 Jun 2015 14:40:07 -0400 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.13,588,1427785200"; d="scan'208";a="585550326" From: Andi Kleen To: Josh Poimboeuf 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 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> Date: Wed, 10 Jun 2015 11:40:06 -0700 In-Reply-To: <20150610153155.GB29724@treble.redhat.com> (Josh Poimboeuf's message of "Wed, 10 Jun 2015 10:31:55 -0500") Message-ID: <87lhfr5rw9.fsf@tassilo.jf.intel.com> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/24.5 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1768 Lines: 54 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. > 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) -Andi -- ak@linux.intel.com -- Speaking for myself only -- 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/