Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751892AbdITPly (ORCPT ); Wed, 20 Sep 2017 11:41:54 -0400 Received: from pandora.armlinux.org.uk ([78.32.30.218]:39314 "EHLO pandora.armlinux.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751622AbdITPlx (ORCPT ); Wed, 20 Sep 2017 11:41:53 -0400 Date: Wed, 20 Sep 2017 16:41:43 +0100 From: Russell King - ARM Linux To: Ard Biesheuvel Cc: Romain Izard , "linux-arm-kernel@lists.infradead.org" , "linux-kernel@vger.kernel.org" , Arnd Bergmann , Al Viro Subject: Re: [PATCH] ARM: unaligned.h: Use an arch-specific version Message-ID: <20170920154143.GO20805@n2100.armlinux.org.uk> References: <20170920151802.7609-1-romain.izard.pro@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1990 Lines: 43 On Wed, Sep 20, 2017 at 08:26:09AM -0700, Ard Biesheuvel wrote: > Hi Romain, > > On 20 September 2017 at 08:18, Romain Izard wrote: > > For the 32-bit ARM architecture, unaligned access support is variable. > > On a chip without a MMU, an unaligned access returns a rotated data word > > and must be avoided. > > > > When a MMU is available, it can be trapped. On ARMv6 or ARMv7, it can also > > be handled by the hardware, depending on the type of access instruction. > > Unaligned access of 32 bits or less are corrected, while larger access > > provoke a trap. > > > > Unfortunately, the compiler is able to merge two 32-bit access that > > would generate a LDR instruction, that works on unaligned access, into a > > single LDM access that will not work. This is not a common situation, > > but it has been observed in the LZ4 decompression code. > > > > To prevent this type of optimization, it is necessary to change the > > explicit accessors for unaligned addresses from those defined in the > > access_ok.h header, to those defined in the packed_struct.h header. > > > > Add an arch-specific header to ARM, to retain other optimizations that > > rely on HAVE_EFFICIENT_UNALIGNED_ACCESS, while making sure that access > > that explicitly rely on the unaligned accessors are correctly handled by > > the compiler. > > > > Signed-off-by: Romain Izard > > --- > > > > If access_ok.h has been observed to produce different output from the > struct versions (using any compiler), I guess we cannot simply change > the asm-generic default and expect everybody to be ok with that. So I > agree this is the most appropriate course of action. However, what effect does this have on the code generated for the rest of the kernel? -- RMK's Patch system: http://www.armlinux.org.uk/developer/patches/ FTTC broadband for 0.8mile line in suburbia: sync at 8.8Mbps down 630kbps up According to speedtest.net: 8.21Mbps down 510kbps up