From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758757Ab2JKOKw (ORCPT ); Thu, 11 Oct 2012 10:10:52 -0400 Received: from multi.imgtec.com ([194.200.65.239]:43164 "EHLO multi.imgtec.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1758742Ab2JKOKu (ORCPT ); Thu, 11 Oct 2012 10:10:50 -0400 Message-ID: <5076D360.2010804@imgtec.com> Date: Thu, 11 Oct 2012 15:10:40 +0100 From: James Hogan User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:15.0) Gecko/20120911 Thunderbird/15.0.1 MIME-Version: 1.0 To: James Bottomley CC: , Subject: Re: [RESEND PATCH] scsi: make struct scsi_varlen_cdb_hdr packed References: <1349946933-30314-1-git-send-email-james.hogan@imgtec.com> <1349951069.2425.44.camel@dabdike.int.hansenpartnership.com> <5076AE3B.5040509@imgtec.com> <1349960296.2425.53.camel@dabdike.int.hansenpartnership.com> In-Reply-To: <1349960296.2425.53.camel@dabdike.int.hansenpartnership.com> X-Enigmail-Version: 1.4.4 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit X-Originating-IP: [192.168.154.65] X-SEF-Processed: 7_3_0_01181__2012_10_11_15_10_42 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 11/10/12 13:58, James Bottomley wrote: > On Thu, 2012-10-11 at 12:32 +0100, James Hogan wrote: >> On 11/10/12 11:24, James Bottomley wrote: >>> On Thu, 2012-10-11 at 10:15 +0100, James Hogan wrote: >>>> The struct scsi_varlen_cdb_hdr is expected to be exactly 10 bytes when >>>> used in struct osd_cdb_head, but it isn't marked as packed. Some >>>> architectures will round the struct size up which triggers BUILD_BUG_ON >>>> compile errors in osd_initiator.c when the outer structs are unexpected >>>> sizes. This is fixed by marking struct scsi_varlen_cdb_hdr as __packed. >>> >>> What actual problem have you encountered? The structure is {u8[8], u16} >>> which is naturally packed on every architecture I know about. I've even >>> built osd_initiator without problem on parisc, which has some of the >>> most rigid alignment rules I've seen. >> >> Hi James, >> >> The alignment is fine (the offset of the u16 is 8 bytes), but >> unfortunately with the metag port of gcc, sizeof(struct >> scsi_varlen_cdb_hdr) is rounded up to a 4 byte boundary (even though the >> largest data member alignment is only 2 bytes), which is 12 bytes >> instead of 10. > > That sounds to be a bug in your compiler ... it shouldn't be rounding up > structure sizes if the structure can fit in 10 bytes. This isn't > happening in any other architecture that I know of (otherwise we'd have > had a reported build break). This was my initial thought, and I share your feeling that this isn't ideal compiler behaviour, however it's pretty much set in stone as part of our ABI now. Having talked to one of our compiler folk about it here's what he had to say: > In GCC 4.2 the following backends have STRUCTURE_SIZE_BOUNDARY set to >=32 which will trigger this behaviour: > > Alpha+unicos > Arm prior to AAPCS > Mt > > This is within standards to my knowledge. Cheers James