From owner-freebsd-hackers@FreeBSD.ORG Wed Apr 18 17:48:10 2012 Return-Path: Delivered-To: freebsd-hackers@freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:4f8:fff6::34]) by hub.freebsd.org (Postfix) with ESMTP id AE54A106566C; Wed, 18 Apr 2012 17:48:10 +0000 (UTC) (envelope-from jhb@freebsd.org) Received: from bigwig.baldwin.cx (bigknife-pt.tunnel.tserv9.chi1.ipv6.he.net [IPv6:2001:470:1f10:75::2]) by mx1.freebsd.org (Postfix) with ESMTP id 81F8B8FC18; Wed, 18 Apr 2012 17:48:10 +0000 (UTC) Received: from jhbbsd.localnet (unknown [209.249.190.124]) by bigwig.baldwin.cx (Postfix) with ESMTPSA id D0B8BB958; Wed, 18 Apr 2012 13:48:09 -0400 (EDT) From: John Baldwin To: Andriy Gapon Date: Wed, 18 Apr 2012 13:47:59 -0400 User-Agent: KMail/1.13.5 (FreeBSD/8.2-CBSD-20110714-p13; KDE/4.5.5; amd64; ; ) References: <4F8999D2.1080902@FreeBSD.org> <1334760007.1082.243.camel@revolution.hippie.lan> <4F8ED702.4020803@FreeBSD.org> In-Reply-To: <4F8ED702.4020803@FreeBSD.org> MIME-Version: 1.0 Content-Type: Text/Plain; charset="iso-8859-1" Content-Transfer-Encoding: 7bit Message-Id: <201204181347.59109.jhb@freebsd.org> X-Greylist: Sender succeeded SMTP AUTH, not delayed by milter-greylist-4.2.7 (bigwig.baldwin.cx); Wed, 18 Apr 2012 13:48:10 -0400 (EDT) Cc: freebsd-fs@freebsd.org, Ian Lepore , freebsd-hackers@freebsd.org Subject: Re: [review request] zfsboot/zfsloader: support accessing filesystems within a pool X-BeenThere: freebsd-hackers@freebsd.org X-Mailman-Version: 2.1.5 Precedence: list List-Id: Technical Discussions relating to FreeBSD List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Wed, 18 Apr 2012 17:48:10 -0000 On Wednesday, April 18, 2012 11:00:18 am Andriy Gapon wrote: > on 18/04/2012 17:40 Ian Lepore said the following: > > On Wed, 2012-04-18 at 17:36 +0300, Andriy Gapon wrote: > >> on 18/04/2012 17:22 Ian Lepore said the following: > >>> YES! A size field (preferably as the first field in the struct) along > >>> with a flag to indicate that it's a new-style boot info struct that > >>> starts with a size field, will allow future changes without a lot of > >>> drama. It can allow code that has to deal with the struct without > >>> interpretting it (such as trampoline code that has to copy it to a new > >>> stack or memory area as part of loading the kernel) to be immune to > >>> future changes. > >> > >> Yeah, placing the new field at front would immediately break compatibility and > >> even access to the flags field :-) > >> > > > > Code would only assume the new field was at the front of the struct if > > the new flag is set, otherwise it would use the historical struct > > layout. > > Right, but where the flag would reside? > And how the older code that is not aware of the new flag would cope with the new > layout? I think the size should be appended to the end of the current structure. However, it will buy us more flexibility in the future. -- John Baldwin