From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mailman by lists.gnu.org with tmda-scanned (Exim 4.43) id 1KQGXp-0005id-Nw for qemu-devel@nongnu.org; Tue, 05 Aug 2008 03:01:21 -0400 Received: from exim by lists.gnu.org with spam-scanned (Exim 4.43) id 1KQGXo-0005iR-7o for qemu-devel@nongnu.org; Tue, 05 Aug 2008 03:01:21 -0400 Received: from [199.232.76.173] (port=36052 helo=monty-python.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1KQGXo-0005iO-19 for qemu-devel@nongnu.org; Tue, 05 Aug 2008 03:01:20 -0400 Received: from mx20.gnu.org ([199.232.41.8]:6229) by monty-python.gnu.org with esmtps (TLS-1.0:RSA_AES_256_CBC_SHA1:32) (Exim 4.60) (envelope-from ) id 1KQGXn-0007pv-Al for qemu-devel@nongnu.org; Tue, 05 Aug 2008 03:01:19 -0400 Received: from mx1.redhat.com ([66.187.233.31]) by mx20.gnu.org with esmtp (Exim 4.60) (envelope-from ) id 1KQGXj-0005nr-Rd for qemu-devel@nongnu.org; Tue, 05 Aug 2008 03:01:16 -0400 Message-ID: <4897FAB0.30701@redhat.com> Date: Tue, 05 Aug 2008 09:01:04 +0200 From: Gerd Hoffmann MIME-Version: 1.0 Subject: Re: [Qemu-devel] [PATCH 5/7] xen: add block device backend driver. References: <1217865045-10722-1-git-send-email-kraxel@redhat.com> <1217865045-10722-6-git-send-email-kraxel@redhat.com> <48975D9A.7070109@redhat.com> In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Reply-To: qemu-devel@nongnu.org List-Id: qemu-devel.nongnu.org List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Blue Swirl Cc: xen-devel@lists.xensource.com, qemu-devel@nongnu.org Blue Swirl wrote: > On 8/4/08, Gerd Hoffmann wrote: >> Blue Swirl wrote: >> > On 8/4/08, Gerd Hoffmann wrote: >> >> +/* i386 protocol version */ >> >> +#pragma pack(push, 4) >> > >> > What's wrong with __attribute__(__aligned__)? >> >> >> That one is tricky because we must be binary compatible with xen. And >> the block driver protocol has a small design flaw: It has a 64bit value >> which is not aligned on a 64bit border. This leads to different struct >> layouts on i386 and x86_64 due to different alignment rules. Unless you >> force them into something else, like we do in that header file to deal >> with it. Which is needed to run 32bit guests on 64bit hosts. > > Isn't this internal Xen protocol, so hopefully next version of Xen > could use more efficient structures? No. It is part of the guest <=> host ABI. So changing that would break existing guest kernels. Otherwise we wouldn't hop through these loops in the first place. In case the ABI is changed anyway at some point in the future (like it was done from xen 2.x -> 3.0) this can be cleaned up of course. I doubt that will happen though. cheers, Gerd -- http://kraxel.fedorapeople.org/xenner/