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From: Paul Brook <paul@codesourcery.com>
Subject: Re: [Qemu-devel] [PATCH 5/7] xen: add block device backend driver.
Date: Mon, 4 Aug 2008 21:04:15 +0100
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To: qemu-devel@nongnu.org
Cc: Blue Swirl <blauwirbel@gmail.com>, xen-devel@lists.xensource.com, Gerd Hoffmann <kraxel@redhat.com>

On Monday 04 August 2008, Gerd Hoffmann wrote:
> Blue Swirl wrote:
> > On 8/4/08, Gerd Hoffmann <kraxel@redhat.com> 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.

qemu should be host independent. You need to use packed structures with 
appropriate padding.

Paul