Re: [Qemu-devel] [PATCH 5/7] xen: add block device backend driver.

[Paul Brook]

On Monday 04 August 2008, Gerd Hoffmann wrote:
> Blue Swirl wrote:
> > On 8/4/08, Gerd Hoffmann <address@hidden> 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