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

[Blue Swirl]

On 8/4/08, Gerd Hoffmann <address@hidden> 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.

Isn't this internal Xen protocol, so hopefully next version of Xen
could use more efficient structures?

>  > Would I need to add
>  > Sparc32 and Sparc64 versions some day?
>
>
> Not sure whenever the same 32/64bit ABI issue exists on sparc.  If so,
>  the code handling this on x86 should work for sparc too.

Well, I'd rather define a new, more architecturally neutral structure.