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

[Gerd Hoffmann <kraxel@redhat.com>]

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