Re: [Qemu-devel] [PATCH] cow: make padding in the header explicit

[Eric Blake]

On 09/04/2014 02:58 AM, Stefan Hajnoczi wrote:
> On-disk structures should be marked packed so the compiler does not
> insert padding for field alignment.  Padding should be explicit so
> on-disk layout is obvious and we don't rely on the architecture-specific
> ABI for alignment rules.
> 
> The pahole(1) diff shows that the padding is now explicit and offsets
> are unchanged:
> 
>       char                       backing_file[1024];   /*     8  1024 */
>       /* --- cacheline 16 boundary (1024 bytes) was 8 bytes ago --- */
>       int32_t                    mtime;                /*  1032     4 */
> -
> -     /* XXX 4 bytes hole, try to pack */
> -
> +     uint32_t                   padding;              /*  1036     4 */
>       uint64_t                   size;                 /*  1040     8 */

Was a 32-bit build also inserting this padding, or do we have historical
differences where 32-bit and 64-bit cow files are actually different,
and we may need to be prepared to parse files from both sources?

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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