At 2014-09-04 08:07:32, "Eric Blake" <ebl...@redhat.com> wrote:
>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?
>
Tried on my x86 32bits computer, and seems the padding is not needed:
struct cow_header_v2 {
uint32_t magic; /* 0 4
*/
uint32_t version; /* 4 4
*/
char backing_file[1024]; /* 8 1024
*/
/* --- cacheline 16 boundary (1024 bytes) was 8 bytes ago --- */
int32_t mtime; /* 1032 4
*/
uint64_t size; /* 1036 8
*/
uint32_t sectorsize; /* 1044 4
*/
/* size: 1048, cachelines: 17, members: 6 */
/* last cacheline: 24 bytes */
};
>--
>Eric Blake eblake redhat com +1-919-301-3266
>Libvirt virtualization library http://libvirt.org
>
Regards,
shhuiw
