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

["Benoît Canet" <benoit.canet@irqsave.net>]

The Thursday 04 Sep 2014 à 09:58:41 (+0100), 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 */
> 
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> ---
>  block/cow.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/block/cow.c b/block/cow.c
> index 6ee4833..5ee9363 100644
> --- a/block/cow.c
> +++ b/block/cow.c
> @@ -37,9 +37,10 @@ struct cow_header_v2 {
>      uint32_t version;
>      char backing_file[1024];
>      int32_t mtime;
> +    uint32_t padding;
>      uint64_t size;
>      uint32_t sectorsize;
> -};
> +} QEMU_PACKED;
>  
>  typedef struct BDRVCowState {
>      CoMutex lock;
> -- 
> 1.9.3
> 
> 
Reviewed-by: Benoît Canet <benoit.canet@nodalink.com>