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Re: [Qemu-devel] [PATCH] cow: make padding in the header explicit
From: |
Benoît Canet |
Subject: |
Re: [Qemu-devel] [PATCH] cow: make padding in the header explicit |
Date: |
Thu, 4 Sep 2014 11:47:12 +0200 |
User-agent: |
Mutt/1.5.23 (2014-03-12) |
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 <address@hidden>
> ---
> 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 <address@hidden>