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Date: Fri, 13 Sep 2013 10:00:31 +0200
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Subject: Re: [Qemu-devel] [PULL 39/42] block: look for zero blocks in
	bs->file
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To: Stefan Hajnoczi <stefanha@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>, qemu-devel@nongnu.org, Anthony Liguori <anthony@codemonkey.ws>

On 06.09.2013 17:39, Stefan Hajnoczi wrote:
> From: Paolo Bonzini <pbonzini@redhat.com>
>
> Reviewed-by: Eric Blake <eblake@redhat.com>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> ---
>   block.c | 16 +++++++++++++++-
>   1 file changed, 15 insertions(+), 1 deletion(-)
>
> diff --git a/block.c b/block.c
> index 3c39769..a325efc 100644
> --- a/block.c
> +++ b/block.c
> @@ -3075,7 +3075,7 @@ static int64_t coroutine_fn bdrv_co_get_block_status(BlockDriverState *bs,
>   {
>       int64_t length;
>       int64_t n;
> -    int64_t ret;
> +    int64_t ret, ret2;
>   
>       length = bdrv_getlength(bs);
>       if (length < 0) {
> @@ -3117,6 +3117,20 @@ static int64_t coroutine_fn bdrv_co_get_block_status(BlockDriverState *bs,
>               }
>           }
>       }
> +
> +    if (bs->file &&
> +        (ret & BDRV_BLOCK_DATA) && !(ret & BDRV_BLOCK_ZERO) &&
> +        (ret & BDRV_BLOCK_OFFSET_VALID)) {
> +        ret2 = bdrv_co_get_block_status(bs->file, ret >> BDRV_SECTOR_BITS,
> +                                        *pnum, pnum);
> +        if (ret2 >= 0) {
> +            /* Ignore errors.  This is just providing extra information, it
> +             * is useful but not necessary.
> +             */
> +            ret |= (ret2 & BDRV_BLOCK_ZERO);
> +        }
> +    }
> +
>       return ret;
>   }
>   
for allocated blocks in iscsi this actually leads to 2 call outs to iscsi_get_block_status.

because the raw driver passes the get_block_status request to bs->file.