Re: [Qemu-devel] [PATCHv4 14/17] block/get_block_status: fix BDRV_BLOCK_ZERO for unallocated blocks

[Stefan Hajnoczi]

On Tue, Oct 08, 2013 at 01:58:08PM +0200, Peter Lieven wrote:
> this patch does 2 things:
> a) only do additional call outs if BDRV_BLOCK_ZERO is not already set.
> b) use the newly introduced bdrv_has_discard_zeroes() to return the
>    zero state of an unallocated block. the used callout to
>    bdrv_has_zero_init() is only valid right after bdrv_create.
> 
> Reviewed-by: Eric Blake <address@hidden>
> Signed-off-by: Peter Lieven <address@hidden>
> ---
>  block.c |    4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/block.c b/block.c
> index fc931e3..1be4418 100644
> --- a/block.c
> +++ b/block.c
> @@ -3247,8 +3247,8 @@ static int64_t coroutine_fn 
> bdrv_co_get_block_status(BlockDriverState *bs,
>          return ret;
>      }
>  
> -    if (!(ret & BDRV_BLOCK_DATA)) {
> -        if (bdrv_has_zero_init(bs)) {
> +    if (!(ret & BDRV_BLOCK_DATA) && !(ret & BDRV_BLOCK_ZERO)) {
> +        if (bdrv_has_discard_zeroes(bs)) {

I'm a little unclear about the semantics of bdrv_has_discard_zeroes().
Originally I thought it just meant any blocks discarded will read back
as zeroes.  But here it implies that any unallocated block reads
back as zeroes too?

In other words, this patch assumes unallocated blocks behave the same as
discarded blocks wrt to zeroes.

Stefan