Re: [Qemu-devel] [PATCH 12/12] block/get_block_status: fix BDRV_BLOCK_ZERO for unallocated blocks

[Eric Blake]

On 09/13/2013 04:25 AM, Peter Lieven wrote:
> this patch does 2 things:
> a) only do additional call outs if BDRV_BLOCK_ZERO is not already set.
> b) use bdi.discard_zeroes to return the zero state of an unallocated block.
>    the callout to bdrv_has_zero_init() is only valid right after bdrv_create.
> 
> Signed-off-by: Peter Lieven <address@hidden>
> ---
>  block.c |    6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/block.c b/block.c
> index 282943b..6698c10 100644
> --- a/block.c
> +++ b/block.c
> @@ -3153,8 +3153,10 @@ 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)) {

Why not avoid the &&:

if ((ret & (BDRV_BLOCK_DATA | BDRV_BLOCK_ZERO)) == 0) {

Then again, any decent compiler will probably already do that
optimization on your behalf.

> +        BlockDriverInfo bdi;
> +        if (bdrv_get_info(bs, &bdi) == 0 &&
> +            bdi.discard_zeroes) {
>              ret |= BDRV_BLOCK_ZERO;
>          } else if (bs->backing_hd) {
>              BlockDriverState *bs2 = bs->backing_hd;
> 

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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