Re: [PATCH v2 1/2] block/null: Report DATA if not reading zeroes

[Markus Armbruster <armbru@redhat.com>]

Nir Soffer <nirsof@gmail.com> writes:

> If read-zeroes is not set, we did not report BDRV_BLOCK_DATA or
> BDRV_BLOCK_ZERO. This is not consistent with other drivers and can
> confuse users or other programs:
>
>     % qemu-img map --output json "json:{'driver': 'raw', 'file': {'driver': 'null-co', 'size': '1g'}}"
>     [{ "start": 0, "length": 1073741824, "depth": 0, "present": false, "zero": false, "data": false, "compressed": false}]
>
>     % qemu-nbd "json:{'driver': 'raw', 'file': {'driver': 'null-co', 'size': '1g'}}" &
>
>     % nbdinfo --map nbd://127.0.0.1
>              0  1073741824    1  hole
>
> With this change we report DATA in this case:
>
>     % ./qemu-img map --output json "json:{'driver': 'raw', 'file': {'driver': 'null-co', 'size': '1g'}}"
>     [{ "start": 0, "length": 1073741824, "depth": 0, "present": true, "zero": false, "data": true, "compressed": false, "offset": 0}]
>
>     % ./qemu-nbd "json:{'driver': 'raw', 'file': {'driver': 'null-co', 'size': '1g'}}" &
>
>     % nbdinfo --map nbd://127.0.0.1
>              0  1073741824    0  data
>
> Signed-off-by: Nir Soffer <nirsof@gmail.com>
> ---
>  block/null.c         | 4 +---
>  qapi/block-core.json | 5 +++--
>  2 files changed, 4 insertions(+), 5 deletions(-)
>
> diff --git a/block/null.c b/block/null.c
> index dc0b1fdbd9..7ba87bd9a9 100644
> --- a/block/null.c
> +++ b/block/null.c
> @@ -239,9 +239,7 @@ static int coroutine_fn null_co_block_status(BlockDriverState *bs,
>      *map = offset;
>      *file = bs;
>  
> -    if (s->read_zeroes) {
> -        ret |= BDRV_BLOCK_ZERO;
> -    }
> +    ret |= s->read_zeroes ? BDRV_BLOCK_ZERO : BDRV_BLOCK_DATA;
>      return ret;
>  }
>  
> diff --git a/qapi/block-core.json b/qapi/block-core.json
> index b1937780e1..7c95c9e36a 100644
> --- a/qapi/block-core.json
> +++ b/qapi/block-core.json
> @@ -3293,8 +3293,9 @@
>  #     requests.  Default to zero which completes requests immediately.
>  #     (Since 2.4)
>  #
> -# @read-zeroes: if true, reads from the device produce zeroes; if
> -#     false, the buffer is left unchanged.
> +# @read-zeroes: if true, emulate a sparse image, and reads from the
> +#     device produce zeroes; if false, emulate an allocated image but
> +#     reads from the device leave the buffer unchanged.
>  #     (default: false; since: 4.1)
>  #
>  # Since: 2.9

Possibly dumb question: how is this doc change related to the code fix?

Suggest to split the sentence for easier reading:

   # @read-zeroes: If true, emulate a sparse image, and reads from the
   #     device produce zeroes.  If false, emulate an allocated image,
   #     but reads from the device leave the buffer unchanged.