Re: [PATCH v2 3/6] block: Clarify that @bytes is no limit on *pnum

[Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>]

23.06.2021 18:01, Max Reitz wrote:
> .bdrv_co_block_status() implementations are free to return a *pnum that
> exceeds @bytes, because bdrv_co_block_status() in block/io.c will clamp
> *pnum as necessary.
> 
> On the other hand, if drivers' implementations return values for *pnum
> that are as large as possible, our recently introduced block-status
> cache will become more effective.
> 
> So, make a note in block_int.h that @bytes is no upper limit for *pnum.
> 
> Suggested-by: Eric Blake <eblake@redhat.com>
> Signed-off-by: Max Reitz <mreitz@redhat.com>
> ---
>   include/block/block_int.h | 5 +++++
>   1 file changed, 5 insertions(+)
> 
> diff --git a/include/block/block_int.h b/include/block/block_int.h
> index fcb599dd1c..f85b96ed99 100644
> --- a/include/block/block_int.h
> +++ b/include/block/block_int.h
> @@ -347,6 +347,11 @@ struct BlockDriver {
>        * clamped to bdrv_getlength() and aligned to request_alignment,
>        * as well as non-NULL pnum, map, and file; in turn, the driver
>        * must return an error or set pnum to an aligned non-zero value.
> +     *
> +     * Note that @bytes is just a hint on how big of a region the
> +     * caller wants to inspect.  It is not a limit on *pnum.
> +     * Implementations are free to return larger values of *pnum if
> +     * doing so does not incur a performance penalty.

Worth mention that the cache will benefit of it?

Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>

>        */
>       int coroutine_fn (*bdrv_co_block_status)(BlockDriverState *bs,
>           bool want_zero, int64_t offset, int64_t bytes, int64_t *pnum,
> 


-- 
Best regards,
Vladimir