Re: [Qemu-devel] [PATCH v3 11/19] block: define get_block_status return value

[Kevin Wolf]

Am 25.07.2013 um 16:23 hat Paolo Bonzini geschrieben:
> Define the return value of get_block_status.  Bits 0, 1, 2 and 9-62
> are valid; bit 63 (the sign bit) is reserved for errors.  Bits 3-8
> are left for future extensions.
> 
> The return code is compatible with the old is_allocated API: if a driver
> only returns 0 or 1 (aka BDRV_BLOCK_DATA) like is_allocated used to,
> clients of is_allocated will not have any change in behavior.  Still,
> we will return more precise information in the next patches and the
> new definition of bdrv_is_allocated is already prepared for this.
> 
> Reviewed-by: Eric Blake <address@hidden>
> Signed-off-by: Paolo Bonzini <address@hidden>
> ---
>  block.c               | 10 ++++++++--
>  include/block/block.h | 26 ++++++++++++++++++++++++++
>  2 files changed, 34 insertions(+), 2 deletions(-)
> 
> diff --git a/block.c b/block.c
> index f533c36..7cfbf71 100644
> --- a/block.c
> +++ b/block.c
> @@ -3004,7 +3004,7 @@ static int64_t coroutine_fn 
> bdrv_co_get_block_status(BlockDriverState *bs,
>  
>      if (!bs->drv->bdrv_co_get_block_status) {
>          *pnum = nb_sectors;
> -        return 1;
> +        return BDRV_BLOCK_DATA;
>      }
>  
>      return bs->drv->bdrv_co_get_block_status(bs, sector_num, nb_sectors, 
> pnum);
> @@ -3054,7 +3054,13 @@ int64_t bdrv_get_block_status(BlockDriverState *bs, 
> int64_t sector_num,
>  int coroutine_fn bdrv_is_allocated(BlockDriverState *bs, int64_t sector_num,
>                                     int nb_sectors, int *pnum)
>  {
> -    return bdrv_get_block_status(bs, sector_num, nb_sectors, pnum);
> +    int64_t ret = bdrv_get_block_status(bs, sector_num, nb_sectors, pnum);
> +    if (ret < 0) {
> +        return ret;
> +    }
> +    return
> +        (ret & BDRV_BLOCK_DATA) ||
> +        ((ret & BDRV_BLOCK_ZERO) && !bdrv_has_zero_init(bs));
>  }
>  
>  /*
> diff --git a/include/block/block.h b/include/block/block.h
> index e41854e..d044b31 100644
> --- a/include/block/block.h
> +++ b/include/block/block.h
> @@ -81,6 +81,32 @@ typedef struct BlockDevOps {
>  #define BDRV_SECTOR_SIZE   (1ULL << BDRV_SECTOR_BITS)
>  #define BDRV_SECTOR_MASK   ~(BDRV_SECTOR_SIZE - 1)
>  
> +/* BDRV_BLOCK_DATA: data is read from bs->file or another file
> + * BDRV_BLOCK_ZERO: sectors read as zero
> + * BDRV_BLOCK_OFFSET_VALID: sector stored in bs->file as raw data
> + *
> + * If BDRV_BLOCK_OFFSET_VALID is set, bits 9-62 represent the offset in
> + * bs->file where sector data can be read from as raw data.
> + *
> + * DATA == 0 && ZERO == 0 means that data is read from backing_hd if present.
> + *
> + * DATA ZERO OFFSET_VALID
> + *  t    t        t       sectors read as zero, bs->file is zero at offset
> + *  t    f        t       sectors read as valid from bs->file at offset
> + *  f    t        t       sectors preallocated, read as zero, bs->file not
> + *                        necessarily zero at offset
> + *  f    f        t       sectors preallocated but read from backing_hd,
> + *                        bs->file contains garbage at offset
> + *  t    t        f       sectors preallocated, read as zero, unknown offset
> + *  t    f        f       sectors read from unknown file or offset
> + *  f    t        f       not allocated or unknown offset, read as zero
> + *  f    f        f       not allocated or unknown offset, read from 
> backing_hd
> + */
> +#define BDRV_BLOCK_DATA         1
> +#define BDRV_BLOCK_ZERO         2
> +#define BDRV_BLOCK_OFFSET_VALID 4
> +#define BDRV_BLOCK_OFFSET_MASK  BDRV_SECTOR_MASK

When are block driver supposed to set the BDRV_BLOCK_OFFSET_VALID flag?

For example, qcow2 could in theory set the flag, it has all of the
information already in memory. But with a fragmented image this might
mean that it returns only one cluster instead of a large area with one
bdrv_get_block_status() call.

Should the caller pass a flag that tells whether he is interested in the
offset or not?

Kevin