Re: [Qemu-devel] [PATCH] vpc: Implement bdrv_co_get_status()

[Max Reitz <mreitz@redhat.com>]

On 2015-02-11 at 11:23, Kevin Wolf wrote:
> This implements bdrv_co_get_status() for VHD images. This can
> significantly speed up qemu-img convert operation because only with this
> function implemented sparseness can be considered. (Before, converting a
> 1 TB empty image took several minutes for me, now it's instantaneous.)
>
> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> ---
>   block/vpc.c | 54 ++++++++++++++++++++++++++++++++++++++++++++++++++++--
>   1 file changed, 52 insertions(+), 2 deletions(-)
>
> diff --git a/block/vpc.c b/block/vpc.c
> index 46803b1..379ff4a 100644
> --- a/block/vpc.c
> +++ b/block/vpc.c
> @@ -597,6 +597,55 @@ static coroutine_fn int vpc_co_write(BlockDriverState *bs, int64_t sector_num,
>       return ret;
>   }
>   
> +static int64_t coroutine_fn vpc_co_get_block_status(BlockDriverState *bs,
> +        int64_t sector_num, int nb_sectors, int *pnum)
> +{
> +    BDRVVPCState *s = bs->opaque;
> +    VHDFooter *footer =  (VHDFooter *) s->footer_buf;

Double space before the left parenthesis.

> +    int64_t start, offset;
> +    bool allocated;
> +    int n;
> +
> +    if (be32_to_cpu(footer->type) == VHD_FIXED) {
> +        *pnum = nb_sectors;
> +        return BDRV_BLOCK_RAW | BDRV_BLOCK_OFFSET_VALID | BDRV_BLOCK_DATA |
> +               (sector_num << BDRV_SECTOR_BITS);
> +    }
> +
> +    offset = get_sector_offset(bs, sector_num, 0);
> +
> +    *pnum = 1;
> +    start = offset;
> +    allocated = (offset != -1);
> +    nb_sectors--;
> +
> +    while (nb_sectors > 0) {
> +        offset = get_sector_offset(bs, sector_num, 0);

During the first iteration, this will is exactly the function call as 
the one before this loop.

> +
> +        if ((offset != -1) != allocated) {
> +            break;
> +        }
> +
> +        if (allocated && offset != start + (*pnum * 512)) {

Therefore, if allocated is true, this check will always fail because 
offset == start.

> +            break;
> +        }
> +
> +        /* All sectors in a block are contiguous (without using the bitmap) */
> +        n = ROUND_UP(sector_num + 1, s->block_size / 512) - sector_num;
> +        n = MIN(n, nb_sectors);

I think you can make use of this assumption even before calling 
get_sector_offset() in the first iteration.

Also, 512 is shorter than BDRV_SECTOR_SIZE and it's nice to get it on a 
single line, but I think not introducing another literal 512 in the code 
would be nicer; the same applies above in "*pnum * 512".

Max

> +
> +        *pnum += n;
> +        sector_num += n;
> +        nb_sectors -= n;
> +    }
> +
> +    if (allocated) {
> +        return BDRV_BLOCK_DATA | BDRV_BLOCK_OFFSET_VALID | start;
> +    } else {
> +        return 0;
> +    }
> +}
> +
>   /*
>    * Calculates the number of cylinders, heads and sectors per cylinder
>    * based on a given number of sectors. This is the algorithm described
> @@ -907,8 +956,9 @@ static BlockDriver bdrv_vpc = {
>       .bdrv_reopen_prepare    = vpc_reopen_prepare,
>       .bdrv_create            = vpc_create,
>   
> -    .bdrv_read              = vpc_co_read,
> -    .bdrv_write             = vpc_co_write,
> +    .bdrv_read                  = vpc_co_read,
> +    .bdrv_write                 = vpc_co_write,
> +    .bdrv_co_get_block_status   = vpc_co_get_block_status,
>   
>       .bdrv_get_info          = vpc_get_info,
>