Re: [Qemu-devel] [PATCHv2 07/20] block: honour alignment and limit in bdrv_co_do_write_zeroes

[Eric Blake <eblake@redhat.com>]

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On 09/17/2013 07:48 AM, Peter Lieven wrote:
> Signed-off-by: Peter Lieven <pl@kamp.de>
> ---
>  block.c |   57 +++++++++++++++++++++++++++++++++++++++++----------------
>  1 file changed, 41 insertions(+), 16 deletions(-)
> 
> diff --git a/block.c b/block.c
> index 177720e..74ec342 100644
> --- a/block.c
> +++ b/block.c
> @@ -2660,28 +2660,53 @@ static int coroutine_fn bdrv_co_do_write_zeroes(BlockDriverState *bs,
>      BlockDriver *drv = bs->drv;
>      QEMUIOVector qiov;
>      struct iovec iov;
> -    int ret;
> +    int ret = 0;
>  
> -    /* TODO Emulate only part of misaligned requests instead of letting block
> -     * drivers return -ENOTSUP and emulate everything */
> +    /* if no limit is specified in the BlockDriverState use a default
> +     * of 32768 512-byte sectors (16 MiB) per request.
> +     */
> +    int max_write_zeroes = bs->max_write_zeroes ? bs->max_write_zeroes : 32768;

Worth having a named constant instead of a magic number?

> +    while (nb_sectors > 0 && !ret) {

> +
> +        if (ret == -ENOTSUP) {
> +            /* Fall back to bounce buffer if write zeroes is unsupported */
> +            iov.iov_len  = nb_sectors * BDRV_SECTOR_SIZE;
> +            iov.iov_base = qemu_blockalign(bs, iov.iov_len);
> +            memset(iov.iov_base, 0, iov.iov_len);

This allocates, clears, and frees iov.iov_len bytes of 0 every iteration
through the loop.  Can you hoist that so you only allocate the bounce
buffer once, and then clean it up after the loop completes?

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


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