Re: [Qemu-devel] [PATCH 1/2] block: Fix unaligned zero write

[Stefan Hajnoczi]

On Mon, Mar 23, 2015 at 12:46:09PM +0800, Fam Zheng wrote:
> @@ -3435,6 +3446,10 @@ static int coroutine_fn 
> bdrv_co_do_pwritev(BlockDriverState *bs,
>          bytes = ROUND_UP(bytes, align);
>      }
>  
> +    if (use_local_qiov) {
> +        /* Local buffer may have non-zero data. */
> +        flags &= ~BDRV_REQ_ZERO_WRITE;
> +    }
>      ret = bdrv_aligned_pwritev(bs, &req, offset, bytes,
>                                 use_local_qiov ? &local_qiov : qiov,
>                                 flags);
> @@ -3475,14 +3490,32 @@ int coroutine_fn 
> bdrv_co_write_zeroes(BlockDriverState *bs,
>                                        int64_t sector_num, int nb_sectors,
>                                        BdrvRequestFlags flags)
>  {
> +    int ret;
> +
>      trace_bdrv_co_write_zeroes(bs, sector_num, nb_sectors, flags);
>  
> -    if (!(bs->open_flags & BDRV_O_UNMAP)) {
> -        flags &= ~BDRV_REQ_MAY_UNMAP;
> -    }

Why is it okay to drop this when the request is aligned?

> +    if (bdrv_req_is_aligned(bs, sector_num << BDRV_SECTOR_BITS,
> +                            nb_sectors << BDRV_SECTOR_BITS)) {
> +        ret = bdrv_co_do_writev(bs, sector_num, nb_sectors, NULL,
> +                                BDRV_REQ_ZERO_WRITE | flags);
> +    } else {

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