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Date: Fri, 25 Mar 2016 10:01:15 +0800
From: Fam Zheng <famz@redhat.com>
Message-ID: <20160325020115.GA29171@ad.usersys.redhat.com>
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Subject: Re: [Qemu-devel] [PATCH v2 2/4] block/null-{co,
	aio}: Allow reading zeroes
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To: Max Reitz <mreitz@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>, qemu-devel@nongnu.org, qemu-block@nongnu.org

On Thu, 03/24 23:33, Max Reitz wrote:
> This is optional so that it does not impede the null block driver's
> performance unless this behavior is desired.
> 
> Signed-off-by: Max Reitz <mreitz@redhat.com>
> ---
>  block/null.c | 20 ++++++++++++++++++++
>  1 file changed, 20 insertions(+)
> 
> diff --git a/block/null.c b/block/null.c
> index d90165d..a7df386 100644
> --- a/block/null.c
> +++ b/block/null.c
> @@ -14,10 +14,12 @@
>  #include "block/block_int.h"
>  
>  #define NULL_OPT_LATENCY "latency-ns"
> +#define NULL_OPT_ZEROES  "read-zeroes"
>  
>  typedef struct {
>      int64_t length;
>      int64_t latency_ns;
> +    bool read_zeroes;
>  } BDRVNullState;
>  
>  static QemuOptsList runtime_opts = {
> @@ -40,6 +42,11 @@ static QemuOptsList runtime_opts = {
>              .help = "nanoseconds (approximated) to wait "
>                      "before completing request",
>          },
> +        {
> +            .name = NULL_OPT_ZEROES,
> +            .type = QEMU_OPT_BOOL,
> +            .help = "return zeroes when read",
> +        },
>          { /* end of list */ }
>      },
>  };
> @@ -61,6 +68,7 @@ static int null_file_open(BlockDriverState *bs, QDict *options, int flags,
>          error_setg(errp, "latency-ns is invalid");
>          ret = -EINVAL;
>      }
> +    s->read_zeroes = qemu_opt_get_bool(opts, NULL_OPT_ZEROES, false);
>      qemu_opts_del(opts);
>      return ret;
>  }
> @@ -90,6 +98,12 @@ static coroutine_fn int null_co_readv(BlockDriverState *bs,
>                                        int64_t sector_num, int nb_sectors,
>                                        QEMUIOVector *qiov)
>  {
> +    BDRVNullState *s = bs->opaque;
> +
> +    if (s->read_zeroes) {
> +        qemu_iovec_memset(qiov, 0, 0, nb_sectors * BDRV_SECTOR_SIZE);
> +    }
> +
>      return null_co_common(bs);
>  }
>  
> @@ -159,6 +173,12 @@ static BlockAIOCB *null_aio_readv(BlockDriverState *bs,
>                                    BlockCompletionFunc *cb,
>                                    void *opaque)
>  {
> +    BDRVNullState *s = bs->opaque;
> +
> +    if (s->read_zeroes) {
> +        qemu_iovec_memset(qiov, 0, 0, nb_sectors * BDRV_SECTOR_SIZE);
> +    }
> +
>      return null_aio_common(bs, cb, opaque);
>  }
>  
> -- 
> 2.7.4
> 

Acked-by: Fam Zheng <famz@redhat.com>