Re: [Qemu-devel] [PATCH 2/4] block/null-{co, aio}: Return zeros when read

From: Eric Blake <eblake@redhat.com>
To: Max Reitz <mreitz@redhat.com>, qemu-block@nongnu.org
Cc: Kevin Wolf <kwolf@redhat.com>, Fam Zheng <famz@redhat.com>,
	qemu-devel@nongnu.org
Subject: Re: [Qemu-devel] [PATCH 2/4] block/null-{co, aio}: Return zeros when read
Date: Mon, 22 Feb 2016 09:31:44 -0700	[thread overview]
Message-ID: <56CB37F0.8080805@redhat.com> (raw)
In-Reply-To: <1455989993-1917-3-git-send-email-mreitz@redhat.com>

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On 02/20/2016 10:39 AM, Max Reitz wrote:
> Currently, we do not define exactly what is returned when read, but
> having a reliable source of zeros is always nice.
> 
> Signed-off-by: Max Reitz <mreitz@redhat.com>
> ---
>  block/null.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/block/null.c b/block/null.c
> index d90165d..ad883d9 100644
> --- a/block/null.c
> +++ b/block/null.c
> @@ -90,6 +90,7 @@ static coroutine_fn int null_co_readv(BlockDriverState *bs,
>                                        int64_t sector_num, int nb_sectors,
>                                        QEMUIOVector *qiov)
>  {
> +    qemu_iovec_memset(qiov, 0, 0, nb_sectors * BDRV_SECTOR_SIZE);
>      return null_co_common(bs);
>  }

Shouldn't this be made optional?  That is, one of the points of null-co
was to be as fast as possible, and the memset will slow us down.  So
having an optional bool as part of the null-co setup, which defaults off
for back-compat but can be turned on for guaranteed all-zero reads,
sounds like it would be nicer than forcing all-zero reads.

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

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