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

[Max Reitz]

On 22.02.2016 17:31, Eric Blake wrote:
> 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 <address@hidden>
>> ---
>>  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.

Can do, will do, thanks.

Max

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