Re: [Qemu-devel] [PATCHv4 14/17] block/get_block_status: fix BDRV_BLOCK_ZERO for unallocated blocks

[Peter Lieven <pl@kamp.de>]

On 18.10.2013 14:38, Stefan Hajnoczi wrote:
> On Tue, Oct 08, 2013 at 01:58:08PM +0200, Peter Lieven wrote:
>> this patch does 2 things:
>> a) only do additional call outs if BDRV_BLOCK_ZERO is not already set.
>> b) use the newly introduced bdrv_has_discard_zeroes() to return the
>>     zero state of an unallocated block. the used callout to
>>     bdrv_has_zero_init() is only valid right after bdrv_create.
>>
>> Reviewed-by: Eric Blake <eblake@redhat.com>
>> Signed-off-by: Peter Lieven <pl@kamp.de>
>> ---
>>   block.c |    4 ++--
>>   1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/block.c b/block.c
>> index fc931e3..1be4418 100644
>> --- a/block.c
>> +++ b/block.c
>> @@ -3247,8 +3247,8 @@ static int64_t coroutine_fn bdrv_co_get_block_status(BlockDriverState *bs,
>>           return ret;
>>       }
>>   
>> -    if (!(ret & BDRV_BLOCK_DATA)) {
>> -        if (bdrv_has_zero_init(bs)) {
>> +    if (!(ret & BDRV_BLOCK_DATA) && !(ret & BDRV_BLOCK_ZERO)) {
>> +        if (bdrv_has_discard_zeroes(bs)) {
> I'm a little unclear about the semantics of bdrv_has_discard_zeroes().
> Originally I thought it just meant any blocks discarded will read back
> as zeroes.  But here it implies that any unallocated block reads
> back as zeroes too?
>
> In other words, this patch assumes unallocated blocks behave the same as
> discarded blocks wrt to zeroes.
>
> Stefan
bdrv_has_discard_zeroes() indicates if unallocated blocks read back as zeroes.
as a discard may silently fail this is no guarantee that a discard will result
in a properly zeroed area.

bdrv_has_discard_write_zeroes() indicates that the driver is able to
honour the BDRV_REQ_MAY_UNMAP flag on bdrv_write_zeroes().

see documentation in block_int.h

     /*
      * Returns true if discarded blocks read back as zeroes.
      */
     bool (*bdrv_has_discard_zeroes)(BlockDriverState *bs);

     /*
      * Returns true if the driver can optimize writing zeroes by discarding
      * sectors. It is additionally required that the block device is
      * opened with BDRV_O_UNMAP and the that write zeroes request carries
      * the BDRV_REQ_MAY_UNMAP flag for this to work.
      */
     bool (*bdrv_has_discard_write_zeroes)(BlockDriverState *bs);


Peter