Re: [Qemu-devel] [PATCH 05/10] block/json: Add bdrv_co_get_block_status()

[Max Reitz <mreitz@redhat.com>]

On 05.03.2014 17:11, Benoît Canet wrote:
> The Monday 03 Mar 2014 à 16:28:48 (+0100), Max Reitz wrote :
>> Implement this function in the same way as raw_bsd does: Acknowledge
>> that this is a passthrough driver (always return BDRV_BLOCK_OFFSET_VALID
>> and BDRV_BLOCK_DATA and derive the offset directly from the sector
>> index) and add BDRV_BLOCK_RAW to the returned value.
>>
>> Signed-off-by: Max Reitz <mreitz@redhat.com>
>> ---
>>   block/json.c | 9 +++++++++
>>   1 file changed, 9 insertions(+)
>>
>> diff --git a/block/json.c b/block/json.c
>> index a2f4691..7392802 100644
>> --- a/block/json.c
>> +++ b/block/json.c
>> @@ -113,6 +113,14 @@ static coroutine_fn int json_co_write_zeroes(BlockDriverState *bs,
>>       return bdrv_co_write_zeroes(bs, sector_num, nb_sectors, flags);
>>   }
>>   
>> +static coroutine_fn int64_t json_co_get_block_status(BlockDriverState *bs,
>> +                                                     int64_t sector_num,
>> +                                                     int nb_sectors, int *pnum)
>> +{
>> +    return BDRV_BLOCK_RAW | BDRV_BLOCK_OFFSET_VALID | BDRV_BLOCK_DATA |
>> +           (sector_num << BDRV_SECTOR_BITS);
>> +}
> I don't understand what is the selling point of this method instead of calling
> bdrv_co_get_block_status on bs->file.
> Some information risk to be lost and it does look like magic.

This is the same what "raw" does. It just is more meaningful: This way, 
this function does not pretend to provide the blocks itself but instead 
tells the truth; that is, the blocks are provided by an underlying BDS 
(bs->file).

I wasn't really sure what to do myself. Generally, this driver is 
actually meant to pretend that it provides the blocks itself. On the 
other hand, I tried to imitate the behavior or "raw", since this is 
something I can hope to be approximately correct. Also, as I've said 
before, the value returned here is in fact at least technically correct.


Max

> Best regards
>
> Benoît
>
>> +
>>   static void json_invalidate_cache(BlockDriverState *bs)
>>   {
>>       return bdrv_invalidate_cache(bs->file);
>> @@ -159,6 +167,7 @@ static BlockDriver bdrv_json = {
>>       .bdrv_aio_discard           = json_aio_discard,
>>   
>>       .bdrv_co_write_zeroes       = json_co_write_zeroes,
>> +    .bdrv_co_get_block_status   = json_co_get_block_status,
>>   
>>       .bdrv_invalidate_cache      = json_invalidate_cache,
>>   
>> -- 
>> 1.9.0
>>
>>