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

[Max Reitz <mreitz@redhat.com>]

On 05.03.2014 21:41, Benoît Canet wrote:
> The Wednesday 05 Mar 2014 à 21:10:03 (+0100), Max Reitz wrote :
>> 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.
> The raw_bsd driver have an additional *pnum = nb_sectors; at the begining of the
> function. Did you left it on purpose ?

Oops, no, I did not. Okay, if I even fail at copying code, the argument 
about "raw" being at least probably correct isn't worth anything here, I 
guess. ;-)

Max

> Best regards
>
> Benoît
>>
>> 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
>>>>
>>>>