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

["Benoît Canet" <benoit.canet@irqsave.net>]

The Wednesday 05 Mar 2014 à 21:44:57 (+0100), Max Reitz wrote :
> 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. ;-)

In fact I wonder i posix_raw act this way because one of the BDRV it does wrap
have simply no clue on how to answer to this request.

Best regards

Benoît

> 
> 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
> >>>>
> >>>>
>