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 21:10:03 (+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 ?

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
> >>
> >>
>