block/vdi: return ZERO block-status when appropriate

In case of !VDI_IS_ALLOCATED[], we do zero out the corresponding chunk
of qiov. So, this should be reported as ZERO.

Note that this changes visible output of "qemu-img map --output=json"
and "qemu-io -c map" commands. For qemu-img map, the change is obvious:
we just mark as zero what is really zero. For qemu-io it's less
obvious: what was unallocated now is allocated.

There is an inconsistency in understanding of unallocated regions in
Qemu: backing-supporting format-drivers return 0 block-status to report
go-to-backing logic for this area. Some protocol-drivers (iscsi) return
0 to report fs-unallocated-non-zero status (i.e., don't occupy space on
disk, read result is undefined).

BDRV_BLOCK_ALLOCATED is defined as something more close to
go-to-backing logic. Still it is calculated as ZERO | DATA, so 0 from
iscsi is treated as unallocated. It doesn't influence backing-chain
behavior, as iscsi can't have backing file. But it does influence
"qemu-io -c map".

We should solve this inconsistency at some future point. Now, let's
just make backing-not-supporting format drivers (vdi at this patch and
vpc with the following) to behave more like backing-supporting drivers
and not report 0 block-status. More over, returning ZERO status is
absolutely valid thing, and again, corresponds to how the other
format-drivers (backing-supporting) work.

After block-status update, it never reports 0, so setting
unallocated_blocks_are_zero doesn't make sense (as the only user of it
is bdrv_co_block_status and it checks unallocated_blocks_are_zero only
for unallocated areas). Drop it.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20200528094405.145708-4-vsementsov@virtuozzo.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>

1 files changed, 1 insertions, 2 deletions

diff --git a/block/vdi.c b/block/vdi.c
index 2f506a01ba..c4527a9d8c 100644
--- a/block/vdi.c
+++ b/block/vdi.c
@@ -334,7 +334,6 @@ static int vdi_get_info(BlockDriverState *bs, BlockDriverInfo *bdi)
     logout("\n");
     bdi->cluster_size = s->block_size;
     bdi->vm_state_offset = 0;
-    bdi->unallocated_blocks_are_zero = true;
     return 0;
 }
 
@@ -536,7 +535,7 @@ static int coroutine_fn vdi_co_block_status(BlockDriverState *bs,
     *pnum = MIN(s->block_size - index_in_block, bytes);
     result = VDI_IS_ALLOCATED(bmap_entry);
     if (!result) {
-        return 0;
+        return BDRV_BLOCK_ZERO;
     }
 
     *map = s->header.offset_data + (uint64_t)bmap_entry * s->block_size +