block: drop unallocated_blocks_are_zero

Currently this field only set by qed and qcow2. But in fact, all
backing-supporting formats (parallels, qcow, qcow2, qed, vmdk) share
these semantics: on unallocated blocks, if there is no backing file they
just memset the buffer with zeroes.

So, document this behavior for .supports_backing and drop
.unallocated_blocks_are_zero

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20200528094405.145708-10-vsementsov@virtuozzo.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>

3 files changed, 2 insertions, 9 deletions

diff --git a/block/io.c b/block/io.c
index 36d4d56..b6564e3 100644
--- a/block/io.c
+++ b/block/io.c
@@ -2406,7 +2406,7 @@ static int coroutine_fn bdrv_co_block_status(BlockDriverState *bs,
 
     if (ret & (BDRV_BLOCK_DATA | BDRV_BLOCK_ZERO)) {
         ret |= BDRV_BLOCK_ALLOCATED;
-    } else if (want_zero) {
+    } else if (want_zero && bs->drv->supports_backing) {
         if (bs->backing) {
             BlockDriverState *bs2 = bs->backing->bs;
             int64_t size2 = bdrv_getlength(bs2);
@@ -2415,12 +2415,7 @@ static int coroutine_fn bdrv_co_block_status(BlockDriverState *bs,
                 ret |= BDRV_BLOCK_ZERO;
             }
         } else {
-            BlockDriverInfo bdi;
-            int ret2 = bdrv_get_info(bs, &bdi);
-
-            if (ret2 == 0 && bdi.unallocated_blocks_are_zero) {
-                ret |= BDRV_BLOCK_ZERO;
-            }
+            ret |= BDRV_BLOCK_ZERO;
         }
     }
 
diff --git a/block/qcow2.c b/block/qcow2.c
index 30f073c..38198b4 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -4987,7 +4987,6 @@ err:
 static int qcow2_get_info(BlockDriverState *bs, BlockDriverInfo *bdi)
 {
     BDRVQcow2State *s = bs->opaque;
-    bdi->unallocated_blocks_are_zero = true;
     bdi->cluster_size = s->cluster_size;
     bdi->vm_state_offset = qcow2_vm_state_offset(s);
     return 0;
diff --git a/block/qed.c b/block/qed.c
index c0c6501..a2dd952 100644
--- a/block/qed.c
+++ b/block/qed.c
@@ -1514,7 +1514,6 @@ static int bdrv_qed_get_info(BlockDriverState *bs, BlockDriverInfo *bdi)
     memset(bdi, 0, sizeof(*bdi));
     bdi->cluster_size = s->header.cluster_size;
     bdi->is_dirty = s->header.features & QED_F_NEED_CHECK;
-    bdi->unallocated_blocks_are_zero = true;
     return 0;
 }