block: make bdrv_has_zero_init return false for copy-on-write-images

This helps implementing is_allocated on top of get_block_status.

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

1 files changed, 5 insertions, 0 deletions

diff --git a/block.c b/block.c
index 5df17de5a4..2a90893464 100644
--- a/block.c
+++ b/block.c
@@ -3031,6 +3031,11 @@ int bdrv_has_zero_init(BlockDriverState *bs)
 {
     assert(bs->drv);
 
+    /* If BS is a copy on write image, it is initialized to
+       the contents of the base image, which may not be zeroes.  */
+    if (bs->backing_hd) {
+        return 0;
+    }
     if (bs->drv->bdrv_has_zero_init) {
         return bs->drv->bdrv_has_zero_init(bs);
     }