[PATCH for-5.1 1/2] qcow2: Implement v2 zero writes with discard if possible

[Kevin Wolf <kwolf@redhat.com>]

qcow2 version 2 images don't support the zero flag for clusters, so for
write_zeroes requests, we return -ENOTSUP and get explicit zero buffer
writes. If the image doesn't have a backing file, we can do better: Just
discard the respective clusters.

This is relevant for 'qemu-img convert -O qcow2 -n', where qemu-img has
to assume that the existing target image may contain any data, so it has
to write zeroes. Without this patch, this results in a fully allocated
target image, even if the source image was empty.

Reported-by: Nir Soffer <nsoffer@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
---
 block/qcow2-cluster.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c
index 4b5fc8c4a7..a677ba9f5c 100644
--- a/block/qcow2-cluster.c
+++ b/block/qcow2-cluster.c
@@ -1797,8 +1797,15 @@ int qcow2_cluster_zeroize(BlockDriverState *bs, uint64_t offset,
     assert(QEMU_IS_ALIGNED(end_offset, s->cluster_size) ||
            end_offset >= bs->total_sectors << BDRV_SECTOR_BITS);
 
-    /* The zero flag is only supported by version 3 and newer */
+    /*
+     * The zero flag is only supported by version 3 and newer. However, if we
+     * have no backing file, we can resort to discard in version 2.
+     */
     if (s->qcow_version < 3) {
+        if (!bs->backing) {
+            return qcow2_cluster_discard(bs, offset, bytes,
+                                         QCOW2_DISCARD_REQUEST, false);
+        }
         return -ENOTSUP;
     }
 
-- 
2.25.4