block: add subcluster_size field to BlockDriverInfo

This is going to be used in the subsequent commit as requests alignment
(in particular, during copy-on-read).  This value only makes sense for
the formats which support subclusters (currently QCOW2 only).  If this
field isn't set by driver's own bdrv_get_info() implementation, we
simply set it equal to the cluster size thus treating each cluster as
having a single subcluster.

Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Denis V. Lunev <den@openvz.org>
Signed-off-by: Andrey Drobyshev <andrey.drobyshev@virtuozzo.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-ID: <20230711172553.234055-2-andrey.drobyshev@virtuozzo.com>

1 files changed, 7 insertions, 0 deletions

diff --git a/block.c b/block.c
index a307c151a8..0af890f647 100644
--- a/block.c
+++ b/block.c
@@ -6480,6 +6480,13 @@ int coroutine_fn bdrv_co_get_info(BlockDriverState *bs, BlockDriverInfo *bdi)
     }
     memset(bdi, 0, sizeof(*bdi));
     ret = drv->bdrv_co_get_info(bs, bdi);
+    if (bdi->subcluster_size == 0) {
+        /*
+         * If the driver left this unset, subclusters are not supported.
+         * Then it is safe to treat each cluster as having only one subcluster.
+         */
+        bdi->subcluster_size = bdi->cluster_size;
+    }
     if (ret < 0) {
         return ret;
     }