qcow2: Add subcluster-related fields to BDRVQcow2State

This patch adds the following new fields to BDRVQcow2State:

- subclusters_per_cluster: Number of subclusters in a cluster
- subcluster_size: The size of each subcluster, in bytes
- subcluster_bits: No. of bits so 1 << subcluster_bits = subcluster_size

Images without subclusters are treated as if they had exactly one
subcluster per cluster (i.e. subcluster_size = cluster_size).

Signed-off-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <55bfeac86b092fa2c9d182a95cbeb479ff7eca4f.1594396418.git.berto@igalia.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>

2 files changed, 10 insertions, 0 deletions

diff --git a/block/qcow2.c b/block/qcow2.c
index 6738daa..fb4584d 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -1444,6 +1444,11 @@ static int coroutine_fn qcow2_do_open(BlockDriverState *bs, QDict *options,
         }
     }
 
+    s->subclusters_per_cluster =
+        has_subclusters(s) ? QCOW_EXTL2_SUBCLUSTERS_PER_CLUSTER : 1;
+    s->subcluster_size = s->cluster_size / s->subclusters_per_cluster;
+    s->subcluster_bits = ctz32(s->subcluster_size);
+
     /* Check support for various header values */
     if (header.refcount_order > 6) {
         error_setg(errp, "Reference count entry width too large; may not "
diff --git a/block/qcow2.h b/block/qcow2.h
index 2064dd3..eee4c8d 100644
--- a/block/qcow2.h
+++ b/block/qcow2.h
@@ -78,6 +78,8 @@
 /* The cluster reads as all zeros */
 #define QCOW_OFLAG_ZERO (1ULL << 0)
 
+#define QCOW_EXTL2_SUBCLUSTERS_PER_CLUSTER 32
+
 #define MIN_CLUSTER_BITS 9
 #define MAX_CLUSTER_BITS 21
 
@@ -295,6 +297,9 @@ typedef struct BDRVQcow2State {
     int cluster_bits;
     int cluster_size;
     int l2_slice_size;
+    int subcluster_bits;
+    int subcluster_size;
+    int subclusters_per_cluster;
     int l2_bits;
     int l2_size;
     int l1_size;