diff --git a/block/qcow2.h b/block/qcow2.h
index a99d51b..c4eaf67 100644
--- a/block/qcow2.h
+++ b/block/qcow2.h
@@ -266,6 +266,11 @@
 
 #define REFT_OFFSET_MASK 0xffffffffffffff00ULL
 
+static inline int64_t start_of_cluster(BDRVQcowState *s, int64_t offset)
+{
+    return offset & ~(s->cluster_size - 1);
+}
+
 static inline int size_to_clusters(BDRVQcowState *s, int64_t size)
 {
     return (size + (s->cluster_size - 1)) >> s->cluster_bits;