3 files changed, 92 insertions, 0 deletions

diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c
index 1c2003a8a4..5fb8c6695a 100644
--- a/block/qcow2-cluster.c
+++ b/block/qcow2-cluster.c
@@ -888,3 +888,85 @@ int qcow2_decompress_cluster(BlockDriverState *bs, uint64_t cluster_offset)
     }
     return 0;
 }
+
+/*
+ * This discards as many clusters of nb_clusters as possible at once (i.e.
+ * all clusters in the same L2 table) and returns the number of discarded
+ * clusters.
+ */
+static int discard_single_l2(BlockDriverState *bs, uint64_t offset,
+    unsigned int nb_clusters)
+{
+    BDRVQcowState *s = bs->opaque;
+    uint64_t l2_offset, *l2_table;
+    int l2_index;
+    int ret;
+    int i;
+
+    ret = get_cluster_table(bs, offset, &l2_table, &l2_offset, &l2_index);
+    if (ret < 0) {
+        return ret;
+    }
+
+    /* Limit nb_clusters to one L2 table */
+    nb_clusters = MIN(nb_clusters, s->l2_size - l2_index);
+
+    for (i = 0; i < nb_clusters; i++) {
+        uint64_t old_offset;
+
+        old_offset = be64_to_cpu(l2_table[l2_index + i]);
+        old_offset &= ~QCOW_OFLAG_COPIED;
+
+        if (old_offset == 0) {
+            continue;
+        }
+
+        /* First remove L2 entries */
+        qcow2_cache_entry_mark_dirty(s->l2_table_cache, l2_table);
+        l2_table[l2_index + i] = cpu_to_be64(0);
+
+        /* Then decrease the refcount */
+        qcow2_free_any_clusters(bs, old_offset, 1);
+    }
+
+    ret = qcow2_cache_put(bs, s->l2_table_cache, (void**) &l2_table);
+    if (ret < 0) {
+        return ret;
+    }
+
+    return nb_clusters;
+}
+
+int qcow2_discard_clusters(BlockDriverState *bs, uint64_t offset,
+    int nb_sectors)
+{
+    BDRVQcowState *s = bs->opaque;
+    uint64_t end_offset;
+    unsigned int nb_clusters;
+    int ret;
+
+    end_offset = offset + (nb_sectors << BDRV_SECTOR_BITS);
+
+    /* Round start up and end down */
+    offset = align_offset(offset, s->cluster_size);
+    end_offset &= ~(s->cluster_size - 1);
+
+    if (offset > end_offset) {
+        return 0;
+    }
+
+    nb_clusters = size_to_clusters(s, end_offset - offset);
+
+    /* Each L2 table is handled by its own loop iteration */
+    while (nb_clusters > 0) {
+        ret = discard_single_l2(bs, offset, nb_clusters);
+        if (ret < 0) {
+            return ret;
+        }
+
+        nb_clusters -= ret;
+        offset += (ret * s->cluster_size);
+    }
+
+    return 0;
+}
diff --git a/block/qcow2.c b/block/qcow2.c
index 49bf7b9d3d..dbe4fdd44d 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -1084,6 +1084,13 @@ static int qcow2_make_empty(BlockDriverState *bs)
     return 0;
 }
 
+static int qcow2_discard(BlockDriverState *bs, int64_t sector_num,
+    int nb_sectors)
+{
+    return qcow2_discard_clusters(bs, sector_num << BDRV_SECTOR_BITS,
+        nb_sectors);
+}
+
 static int qcow2_truncate(BlockDriverState *bs, int64_t offset)
 {
     BDRVQcowState *s = bs->opaque;
@@ -1349,6 +1356,7 @@ static BlockDriver bdrv_qcow2 = {
     .bdrv_aio_writev    = qcow2_aio_writev,
     .bdrv_aio_flush     = qcow2_aio_flush,
 
+    .bdrv_discard           = qcow2_discard,
     .bdrv_truncate          = qcow2_truncate,
     .bdrv_write_compressed  = qcow2_write_compressed,
 
diff --git a/block/qcow2.h b/block/qcow2.h
index 6d801206e2..a019831838 100644
--- a/block/qcow2.h
+++ b/block/qcow2.h
@@ -209,6 +209,8 @@ uint64_t qcow2_alloc_compressed_cluster_offset(BlockDriverState *bs,
                                          int compressed_size);
 
 int qcow2_alloc_cluster_link_l2(BlockDriverState *bs, QCowL2Meta *m);
+int qcow2_discard_clusters(BlockDriverState *bs, uint64_t offset,
+    int nb_sectors);
 
 /* qcow2-snapshot.c functions */
 int qcow2_snapshot_create(BlockDriverState *bs, QEMUSnapshotInfo *sn_info);