[Qemu-devel] [PATCH v6 01/14] qcow2: Allow "full" discard

[Max Reitz]

Normally, discarded sectors should read back as zero. However, there are
cases in which a sector (or rather cluster) should be discarded as if
they were never written in the first place, that is, reading them should
fall through to the backing file again.

Signed-off-by: Max Reitz <address@hidden>
---
 block/qcow2-cluster.c  | 27 +++++++++++++++++----------
 block/qcow2-snapshot.c |  2 +-
 block/qcow2.c          |  2 +-
 block/qcow2.h          |  2 +-
 4 files changed, 20 insertions(+), 13 deletions(-)

diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c
index b746429..f131562 100644
--- a/block/qcow2-cluster.c
+++ b/block/qcow2-cluster.c
@@ -1343,7 +1343,7 @@ int qcow2_decompress_cluster(BlockDriverState *bs, 
uint64_t cluster_offset)
  * clusters.
  */
 static int discard_single_l2(BlockDriverState *bs, uint64_t offset,
-    unsigned int nb_clusters, enum qcow2_discard_type type)
+    unsigned int nb_clusters, enum qcow2_discard_type type, bool full_discard)
 {
     BDRVQcowState *s = bs->opaque;
     uint64_t *l2_table;
@@ -1365,23 +1365,30 @@ static int discard_single_l2(BlockDriverState *bs, 
uint64_t offset,
         old_l2_entry = be64_to_cpu(l2_table[l2_index + i]);
 
         /*
-         * Make sure that a discarded area reads back as zeroes for v3 images
-         * (we cannot do it for v2 without actually writing a zero-filled
-         * buffer). We can skip the operation if the cluster is already marked
-         * as zero, or if it's unallocated and we don't have a backing file.
+         * If full_discard is false, make sure that a discarded area reads back
+         * as zeroes for v3 images (we cannot do it for v2 without actually
+         * writing a zero-filled buffer). We can skip the operation if the
+         * cluster is already marked as zero, or if it's unallocated and we
+         * don't have a backing file.
          *
          * TODO We might want to use bdrv_get_block_status(bs) here, but we're
          * holding s->lock, so that doesn't work today.
+         *
+         * In case of full_discard being true, the sector should not be read
+         * back as zeroes, but rather fall through to the backing file.
          */
         switch (qcow2_get_cluster_type(old_l2_entry)) {
             case QCOW2_CLUSTER_UNALLOCATED:
-                if (!bs->backing_hd) {
+                if (full_discard || !bs->backing_hd) {
                     continue;
                 }
                 break;
 
             case QCOW2_CLUSTER_ZERO:
-                continue;
+                if (!full_discard) {
+                    continue;
+                }
+                break;
 
             case QCOW2_CLUSTER_NORMAL:
             case QCOW2_CLUSTER_COMPRESSED:
@@ -1393,7 +1400,7 @@ static int discard_single_l2(BlockDriverState *bs, 
uint64_t offset,
 
         /* First remove L2 entries */
         qcow2_cache_entry_mark_dirty(s->l2_table_cache, l2_table);
-        if (s->qcow_version >= 3) {
+        if (!full_discard && s->qcow_version >= 3) {
             l2_table[l2_index + i] = cpu_to_be64(QCOW_OFLAG_ZERO);
         } else {
             l2_table[l2_index + i] = cpu_to_be64(0);
@@ -1412,7 +1419,7 @@ static int discard_single_l2(BlockDriverState *bs, 
uint64_t offset,
 }
 
 int qcow2_discard_clusters(BlockDriverState *bs, uint64_t offset,
-    int nb_sectors, enum qcow2_discard_type type)
+    int nb_sectors, enum qcow2_discard_type type, bool full_discard)
 {
     BDRVQcowState *s = bs->opaque;
     uint64_t end_offset;
@@ -1435,7 +1442,7 @@ int qcow2_discard_clusters(BlockDriverState *bs, uint64_t 
offset,
 
     /* Each L2 table is handled by its own loop iteration */
     while (nb_clusters > 0) {
-        ret = discard_single_l2(bs, offset, nb_clusters, type);
+        ret = discard_single_l2(bs, offset, nb_clusters, type, full_discard);
         if (ret < 0) {
             goto fail;
         }
diff --git a/block/qcow2-snapshot.c b/block/qcow2-snapshot.c
index 0aa9def..c5ea2cd 100644
--- a/block/qcow2-snapshot.c
+++ b/block/qcow2-snapshot.c
@@ -436,7 +436,7 @@ int qcow2_snapshot_create(BlockDriverState *bs, 
QEMUSnapshotInfo *sn_info)
     qcow2_discard_clusters(bs, qcow2_vm_state_offset(s),
                            align_offset(sn->vm_state_size, s->cluster_size)
                                 >> BDRV_SECTOR_BITS,
-                           QCOW2_DISCARD_NEVER);
+                           QCOW2_DISCARD_NEVER, false);
 
 #ifdef DEBUG_ALLOC
     {
diff --git a/block/qcow2.c b/block/qcow2.c
index e903d97..1e7b7d5 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -1864,7 +1864,7 @@ static coroutine_fn int qcow2_co_discard(BlockDriverState 
*bs,
 
     qemu_co_mutex_lock(&s->lock);
     ret = qcow2_discard_clusters(bs, sector_num << BDRV_SECTOR_BITS,
-        nb_sectors, QCOW2_DISCARD_REQUEST);
+        nb_sectors, QCOW2_DISCARD_REQUEST, false);
     qemu_co_mutex_unlock(&s->lock);
     return ret;
 }
diff --git a/block/qcow2.h b/block/qcow2.h
index b49424b..2332634 100644
--- a/block/qcow2.h
+++ b/block/qcow2.h
@@ -519,7 +519,7 @@ uint64_t 
qcow2_alloc_compressed_cluster_offset(BlockDriverState *bs,
 
 int qcow2_alloc_cluster_link_l2(BlockDriverState *bs, QCowL2Meta *m);
 int qcow2_discard_clusters(BlockDriverState *bs, uint64_t offset,
-    int nb_sectors, enum qcow2_discard_type type);
+    int nb_sectors, enum qcow2_discard_type type, bool full_discard);
 int qcow2_zero_clusters(BlockDriverState *bs, uint64_t offset, int nb_sectors);
 
 int qcow2_expand_zero_clusters(BlockDriverState *bs);
-- 
1.9.2