2 files changed, 63 insertions, 20 deletions

diff --git a/block/io.c b/block/io.c
index 02528b3823..9918f2499c 100644
--- a/block/io.c
+++ b/block/io.c
@@ -633,6 +633,19 @@ void bdrv_drain_all_begin(void)
     }
 }
 
+void bdrv_drain_all_end_quiesce(BlockDriverState *bs)
+{
+    int drained_end_counter = 0;
+
+    g_assert(bs->quiesce_counter > 0);
+    g_assert(!bs->refcnt);
+
+    while (bs->quiesce_counter) {
+        bdrv_do_drained_end(bs, false, NULL, true, &drained_end_counter);
+    }
+    BDRV_POLL_WHILE(bs, qatomic_read(&drained_end_counter) > 0);
+}
+
 void bdrv_drain_all_end(void)
 {
     BlockDriverState *bs = NULL;
@@ -2282,17 +2295,17 @@ static int coroutine_fn bdrv_co_block_status(BlockDriverState *bs,
 
     if (ret & (BDRV_BLOCK_DATA | BDRV_BLOCK_ZERO)) {
         ret |= BDRV_BLOCK_ALLOCATED;
-    } else if (want_zero && bs->drv->supports_backing) {
+    } else if (bs->drv->supports_backing) {
         BlockDriverState *cow_bs = bdrv_cow_bs(bs);
 
-        if (cow_bs) {
+        if (!cow_bs) {
+            ret |= BDRV_BLOCK_ZERO;
+        } else if (want_zero) {
             int64_t size2 = bdrv_getlength(cow_bs);
 
             if (size2 >= 0 && offset >= size2) {
                 ret |= BDRV_BLOCK_ZERO;
             }
-        } else {
-            ret |= BDRV_BLOCK_ZERO;
         }
     }
 
@@ -2447,6 +2460,33 @@ int bdrv_block_status(BlockDriverState *bs, int64_t offset, int64_t bytes,
                                    offset, bytes, pnum, map, file);
 }
 
+/*
+ * Check @bs (and its backing chain) to see if the range defined
+ * by @offset and @bytes is known to read as zeroes.
+ * Return 1 if that is the case, 0 otherwise and -errno on error.
+ * This test is meant to be fast rather than accurate so returning 0
+ * does not guarantee non-zero data.
+ */
+int coroutine_fn bdrv_co_is_zero_fast(BlockDriverState *bs, int64_t offset,
+                                      int64_t bytes)
+{
+    int ret;
+    int64_t pnum = bytes;
+
+    if (!bytes) {
+        return 1;
+    }
+
+    ret = bdrv_common_block_status_above(bs, NULL, false, false, offset,
+                                         bytes, &pnum, NULL, NULL);
+
+    if (ret < 0) {
+        return ret;
+    }
+
+    return (pnum == bytes) && (ret & BDRV_BLOCK_ZERO);
+}
+
 int coroutine_fn bdrv_is_allocated(BlockDriverState *bs, int64_t offset,
                                    int64_t bytes, int64_t *pnum)
 {
diff --git a/block/qcow2.c b/block/qcow2.c
index b6cb4db8bb..4274806a2a 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -2387,26 +2387,26 @@ static bool merge_cow(uint64_t offset, unsigned bytes,
     return false;
 }
 
-static bool is_unallocated(BlockDriverState *bs, int64_t offset, int64_t bytes)
-{
-    int64_t nr;
-    return !bytes ||
-        (!bdrv_is_allocated_above(bs, NULL, false, offset, bytes, &nr) &&
-         nr == bytes);
-}
-
-static bool is_zero_cow(BlockDriverState *bs, QCowL2Meta *m)
+/*
+ * Return 1 if the COW regions read as zeroes, 0 if not, < 0 on error.
+ * Note that returning 0 does not guarantee non-zero data.
+ */
+static int is_zero_cow(BlockDriverState *bs, QCowL2Meta *m)
 {
     /*
      * This check is designed for optimization shortcut so it must be
      * efficient.
-     * Instead of is_zero(), use is_unallocated() as it is faster (but not
-     * as accurate and can result in false negatives).
+     * Instead of is_zero(), use bdrv_co_is_zero_fast() as it is
+     * faster (but not as accurate and can result in false negatives).
      */
-    return is_unallocated(bs, m->offset + m->cow_start.offset,
-                          m->cow_start.nb_bytes) &&
-           is_unallocated(bs, m->offset + m->cow_end.offset,
-                          m->cow_end.nb_bytes);
+    int ret = bdrv_co_is_zero_fast(bs, m->offset + m->cow_start.offset,
+                                   m->cow_start.nb_bytes);
+    if (ret <= 0) {
+        return ret;
+    }
+
+    return bdrv_co_is_zero_fast(bs, m->offset + m->cow_end.offset,
+                                m->cow_end.nb_bytes);
 }
 
 static int handle_alloc_space(BlockDriverState *bs, QCowL2Meta *l2meta)
@@ -2432,7 +2432,10 @@ static int handle_alloc_space(BlockDriverState *bs, QCowL2Meta *l2meta)
             continue;
         }
 
-        if (!is_zero_cow(bs, m)) {
+        ret = is_zero_cow(bs, m);
+        if (ret < 0) {
+            return ret;
+        } else if (ret == 0) {
             continue;
         }