stream: do not copy unallocated sectors from the base

Unallocated sectors should really never be accessed by the guest,
so there's no need to copy them during the streaming process.
If they are read by the guest during streaming, guest-initiated
copy-on-read will copy them (we're in the base == NULL case, which
enables copy on read).  If they are read after we disconnect the
image from the base, they will read as zeroes anyway.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

1 files changed, 4 insertions, 14 deletions

diff --git a/block/stream.c b/block/stream.c
index a2c8f67711..608a860aa2 100644
--- a/block/stream.c
+++ b/block/stream.c
@@ -130,14 +130,9 @@ static int coroutine_fn is_allocated_base(BlockDriverState *top,
      */
     intermediate = top->backing_hd;
 
-    while (intermediate) {
+    while (intermediate != base) {
         int pnum_inter;
 
-        /* reached base */
-        if (intermediate == base) {
-            *pnum = n;
-            return 1;
-        }
         ret = bdrv_co_is_allocated(intermediate, sector_num, nb_sectors,
                                    &pnum_inter);
         if (ret < 0) {
@@ -160,6 +155,7 @@ static int coroutine_fn is_allocated_base(BlockDriverState *top,
         intermediate = intermediate->backing_hd;
     }
 
+    *pnum = n;
     return 1;
 }
 
@@ -203,14 +199,8 @@ wait:
             break;
         }
 
-        if (base) {
-            ret = is_allocated_base(bs, base, sector_num,
-                                    STREAM_BUFFER_SIZE / BDRV_SECTOR_SIZE, &n);
-        } else {
-            ret = bdrv_co_is_allocated(bs, sector_num,
-                                       STREAM_BUFFER_SIZE / BDRV_SECTOR_SIZE,
-                                       &n);
-        }
+        ret = is_allocated_base(bs, base, sector_num,
+                                STREAM_BUFFER_SIZE / BDRV_SECTOR_SIZE, &n);
         trace_stream_one_iteration(s, sector_num, n, ret);
         if (ret == 0) {
             if (s->common.speed) {