1 files changed, 25 insertions, 1 deletions

diff --git a/qemu-img.c b/qemu-img.c
index ebe1b866da..ae4acb655b 100644
--- a/qemu-img.c
+++ b/qemu-img.c
@@ -1556,7 +1556,9 @@ typedef struct ImgConvertState {
     BlockBackend *target;
     bool has_zero_init;
     bool compressed;
+    bool unallocated_blocks_are_zero;
     bool target_has_backing;
+    int64_t target_backing_sectors; /* negative if unknown */
     bool wr_in_order;
     bool copy_range;
     int min_sparse;
@@ -1586,12 +1588,23 @@ static int convert_iteration_sectors(ImgConvertState *s, int64_t sector_num)
 {
     int64_t src_cur_offset;
     int ret, n, src_cur;
+    bool post_backing_zero = false;
 
     convert_select_part(s, sector_num, &src_cur, &src_cur_offset);
 
     assert(s->total_sectors > sector_num);
     n = MIN(s->total_sectors - sector_num, BDRV_REQUEST_MAX_SECTORS);
 
+    if (s->target_backing_sectors >= 0) {
+        if (sector_num >= s->target_backing_sectors) {
+            post_backing_zero = s->unallocated_blocks_are_zero;
+        } else if (sector_num + n > s->target_backing_sectors) {
+            /* Split requests around target_backing_sectors (because
+             * starting from there, zeros are handled differently) */
+            n = s->target_backing_sectors - sector_num;
+        }
+    }
+
     if (s->sector_next_status <= sector_num) {
         int64_t count = n * BDRV_SECTOR_SIZE;
 
@@ -1613,7 +1626,7 @@ static int convert_iteration_sectors(ImgConvertState *s, int64_t sector_num)
         n = DIV_ROUND_UP(count, BDRV_SECTOR_SIZE);
 
         if (ret & BDRV_BLOCK_ZERO) {
-            s->status = BLK_ZERO;
+            s->status = post_backing_zero ? BLK_BACKING_FILE : BLK_ZERO;
         } else if (ret & BDRV_BLOCK_DATA) {
             s->status = BLK_DATA;
         } else {
@@ -2379,6 +2392,16 @@ static int img_convert(int argc, char **argv)
         }
     }
 
+    if (s.target_has_backing) {
+        /* Errors are treated as "backing length unknown" (which means
+         * s.target_backing_sectors has to be negative, which it will
+         * be automatically).  The backing file length is used only
+         * for optimizations, so such a case is not fatal. */
+        s.target_backing_sectors = bdrv_nb_sectors(out_bs->backing->bs);
+    } else {
+        s.target_backing_sectors = -1;
+    }
+
     ret = bdrv_get_info(out_bs, &bdi);
     if (ret < 0) {
         if (s.compressed) {
@@ -2388,6 +2411,7 @@ static int img_convert(int argc, char **argv)
     } else {
         s.compressed = s.compressed || bdi.needs_compressed_writes;
         s.cluster_sectors = bdi.cluster_size / BDRV_SECTOR_SIZE;
+        s.unallocated_blocks_are_zero = bdi.unallocated_blocks_are_zero;
     }
 
     ret = convert_do_copy(&s);