3 files changed, 35 insertions, 4 deletions

diff --git a/block.c b/block.c
index 7230f04e40..823841df03 100644
--- a/block.c
+++ b/block.c
@@ -181,6 +181,20 @@ BlockDriver *bdrv_find_format(const char *format_name)
     return NULL;
 }
 
+int bdrv_create2(BlockDriver *drv,
+                const char *filename, int64_t size_in_sectors,
+                const char *backing_file, const char *backing_format,
+                int flags)
+{
+    if (drv->bdrv_create2)
+        return drv->bdrv_create2(filename, size_in_sectors, backing_file,
+                                 backing_format, flags);
+    if (drv->bdrv_create)
+        return drv->bdrv_create(filename, size_in_sectors, backing_file,
+                                flags);
+    return -ENOTSUP;
+}
+
 int bdrv_create(BlockDriver *drv,
                 const char *filename, int64_t size_in_sectors,
                 const char *backing_file, int flags)
@@ -357,7 +371,7 @@ int bdrv_open2(BlockDriverState *bs, const char *filename, int flags,
 
         /* if there is a backing file, use it */
         bs1 = bdrv_new("");
-        ret = bdrv_open(bs1, filename, 0);
+        ret = bdrv_open2(bs1, filename, 0, drv);
         if (ret < 0) {
             bdrv_delete(bs1);
             return ret;
@@ -378,12 +392,14 @@ int bdrv_open2(BlockDriverState *bs, const char *filename, int flags,
         else
             realpath(filename, backing_filename);
 
-        ret = bdrv_create(&bdrv_qcow2, tmp_filename,
-                          total_size, backing_filename, 0);
+        ret = bdrv_create2(&bdrv_qcow2, tmp_filename,
+                           total_size, backing_filename, 
+                           (drv ? drv->format_name : NULL), 0);
         if (ret < 0) {
             return ret;
         }
         filename = tmp_filename;
+        drv = &bdrv_qcow2;
         bs->is_temporary = 1;
     }
 
@@ -429,10 +445,14 @@ int bdrv_open2(BlockDriverState *bs, const char *filename, int flags,
 #endif
     if (bs->backing_file[0] != '\0') {
         /* if there is a backing file, use it */
+        BlockDriver *back_drv = NULL;
         bs->backing_hd = bdrv_new("");
         path_combine(backing_filename, sizeof(backing_filename),
                      filename, bs->backing_file);
-        ret = bdrv_open(bs->backing_hd, backing_filename, open_flags);
+        if (bs->backing_format[0] != '\0')
+            back_drv = bdrv_find_format(bs->backing_format);
+        ret = bdrv_open2(bs->backing_hd, backing_filename, open_flags,
+                         back_drv);
         if (ret < 0) {
             bdrv_close(bs);
             return ret;
diff --git a/block.h b/block.h
index 750816e55d..6701d34502 100644
--- a/block.h
+++ b/block.h
@@ -62,6 +62,10 @@ BlockDriver *bdrv_find_format(const char *format_name);
 int bdrv_create(BlockDriver *drv,
                 const char *filename, int64_t size_in_sectors,
                 const char *backing_file, int flags);
+int bdrv_create2(BlockDriver *drv,
+                 const char *filename, int64_t size_in_sectors,
+                 const char *backing_file, const char *backing_format,
+                 int flags);
 BlockDriverState *bdrv_new(const char *device_name);
 void bdrv_delete(BlockDriverState *bs);
 int bdrv_file_open(BlockDriverState **pbs, const char *filename, int flags);
diff --git a/block_int.h b/block_int.h
index b45bde04d4..af6dff6bd6 100644
--- a/block_int.h
+++ b/block_int.h
@@ -91,6 +91,12 @@ struct BlockDriver {
         BlockDriverCompletionFunc *cb, void *opaque);
 
     AIOPool aio_pool;
+
+    /* new create with backing file format */
+    int (*bdrv_create2)(const char *filename, int64_t total_sectors,
+                        const char *backing_file, const char *backing_format,
+                        int flags);
+
     struct BlockDriver *next;
 };
 
@@ -113,6 +119,7 @@ struct BlockDriverState {
     char filename[1024];
     char backing_file[1024]; /* if non zero, the image is a diff of
                                 this file image */
+    char backing_format[16]; /* if non-zero and backing_file exists */
     int is_temporary;
     int media_changed;