[Qemu-devel] [PATCH v2 10/11] block: add option 'backing' to -drive options

[Fam Zheng <famz@redhat.com>]

This option allows overriding backing hd of drive. If the target drive
exists, it's referenced as the backing file and refcount incremented.

Example:
    qemu-system-x86_64 -drive \
        file.filename=foo.qcow2,if=none,id=foo \
        -drive file=bar.qcow2,backing=foo

Signed-off-by: Fam Zheng <famz@redhat.com>
---
 block.c | 30 ++++++++++++++++++++++++------
 1 file changed, 24 insertions(+), 6 deletions(-)

diff --git a/block.c b/block.c
index 147a448..31fab07 100644
--- a/block.c
+++ b/block.c
@@ -1083,12 +1083,30 @@ int bdrv_open(BlockDriverState *bs, const char *filename, QDict *options,
 
     /* If there is a backing file, use it */
     if ((flags & BDRV_O_NO_BACKING) == 0) {
-        QDict *backing_options;
-
-        extract_subqdict(options, &backing_options, "backing.");
-        ret = bdrv_open_backing_file(bs, backing_options);
-        if (ret < 0) {
-            goto close_and_fail;
+        const char *backing_drive;
+        backing_drive = qdict_get_try_str(options, "backing");
+        if (backing_drive) {
+            bs->backing_hd = bdrv_find(backing_drive);
+            if (bs->backing_hd) {
+                bdrv_ref(bs->backing_hd, false);
+                pstrcpy(bs->backing_file, sizeof(bs->backing_file),
+                        bs->backing_hd->filename);
+                pstrcpy(bs->backing_format, sizeof(bs->backing_format),
+                        bs->backing_hd->drv->format_name);
+            } else {
+                qerror_report(ERROR_CLASS_DEVICE_NOT_FOUND,
+                        "Can't find drive with name %s\n", backing_drive);
+                ret = -EINVAL;
+                goto close_and_fail;
+            }
+            qdict_del(options, "backing");
+        } else {
+            QDict *backing_options;
+            extract_subqdict(options, &backing_options, "backing.");
+            ret = bdrv_open_backing_file(bs, backing_options);
+            if (ret < 0) {
+                goto close_and_fail;
+            }
         }
     }
 
-- 
1.8.3.2