This is needed to implement image-fleecing scheme, when we create
a temporary node, mark our active node to be backing for the temp,
and start backup(sync=none) from active node to the temp node.
Temp node then represents a kind of snapshot and may be used
for external backup through NBD.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
---
 blockdev.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/blockdev.c b/blockdev.c
index 58d7570932..72f5347df5 100644
--- a/blockdev.c
+++ b/blockdev.c
@@ -1859,7 +1859,7 @@ static void blockdev_backup_prepare(BlkActionState *common, Error **errp)
     assert(common->action->type == TRANSACTION_ACTION_KIND_BLOCKDEV_BACKUP);
     backup = common->action->u.blockdev_backup.data;
 
-    bs = qmp_get_root_bs(backup->device, errp);
+    bs = bdrv_lookup_bs(backup->device, backup->device, errp);
     if (!bs) {
         return;
     }
@@ -3517,7 +3517,7 @@ BlockJob *do_blockdev_backup(BlockdevBackup *backup, JobTxn *txn,
         backup->compress = false;
     }
 
-    bs = qmp_get_root_bs(backup->device, errp);
+    bs = bdrv_lookup_bs(backup->device, backup->device, errp);
     if (!bs) {
         return NULL;
     }
-- 
2.11.1

On 06/29/2018 10:15 AM, Vladimir Sementsov-Ogievskiy wrote:
> This is needed to implement image-fleecing scheme, when we create
> a temporary node, mark our active node to be backing for the temp,
> and start backup(sync=none) from active node to the temp node.
> Temp node then represents a kind of snapshot and may be used
> for external backup through NBD.
> 
> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> ---
>   blockdev.c | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/blockdev.c b/blockdev.c
> index 58d7570932..72f5347df5 100644
> --- a/blockdev.c
> +++ b/blockdev.c
> @@ -1859,7 +1859,7 @@ static void blockdev_backup_prepare(BlkActionState *common, Error **errp)
>       assert(common->action->type == TRANSACTION_ACTION_KIND_BLOCKDEV_BACKUP);
>       backup = common->action->u.blockdev_backup.data;
>   
> -    bs = qmp_get_root_bs(backup->device, errp);
> +    bs = bdrv_lookup_bs(backup->device, backup->device, errp);
>       if (!bs) {
>           return;
>       }

This hunk is new,

> @@ -3517,7 +3517,7 @@ BlockJob *do_blockdev_backup(BlockdevBackup *backup, JobTxn *txn,
>           backup->compress = false;
>       }
>   
> -    bs = qmp_get_root_bs(backup->device, errp);
> +    bs = bdrv_lookup_bs(backup->device, backup->device, errp);
>       if (!bs) {
>           return NULL;
>       }
> 

but this hunk is identical to John's patch:
https://lists.gnu.org/archive/html/qemu-devel/2018-06/msg08593.html

I guess yours adds additional code needed to work under a transaction, 
as opposed to separate QMP commands.

Reviewed-by: Eric Blake <eblake@redhat.com>

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

On 06/29/2018 11:15 AM, Vladimir Sementsov-Ogievskiy wrote:
> This is needed to implement image-fleecing scheme, when we create
> a temporary node, mark our active node to be backing for the temp,
> and start backup(sync=none) from active node to the temp node.
> Temp node then represents a kind of snapshot and may be used
> for external backup through NBD.
> 
> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>

A hair more comprehensive than my recent patch which did the same thing, so:

Reviewed-by: John Snow <jsnow@redhat.com>