block: Reject writethrough mode except at the root

Writethrough mode is going to become a BlockBackend feature rather than
a BDS one, so forbid it in places where we won't be able to support it
when the code finally matches the envisioned design.

We only allowed setting the cache mode of non-root nodes after the 2.5
release, so we're still free to make this change.

The target of block jobs is now always opened in a writeback mode
because it doesn't have a BlockBackend attached. This makes more sense
anyway because block jobs know when to flush. If the graph is modified
on job completion, the original cache mode moves to the new root, so
for the guest device writethough always stays enabled if it was
configured this way.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>

1 files changed, 7 insertions, 0 deletions

diff --git a/block.c b/block.c
index d050c4bb58..4158a3a36f 100644
--- a/block.c
+++ b/block.c
@@ -975,6 +975,13 @@ static int bdrv_open_common(BlockDriverState *bs, BdrvChild *file,
 
     /* Apply cache mode options */
     update_flags_from_options(&bs->open_flags, opts);
+
+    if (!bs->blk && (bs->open_flags & BDRV_O_CACHE_WB) == 0) {
+        error_setg(errp, "Can't set writethrough mode except for the root");
+        ret = -EINVAL;
+        goto free_and_fail;
+    }
+
     bdrv_set_enable_write_cache(bs, bs->open_flags & BDRV_O_CACHE_WB);
 
     /* Open the image, either directly or using a protocol */