tree a86373383babf4627a539d38dd46e4fd6f43e5e8
parent 8546632e610f9b906ff9d34ecba167915fc8157c
author Alberto Garcia <berto@igalia.com> 1552409326 +0200
committer Kevin Wolf <kwolf@redhat.com> 1552419014 +0100

block: Allow omitting the 'backing' option in certain cases

Of all options of type BlockdevRef used to specify children in
BlockdevOptions, 'backing' is the only one that is optional.

For "x-blockdev-reopen" we want that if an option is omitted then it
must be reset to its default value. The default value of 'backing'
means that QEMU opens the backing file specified in the image
metadata, but this is not something that we want to support for the
reopen operation.

Because of this the 'backing' option has to be specified during
reopen, pointing to the existing backing file if we want to keep it,
or pointing to a different one (or NULL) if we want to replace it (to
be implemented in a subsequent patch).

In order to simplify things a bit and not to require that the user
passes the 'backing' option to every single block device even when
it's clearly not necessary, this patch allows omitting this option if
the block device being reopened doesn't have a backing file attached
_and_ no default backing file is specified in the image metadata.

Signed-off-by: Alberto Garcia <berto@igalia.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>