A block device consists of a frontend device model and a backend.
A block backend has a tree of block drivers doing the actual work.
The tree is managed by the block layer.
We currently use a single abstraction BlockDriverState both for tree
nodes and the backend as a whole. Drawbacks:
* Its API includes both stuff that makes sense only at the block
backend level (root of the tree) and stuff that's only for use
within the block layer. This makes the API bigger and more complex
than necessary. Moreover, it's not obvious which interfaces are
meant for device models, and which really aren't.
* Since device models keep a reference to their backend, the backend
object can't just be destroyed. But for media change, we need to
replace the tree. Our solution is to make the BlockDriverState
generic, with actual driver state in a separate object, pointed to
by member opaque. That lets us replace the tree by deinitializing
and reinitializing its root. This special need of the root makes
the data structure awkward everywhere in the tree.
The general plan is to separate the APIs into "block backend", for use
by device models, monitor and whatever other code dealing with block
backends, and "block driver", for use by the block layer and whatever
other code (if any) dealing with trees and tree nodes.
Code dealing with block backends, device models in particular, should
become completely oblivious of BlockDriverState. This should let us
clean up both APIs, and the tree data structures.
This commit is a first step. It creates a minimal "block backend"
API: type BlockBackend and functions to create, destroy and find them.
BlockBackend objects are created and destroyed exactly when root
BlockDriverState objects are created and destroyed. "Root" in the
sense of "in bdrv_states". They're not yet used for anything; that'll
come shortly.
BlockBackend is reference-counted. Its reference count never exceeds
one so far, but that's going to change.
Signed-off-by: Markus Armbruster <address@hidden>
---
block/Makefile.objs | 2 +-
block/block-backend.c | 120 +++++++++++++++++++++++++++++++++++++++++
blockdev.c | 13 ++++-
hw/block/xen_disk.c | 11 ++++
include/qemu/typedefs.h | 1 +
include/sysemu/block-backend.h | 26 +++++++++
qemu-img.c | 70 +++++++++++++++++++++---
qemu-io.c | 8 +++
qemu-nbd.c | 5 +-
9 files changed, 245 insertions(+), 11 deletions(-)
create mode 100644 block/block-backend.c
create mode 100644 include/sysemu/block-backend.h