Re: [Qemu-devel] [PATCH v3 02/23] block: New BlockBackend

[Kevin Wolf <kwolf@redhat.com>]

Am 16.09.2014 um 20:12 hat Markus Armbruster geschrieben:
> 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 <armbru@redhat.com>

> diff --git a/include/sysemu/block-backend.h b/include/sysemu/block-backend.h
> new file mode 100644
> index 0000000..3f8371c
> --- /dev/null
> +++ b/include/sysemu/block-backend.h
> @@ -0,0 +1,26 @@
> +/*
> + * QEMU Block backends
> + *
> + * Copyright (C) 2014 Red Hat, Inc.
> + *
> + * Authors:
> + *  Markus Armbruster <armbru@redhat.com>,
> + *
> + * This work is licensed under the terms of the GNU GPL, version 2 or
> + * later.  See the COPYING file in the top-level directory.
> + */

This one should be LGPL as well.

The distribution of blk_unref() should be good enough. To be correct it
requires the assumption that the blockdev-add reference to any given BDS
is the last one that goes away. I'm not sure if this is the case here (I
suspect it isn't), but at the end of the series, we should be good.

With the license fixed, you can add:

    Reviewed-by Kevin Wolf <kwolf@redhat.com>

Kevin