Re: [Qemu-devel] [PATCH v2 02/23] block: New BlockBackend

[Max Reitz <mreitz@redhat.com>]

On 13.09.2014 17:00, Markus Armbruster wrote:
> 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>
> ---
>   block.c                        |   3 +-
>   block/Makefile.objs            |   2 +-
>   block/block-backend.c          | 119 +++++++++++++++++++++++++++++++++++++++++
>   blockdev.c                     |  21 +++++++-
>   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 +-
>   10 files changed, 253 insertions(+), 13 deletions(-)
>   create mode 100644 block/block-backend.c
>   create mode 100644 include/sysemu/block-backend.h

[snip]

> diff --git a/qemu-img.c b/qemu-img.c
> index 58d59d0..acb272e 100644
> --- a/qemu-img.c
> +++ b/qemu-img.c

[snip]

> @@ -942,6 +952,7 @@ static int check_empty_sectors(BlockDriverState *bs, int64_t sect_num,
>   static int img_compare(int argc, char **argv)
>   {
>       const char *fmt1 = NULL, *fmt2 = NULL, *cache, *filename1, *filename2;
> +    BlockBackend *blk1, *blk2;
>       BlockDriverState *bs1, *bs2;

By initializing all of these to NULL we could remove out1, out2 and 
out3, and just always go to out. But of course it's not wrong this way.

Reviewed-by: Max Reitz <mreitz@redhat.com>