Max Reitz <mre...@redhat.com> writes:
> 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 <arm...@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.
Indeed, and I actually like that way to free stuff on error. But it
would be yet another preliminary cleanup patch. If you guys want it,
I'll cook it up.
> Reviewed-by: Max Reitz <mre...@redhat.com>
Thanks!
