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

[Markus Armbruster]

Benoît Canet <address@hidden> writes:

> The Wednesday 10 Sep 2014  10:13:31 (+0200), 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, but 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          | 110 
>> +++++++++++++++++++++++++++++++++++++++++
>>  blockdev.c                     |  10 +++-
>>  hw/block/xen_disk.c            |  11 +++++
>>  include/qemu/typedefs.h        |   1 +
>>  include/sysemu/block-backend.h |  26 ++++++++++
>>  qemu-img.c                     |  46 +++++++++++++++++
>>  qemu-io.c                      |   8 +++
>>  qemu-nbd.c                     |   3 +-
>>  9 files changed, 214 insertions(+), 3 deletions(-)
>>  create mode 100644 block/block-backend.c
>>  create mode 100644 include/sysemu/block-backend.h
>> 
>> diff --git a/block/Makefile.objs b/block/Makefile.objs
>> index f45f939..a70140b 100644
>> --- a/block/Makefile.objs
>> +++ b/block/Makefile.objs
>> @@ -5,7 +5,7 @@ block-obj-y += qed-check.o
>>  block-obj-$(CONFIG_VHDX) += vhdx.o vhdx-endian.o vhdx-log.o
>>  block-obj-$(CONFIG_QUORUM) += quorum.o
>>  block-obj-y += parallels.o blkdebug.o blkverify.o
>> -block-obj-y += snapshot.o qapi.o
>> +block-obj-y += block-backend.o snapshot.o qapi.o
>>  block-obj-$(CONFIG_WIN32) += raw-win32.o win32-aio.o
>>  block-obj-$(CONFIG_POSIX) += raw-posix.o
>>  block-obj-$(CONFIG_LINUX_AIO) += linux-aio.o
>> diff --git a/block/block-backend.c b/block/block-backend.c
>> new file mode 100644
>> index 0000000..833f7d9
>> --- /dev/null
>> +++ b/block/block-backend.c
>> @@ -0,0 +1,110 @@
>> +/*
>> + * QEMU Block backends
>> + *
>> + * Copyright (C) 2014 Red Hat, Inc.
>> + *
>> + * Authors:
>> + *  Markus Armbruster <address@hidden>,
>> + *
>> + * This work is licensed under the terms of the GNU GPL, version 2 or
>> + * later.  See the COPYING file in the top-level directory.
>> + */
>> +
>> +#include "sysemu/block-backend.h"
>> +#include "block/block_int.h"
>> +
>> +struct BlockBackend {
>> +    char *name;
>> +    int refcnt;
>> +    QTAILQ_ENTRY(BlockBackend) link; /* for blk_backends */
>> +};
>> +
>> +static QTAILQ_HEAD(, BlockBackend) blk_backends =
>> +    QTAILQ_HEAD_INITIALIZER(blk_backends);
>> +
>> +/**
>> + * blk_new:
>> + * @name: name, must not be %NULL or empty
>> + * @errp: return location for an error to be set on failure, or %NULL
>> + *
>> + * Create a new BlockBackend, with a reference count of one.  Fail if
>> + * @name already exists.
>> + *
>> + * Returns: the BlockBackend on success, %NULL on failure
>> + */
>> +BlockBackend *blk_new(const char *name, Error **errp)
>
> I am responding for the easy part first.
>
> So here the blockbackend is identified by a name
>
>> +{
>> +    BlockBackend *blk = g_new0(BlockBackend, 1);
>> +
>> +    assert(name && name[0]);
>> +    if (blk_by_name(name)) {
>
>> +        error_setg(errp, "Device with id '%s' already exists", name);
>
> But here is it an id or a name ?
> Do we need to make a choice everywhere in the code between id and name ?

If we can agree on a convention to use within the block layer, I'll be
happy to follow it.

Right now, we mix "id", "device", "device name" freely.  My patch
mimicks existing usage: "id" in QemuOpts and some error messages,
"device name" and its abbreviated variations in the code, the schema,
and some other error messages.

>> +        return NULL;
>> +    }
>> +    blk->name = g_strdup(name);
>> +    blk->refcnt = 1;
>> +    QTAILQ_INSERT_TAIL(&blk_backends, blk, link);
>> +    return blk;
>> +}
>> +
>> +static void blk_delete(BlockBackend *blk)
>> +{
>> +    assert(!blk->refcnt);
>> +    QTAILQ_REMOVE(&blk_backends, blk, link);
>> +    g_free(blk->name);
>> +    g_free(blk);
>> +}
>> +
>> +/**
>> + * blk_ref:
>> + *
>> + * Increment @blk's reference count.
>> + */
>> +void blk_ref(BlockBackend *blk)
>> +{
>
> if blk_unref you take care of doing
> +    if (blk) {
> to make sur the user does not pass a NULL pointer.
> Transforming blk into a NULL pointer is not a side effect
> of blk_unref so this test is designed to prevent a user
> brain damage.
>
> If the user can be brain damaged to pass a NULL to blk_unref he
> could be equally stupid passing a NULL to blk_ref.
> Why not adding the same test here ?

Kevin already explained this one.

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