Re: [Qemu-devel] [PATCH 03/12] block: Introduce BlockBackendPublic

[Eric Blake]

On 03/22/2016 09:33 AM, Kevin Wolf wrote:
> Some features, like I/O throttling, are implemented outside
> block-backend.c, but still want to keep BlockBackends in a list. In
> order to avoid exposing the whole struct layout in the public header
> file, this patch introduces an embedded public struct where list entry
> structs can be added and a pair of functions to convert between
> BlockBackend and BlockBackendPublic.
> 
> Signed-off-by: Kevin Wolf <address@hidden>
> ---
>  block/block-backend.c          | 17 +++++++++++++++++
>  include/sysemu/block-backend.h |  9 +++++++++
>  2 files changed, 26 insertions(+)
> 
> diff --git a/block/block-backend.c b/block/block-backend.c
> index df8f717..4394950 100644
> --- a/block/block-backend.c
> +++ b/block/block-backend.c
> @@ -33,6 +33,7 @@ struct BlockBackend {
>      DriveInfo *legacy_dinfo;    /* null unless created by drive_new() */
>      QTAILQ_ENTRY(BlockBackend) link;         /* for block_backends */
>      QTAILQ_ENTRY(BlockBackend) monitor_link; /* for monitor_block_backends */
> +    BlockBackendPublic public;

Any reason to not put the public struct at offset 0?

> +
> +/*
> + * Returns a BlockBackend given the associated @public fields.
> + */
> +BlockBackend *blk_by_public(BlockBackendPublic *public)
> +{
> +    return container_of(public, BlockBackend, public);
> +}

At least container_of() doesn't care, so I guess it doesn't matter.

> +/* This struct is embedded in (the private) BlockBackend struct and contains
> + * fields that must be public. This is in particular for QLIST_ENTRY() and
> + * friends so that BlockBackends can be kept in lists outside 
> block-backend.c */
> +typedef struct BlockBackendPublic {
> +} BlockBackendPublic;

No fields?  So really all we need this for is so that we can have an
address of a member of the larger struct, so that we can do list
operations based on that address?

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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