Re: [Qemu-devel] [RFC 1/5] qdev: add child alias properties

[Andreas Färber]

Am 19.03.2014 16:48, schrieb Stefan Hajnoczi:
> Child alias properties allow a parent object to expose child object
> properties.  This makes it possible for a parent object to forward all
> or a subset of a child's properties.
> 
> Currently we achieve similar behavior by duplicating property
> definitions and copying the fields around.  It turns out virtio has
> several double-frees since we didn't get the memory management right.
> 
> Using child alias properties it will be much easier for a parent object
> to set a child's properties without fragile memory management issues
> since the actual field only exists once in the child object.
> 
> Signed-off-by: Stefan Hajnoczi <address@hidden>
> ---
>  hw/core/qdev-properties.c    | 28 ++++++++++++++++++++++++++++
>  include/hw/qdev-properties.h | 28 ++++++++++++++++++++++++++++
>  2 files changed, 56 insertions(+)
> 
> diff --git a/hw/core/qdev-properties.c b/hw/core/qdev-properties.c
> index 77d0c66..e62a4fd 100644
> --- a/hw/core/qdev-properties.c
> +++ b/hw/core/qdev-properties.c
> @@ -1002,3 +1002,31 @@ PropertyInfo qdev_prop_size = {
>      .get = get_size,
>      .set = set_size,
>  };
> +
> +/* --- alias that forwards to a child object's property --- */
> +
> +static void get_child_alias(Object *obj, Visitor *v, void *opaque,
> +                            const char *name, Error **errp)
> +{
> +    DeviceState *dev = DEVICE(obj);
> +    Property *prop = opaque;
> +    Object *child = OBJECT(qdev_get_prop_ptr(dev, prop));
> +
> +    object_property_get(child, v, name, errp);
> +}
> +
> +static void set_child_alias(Object *obj, Visitor *v, void *opaque,
> +                            const char *name, Error **errp)
> +{
> +    DeviceState *dev = DEVICE(obj);
> +    Property *prop = opaque;
> +    Object *child = OBJECT(qdev_get_prop_ptr(dev, prop));
> +
> +    object_property_set(child, v, name, errp);
> +}
> +
> +PropertyInfo qdev_prop_child_alias = {
> +    .name  = "ChildAlias",

Won't this turn into the QMP-exposed type of the property? That would be
wrong then, requiring per-type alias property types. They could be
macro-automated of course, if we need them.

Paolo had cleaned up type names to match QAPI schema for 2.0, CC'ing.

Regards,
Andreas

> +    .get = get_child_alias,
> +    .set = set_child_alias,
> +};
> diff --git a/include/hw/qdev-properties.h b/include/hw/qdev-properties.h
> index 3c000ee..5ab1cac 100644
> --- a/include/hw/qdev-properties.h
> +++ b/include/hw/qdev-properties.h
> @@ -13,6 +13,7 @@ extern PropertyInfo qdev_prop_uint32;
>  extern PropertyInfo qdev_prop_int32;
>  extern PropertyInfo qdev_prop_uint64;
>  extern PropertyInfo qdev_prop_size;
> +extern PropertyInfo qdev_prop_child_alias;
>  extern PropertyInfo qdev_prop_string;
>  extern PropertyInfo qdev_prop_chr;
>  extern PropertyInfo qdev_prop_ptr;
> @@ -61,6 +62,33 @@ extern PropertyInfo qdev_prop_arraylen;
>          .defval    = (bool)_defval,                              \
>          }
>  
> +/**
> + * DEFINE_PROP_CHILD_ALIAS:
> + * @_name: property name
> + * @_state: name of the device state structure type
> + * @_field: name of field in @_state, must be Object subclass
> + *
> + * Define device properties that alias a child object's property.  For 
> example,
> + * use the following to forward the 'baz' property where Foo embeds a Bar
> + * object:
> + *
> + *   typedef struct {
> + *       Object parent_obj;
> + *
> + *       Bar bar_obj;
> + *   } Foo;
> + *
> + *   DEFINE_PROP_CHILD_ALIAS("baz", Foo, bar_obj)
> + *
> + * Any access to Foo's 'baz' property actually accesses bar_obj's 'baz'
> + * property.
> + */
> +#define DEFINE_PROP_CHILD_ALIAS(_name, _state, _field) {         \
> +        .name      = (_name),                                    \
> +        .info      = &(qdev_prop_child_alias),                   \
> +        .offset    = offsetof(_state, _field),                   \
> +        }
> +
>  #define PROP_ARRAY_LEN_PREFIX "len-"
>  
>  /**

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