Re: [Qemu-devel] Object instantiation vs. device realization: what to do when?

[Paolo Bonzini <pbonzini@redhat.com>]

On 14/02/19 17:21, Markus Armbruster wrote:
>  * As an interim step, the #DeviceState:realized property can also be
>  * set with qdev_init_nofail().
>  * In the future, devices will propagate this state change to their children
>  * and along busses they expose.
> 
> This sentence is five years old.  Any progress?  If not, any intentions
> to make progress?

Good news, it's done!  What we still don't do is removing
qdev_init_nofail and realizing the whole machine in one fell swoop.

Bad news, it's been done for five years:

    commit 5c21ce77d7e5643089ceec556c0408445d017f32
    Author: Bandan Das <bsd@redhat.com>
    Date:   Wed Mar 12 21:02:12 2014 +0100

    qdev: Realize buses on device realization

    Integrate (un)realization of child buses with
    realization/unrealization
    of the device hosting them. Code in device_unparent() is reordered
    for unrealization of buses to work as part of device unrealization.

    That way no changes need to be made to bus instantiation.

    Signed-off-by: Bandan Das <bsd@redhat.com>
    Signed-off-by: Andreas Färber <afaerber@suse.de>

so I don't expect that the next step will ever happen...

>  * The point in time will be deferred to machine creation, so that values
>  * set in @realize will not be introspectable beforehand. Therefore devices
>  * must not create children during @realize; they should initialize them via
>  * object_initialize() in their own #TypeInfo.instance_init and forward the
>  * realization events appropriately.
> 
> This is mostly greek to me.  Pity the developer who knows less about
> qdev than I do.

The first part refers to what virtio_instance_init_common does:

    object_initialize(vdev, vdev_size, vdev_name);
    object_property_add_child(proxy_obj, "virtio-backend", OBJECT(vdev),
                              NULL);
    object_unref(OBJECT(vdev));

The second part doesn't apply to virtio because it has a bus (so the
code from the above commit handles recursive realization automatically).

hw/sd/milkymist-memcard.c has an example of this:

    object_property_set_bool(OBJECT(carddev), true, "realized", &err);

but it should create the device in milkymist_memcard_init rather than
milkymist_memcard_realize, in order to obey the directives of the sacred
book of QEMU.

Paolo