[Qemu-devel] [RFC PATCH 0/9] qom: improve reference counting and hotplug

[Anthony Liguori]

Right now, you need to pair up object_new with object_delete.  This is
impractical when using reference counting because we would like to ensure that
object_unref() also frees memory when needed.

The first few patches fix this problem by introducing a release callback so
that objects that need special release behavior (i.e. g_free) can do that.

Since link and child properties all hold references, in order to actually free
an object, we need to break those links.  User created devices end up as
children of a container.  But child properties cannot be removed which means
there's no obvious way to remove the reference and ultimately free the object.

We introduce the concept of "nullable child" properties to solve this.  This is
a child property that can be broken by writing NULL to the child link.  Today
we set all /peripheral* children to be nullable so that they can be deleted by
management tools.

In terms of modeling hotplug, we represent unplug by removing the object from
the parent bus.  We need to register a notifier for when this happens so that
we can also remove the parent's child property to ultimately release the object.

Putting it all together, we have:

1) qmp_device_del will issue a callback to a device.  The default callback will
   do a forced eject (which means writing NULL to the parent_bus link).

2) PCI hotplug is a bit more sophisticated in that it waits for the guest to
   do the ejection.

3) qmp_device_del will register an eject notifier such that the device gets
   completely removed.

There's a slightly change in behavior here.  A device is not automatically
destroyed based on a guest initiated eject.  A management tool must explicitly
break the parent's link to the child in order for the device to disappear
completely.  device_del behaves exactly as it does today though.

This is an RFC.  I've tested the series quite a lot (it was hard to get the
reference counting right) but not enough to apply.  I also don't think the
series is quite split right and may not bisect cleanly.

I also want to write up a document describing object life cycle since admittedly
the above is probably not that easy to follow.

I wanted to share this now though because it works and I think the concepts are
right.