On Wed, 18 Dec 2013 11:36:52 +0100 Paolo Bonzini <pbonz...@redhat.com> wrote:
> Il 17/12/2013 20:38, Anthony Liguori ha scritto: > > On Tue, Dec 17, 2013 at 4:38 AM, Paolo Bonzini <pbonz...@redhat.com> wrote: > >> Il 17/12/2013 00:26, Anthony Liguori ha scritto: > >>> Sharing hot plug code is a good thing. Making hotplug a qdev-level > >>> concept seems like a bad thing to me. > >> > >> Can you explain what you mean? > > > > The question is whether "hotpluggable" as a property applies to all > > devices or not. I think Andreas asked me to provide "hotpluggable" property to distinguish hotpluggable vs not hotpluggable DimmDevice via qom interface. > > > > But hotplug is strictly a bus level concept. It's a sequence of > > events that correspond to what happens when you add a new device to a > > bus after power on. > > Hotplugging a device is a special case of plugging a device. If a bus > or device only supports cold-plug, that can be done using > "bc->allow_hotplug = false" or "dc->hotpluggable = false". Do we need per instance ability to set "hotpluggable" property? For example board might want to mark some CPUs as not hotpluggable. > > Igor's interface applies just as well to the case of plugging a device > at startup; I think separating the two makes little sense. And once you > have cold-plug and hot-plug in qdev core, it makes sense to add unplug > as well. Also because we already have surprise removal in qdev core > (that's unparent) and we have some kind of unplug request support > (device_del/dc->unplug). > > One possibility that remains is to put cold/hot-plug in a "BusDevice" > class rather than in the core qdev: > > Device > BusDevice <-- can be cold/hot-plugged > > but this adds more complication. For example, the same CPU can be > hotpluggable or not depending on the board model, should the superclass > be Device or BusDevice. And if we ever have multi-CPU targets, with the > "core" CPU not hotpluggable and additional hotpluggable ones (e.g. for > GPUs) what would be the superclass of the common CPU superclass? > > > The question is whether there can be code sharing without touching the > > base class. You could certainly have a HotpluggableBusState and then > > a HotpluggableDeviceState. > > > > Interfaces would be another option too. > > Interfaces are fine, but the question is who finds them and calls them. > In this case, the discovery mechanism is a link property, and the > calling mechanism is an explicit hook in the "realized" property. If we don't need per instance "hotpluggable" state and we can call interfaces from generic qdev/device code, then we would need at first only TYPE_HOTPLUGGABLE_BUS_DEVICE_IF and later for link<> based hotplug we could add just TYPE_HOTPLUGGABLE_DEVICE_IF. Difference would be in the way they get access to hotplug device link, former one will use bus for it and second some other way. > > If we had aspect-oriented programming, we would be marking join points > instead of writing "if (dev->foo) bar(dev->foo)" conditionals. But the > idea is the same. > > > The general concern is about polluting widely used base classes. It's > > better if we can avoid adding things to DeviceState and Object > > whenever possible. > > I agree. At the same time we should make base classes as small as > possible, but not smaller than that. > > Paolo >