| Message ID | 20241024165627.1372621-1-peterx@redhat.com (mailing list archive) |
|---|---|
| Headers | show |
| Series | QOM: Singleton interface | expand |
Peter Xu <peterx@redhat.com> writes: > This patchset introduces the singleton interface for QOM. > > The singleton interface is as simple as "this class can only create one > instance". > > We used to have similar demand when working on all kinds of vIOMMUs, > because in most cases that I am aware of, vIOMMU must be a singleton as > it's closely bound to the machine and also the root PCIe systems. We used > to have a bunch of special code guarding those, for example, X86 has > pc_machine_device_pre_plug_cb() just to detect when vIOMMU is created more > than once. > > There is a similar demand raising recently (even if the problem existed > over years) when we were looking at a migration bug, that part of it was > involved with the current_migration global pointer being referenced > anywhere, even after the migration object was finalize()ed. So far without > singleton support, there's no way to reset the variable properly. > Declaring migration object to be a singleton also just makes sense, e.g., > dynamically creating migration objects on the fly with QMP commands doesn't > sound right.. > > The idea behind is pretty simple: any object that can only be created once > can now declare the TYPE_SINGLETON interface, then QOM facilities will make > sure it won't be created more than once. For example, qom-list-properties, > device-list-properties, etc., will be smart enough to not try to create > temporary singleton objects now. QOM design assumption: object_new() followed by object_unref() is always possible and has no side effect. QOM introspection relies on this. Your PATCH 1 makes non-class properties of singletons invisible in introspection. Making something with such properties a singleton would be a regression. Anything that violates the design assumption must be delayed to a suitable state transition. For devices (subtypes of TYPE_DEVICE), this is ->realize(). For user-creatable objects (provide interface TYPE_USER_CREATABLE), this is ->complete(). For anything else, it's something else that probably doesn't even exist, yet. See "Problem 5: QOM lacks a clear life cycle" in Subject: Dynamic & heterogeneous machines, initial configuration: problems Date: Wed, 31 Jan 2024 21:14:21 +0100 Message-ID: <87o7d1i7ky.fsf@pond.sub.org> https://lore.kernel.org/qemu-devel/87o7d1i7ky.fsf@pond.sub.org/ > Meanwhile, we also guard device-add paths > so that it'll fail properly if it's created more than once (and iff it's a > TYPE_DEVICE first). [...]
On Fri, Oct 25, 2024 at 09:38:31AM +0200, Markus Armbruster wrote: > Peter Xu <peterx@redhat.com> writes: > > > This patchset introduces the singleton interface for QOM. > > > > The singleton interface is as simple as "this class can only create one > > instance". > > > > We used to have similar demand when working on all kinds of vIOMMUs, > > because in most cases that I am aware of, vIOMMU must be a singleton as > > it's closely bound to the machine and also the root PCIe systems. We used > > to have a bunch of special code guarding those, for example, X86 has > > pc_machine_device_pre_plug_cb() just to detect when vIOMMU is created more > > than once. > > > > There is a similar demand raising recently (even if the problem existed > > over years) when we were looking at a migration bug, that part of it was > > involved with the current_migration global pointer being referenced > > anywhere, even after the migration object was finalize()ed. So far without > > singleton support, there's no way to reset the variable properly. > > Declaring migration object to be a singleton also just makes sense, e.g., > > dynamically creating migration objects on the fly with QMP commands doesn't > > sound right.. > > > > The idea behind is pretty simple: any object that can only be created once > > can now declare the TYPE_SINGLETON interface, then QOM facilities will make > > sure it won't be created more than once. For example, qom-list-properties, > > device-list-properties, etc., will be smart enough to not try to create > > temporary singleton objects now. > > QOM design assumption: object_new() followed by object_unref() is always > possible and has no side effect. I see. > > QOM introspection relies on this. Your PATCH 1 makes non-class > properties of singletons invisible in introspection. Making something > with such properties a singleton would be a regression. > > Anything that violates the design assumption must be delayed to a > suitable state transition. For devices (subtypes of TYPE_DEVICE), this > is ->realize(). For user-creatable objects (provide interface > TYPE_USER_CREATABLE), this is ->complete(). For anything else, it's > something else that probably doesn't even exist, yet. See "Problem 5: > QOM lacks a clear life cycle" in > > Subject: Dynamic & heterogeneous machines, initial configuration: problems > Date: Wed, 31 Jan 2024 21:14:21 +0100 > Message-ID: <87o7d1i7ky.fsf@pond.sub.org> > https://lore.kernel.org/qemu-devel/87o7d1i7ky.fsf@pond.sub.org/ The major challenge here might be that, in migration's use case we want to do something after the last refcount is released. IOW, I don't yet see an easy way to explicit do qdev_unrealize() (even if migration object will use realize(), while it doesn't yet..), because there can be more than one thread holding refcount of the object, and we don't know where to invoke it, and we don't want to do the cleanup if the other one still holds a refcount. The only sane place now is in instance_finalize(), which is the only clean place so far that is invoked after last refcount dropped. Maybe I can also try do that with a "magic property" with its set/get, as that's indeed the other hook (basically, object_property_del_all()) that is only invoked after the final refcount is released. However I think we still need the singleton idea somehow.. Thanks,