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. 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). The 1st patch introduces the QOM interface, while I made both x86-iommu and migration as the initial users of it, so it's really more about the 1st patch to discuss first. I didn't yet touch ARM/SMMU as it's also more complicated; I also didn't yet dig anything else that may apply, but I do feel like this can apply more than the two attached here. Hopefully the two use cases can be good enough to start the discussion. Thanks, Peter Xu (4): qom: TYPE_SINGLETON interface x86/iommu: Make x86-iommu a singleton object migration: Make migration object a singleton object migration: Reset current_migration properly include/qom/object_interfaces.h | 47 +++++++++++++++++++++++++++++++++ hw/i386/x86-iommu.c | 14 ++++++++++ migration/migration.c | 36 ++++++++++++++++++++++--- qom/object.c | 3 +++ qom/object_interfaces.c | 24 +++++++++++++++++ qom/qom-qmp-cmds.c | 22 ++++++++++++--- system/qdev-monitor.c | 7 +++++ 7 files changed, 147 insertions(+), 6 deletions(-) -- 2.45.0
