[PATCH RFC v2 5/7] x86/iommu: Make x86-iommu a singleton object

[Peter Xu <peterx@redhat.com>]

X86 IOMMUs cannot be created more than one on a system yet.  Make it a
singleton so it guards the system from accidentally create yet another
IOMMU object when one already presents.

Now if someone tries to create more than one, e.g., via:

  ./qemu -M q35 -device intel-iommu -device intel-iommu

The error will change from:

  qemu-system-x86_64: -device intel-iommu: QEMU does not support multiple vIOMMUs for x86 yet.

To:

  qemu-system-x86_64: -device intel-iommu: Class 'intel-iommu' only supports one instance

Unfortunately, yet we can't remove the singleton check in the machine
hook (pc_machine_device_pre_plug_cb), because there can also be
virtio-iommu involved, which doesn't share a common parent class yet.

But with this, it should be closer to reach that goal to check singleton by
QOM one day.

Note: after this patch, x86_iommu_get_default() might be called very early,
even before machine is created (e.g., cmdline "-device intel-iommu,help").
In this case we need to be prepared with such, and that also means the
default iommu is not yet around.

Signed-off-by: Peter Xu <peterx@redhat.com>
---
 hw/i386/x86-iommu.c | 26 +++++++++++++++++++++++---
 1 file changed, 23 insertions(+), 3 deletions(-)

diff --git a/hw/i386/x86-iommu.c b/hw/i386/x86-iommu.c
index 60af896225..ec45b80306 100644
--- a/hw/i386/x86-iommu.c
+++ b/hw/i386/x86-iommu.c
@@ -26,6 +26,7 @@
 #include "qemu/error-report.h"
 #include "trace.h"
 #include "sysemu/kvm.h"
+#include "qom/object_interfaces.h"
 
 void x86_iommu_iec_register_notifier(X86IOMMUState *iommu,
                                      iec_notify_fn fn, void *data)
@@ -79,9 +80,15 @@ void x86_iommu_irq_to_msi_message(X86IOMMUIrq *irq, MSIMessage *msg_out)
 
 X86IOMMUState *x86_iommu_get_default(void)
 {
-    MachineState *ms = MACHINE(qdev_get_machine());
-    PCMachineState *pcms =
-        PC_MACHINE(object_dynamic_cast(OBJECT(ms), TYPE_PC_MACHINE));
+    Object *machine = qdev_get_machine();
+    PCMachineState *pcms;
+
+    /* If machine has not been created, so is the vIOMMU */
+    if (!machine) {
+        return NULL;
+    }
+
+    pcms = PC_MACHINE(object_dynamic_cast(machine, TYPE_PC_MACHINE));
 
     if (pcms &&
         object_dynamic_cast(OBJECT(pcms->iommu), TYPE_X86_IOMMU_DEVICE)) {
@@ -133,10 +140,19 @@ static Property x86_iommu_properties[] = {
     DEFINE_PROP_END_OF_LIST(),
 };
 
+static Object *x86_iommu_get_instance(void)
+{
+    return object_ref(OBJECT(x86_iommu_get_default()));
+}
+
 static void x86_iommu_class_init(ObjectClass *klass, void *data)
 {
     DeviceClass *dc = DEVICE_CLASS(klass);
+    SingletonClass *singleton = SINGLETON_CLASS(klass);
+
     dc->realize = x86_iommu_realize;
+    singleton->get_instance = x86_iommu_get_instance;
+
     device_class_set_props(dc, x86_iommu_properties);
 }
 
@@ -152,6 +168,10 @@ static const TypeInfo x86_iommu_info = {
     .class_init    = x86_iommu_class_init,
     .class_size    = sizeof(X86IOMMUClass),
     .abstract      = true,
+    .interfaces = (InterfaceInfo[]) {
+        { TYPE_SINGLETON },
+        { }
+    }
 };
 
 static void x86_iommu_register_types(void)
-- 
2.45.0