accel/kvm: add a notifier to indicate KVM VM file descriptor has changed

A notifier callback can be used by various subsystems to perform actions when
KVM file descriptor for a virtual machine changes as a part of confidential
guest reset process. This change adds this notifier mechanism. Subsequent
patches will add specific implementations for various notifier callbacks
corresponding to various subsystems that need to take action when KVM VM file
descriptor changed.

Signed-off-by: Ani Sinha <anisinha@redhat.com>
Link: https://lore.kernel.org/r/20260225035000.385950-8-anisinha@redhat.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

accel/kvm/kvm-all.c

@@ -90,6 +90,7 @@ struct KVMParkedVcpu {
 };
 
 KVMState *kvm_state;
+VmfdChangeNotifier vmfd_notifier;
 bool kvm_kernel_irqchip;
 bool kvm_split_irqchip;
 bool kvm_async_interrupts_allowed;
@@ -123,6 +124,9 @@ static const KVMCapabilityInfo kvm_required_capabilites[] = {
 static NotifierList kvm_irqchip_change_notifiers =
     NOTIFIER_LIST_INITIALIZER(kvm_irqchip_change_notifiers);
 
+static NotifierWithReturnList register_vmfd_changed_notifiers =
+    NOTIFIER_WITH_RETURN_LIST_INITIALIZER(register_vmfd_changed_notifiers);
+
 struct KVMResampleFd {
     int gsi;
     EventNotifier *resample_event;
@@ -2173,6 +2177,22 @@ void kvm_irqchip_change_notify(void)
     notifier_list_notify(&kvm_irqchip_change_notifiers, NULL);
 }
 
+void kvm_vmfd_add_change_notifier(NotifierWithReturn *n)
+{
+    notifier_with_return_list_add(&register_vmfd_changed_notifiers, n);
+}
+
+void kvm_vmfd_remove_change_notifier(NotifierWithReturn *n)
+{
+    notifier_with_return_remove(n);
+}
+
+static int kvm_vmfd_change_notify(Error **errp)
+{
+    return notifier_with_return_list_notify(&register_vmfd_changed_notifiers,
+                                            &vmfd_notifier, errp);
+}
+
 int kvm_irqchip_get_virq(KVMState *s)
 {
     int next_virq;
@@ -2671,6 +2691,16 @@ static int kvm_reset_vmfd(MachineState *ms)
         do_kvm_irqchip_create(s);
     }
 
+    /*
+     * notify everyone that vmfd has changed.
+     */
+    vmfd_notifier.vmfd = s->vmfd;
+    ret = kvm_vmfd_change_notify(&err);
+    if (ret < 0) {
+        return ret;
+    }
+    assert(!err);
+
     /* these can be only called after ram_block_rebind() */
     memory_listener_register(&kml->listener, &address_space_memory);
     memory_listener_register(&kvm_io_listener, &address_space_io);

accel/stubs/kvm-stub.c

@@ -79,6 +79,14 @@ void kvm_irqchip_change_notify(void)
 {
 }
 
+void kvm_vmfd_add_change_notifier(NotifierWithReturn *n)
+{
+}
+
+void kvm_vmfd_remove_change_notifier(NotifierWithReturn *n)
+{
+}
+
 int kvm_irqchip_add_irqfd_notifier_gsi(KVMState *s, EventNotifier *n,
                                        EventNotifier *rn, int virq)
 {

include/system/kvm.h

@@ -181,6 +181,7 @@ DECLARE_INSTANCE_CHECKER(KVMState, KVM_STATE,
 
 extern KVMState *kvm_state;
 typedef struct Notifier Notifier;
+typedef struct NotifierWithReturn NotifierWithReturn;
 
 typedef struct KVMRouteChange {
      KVMState *s;
@@ -567,4 +568,24 @@ int kvm_set_memory_attributes_shared(hwaddr start, uint64_t size);
 
 int kvm_convert_memory(hwaddr start, hwaddr size, bool to_private);
 
+/* argument to vmfd change notifier */
+typedef struct VmfdChangeNotifier {
+    int vmfd;
+} VmfdChangeNotifier;
+
+/**
+ * kvm_vmfd_add_change_notifier - register a notifier to get notified when
+ * a KVM vm file descriptor changes as a part of the confidential guest "reset"
+ * process. Various subsystems should use this mechanism to take actions such
+ * as creating new fds against this new vm file descriptor.
+ * @n: notifier with return value.
+ */
+void kvm_vmfd_add_change_notifier(NotifierWithReturn *n);
+/**
+ * kvm_vmfd_remove_change_notifier - de-register a notifer previously
+ * registered with kvm_vmfd_add_change_notifier call.
+ * @n: notifier that was previously registered.
+ */
+void kvm_vmfd_remove_change_notifier(NotifierWithReturn *n);
+
 #endif