kvm/vcpu: add notifiers to inform vcpu file descriptor change

When new vcpu file descriptors are created and bound to the new kvm file
descriptor as a part of the confidential guest reset mechanism, various
subsystems needs to know about it. This change adds notifiers so that various
subsystems can take appropriate actions when vcpu fds change by registering
their handlers to this notifier.
Subsequent changes will register specific handlers to this notifier.

Signed-off-by: Ani Sinha <anisinha@redhat.com>
Link: https://lore.kernel.org/r/20260225035000.385950-31-anisinha@redhat.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

accel/kvm/kvm-all.c

@@ -127,6 +127,9 @@ static NotifierList kvm_irqchip_change_notifiers =
 static NotifierWithReturnList register_vmfd_changed_notifiers =
     NOTIFIER_WITH_RETURN_LIST_INITIALIZER(register_vmfd_changed_notifiers);
 
+static NotifierWithReturnList register_vcpufd_changed_notifiers =
+    NOTIFIER_WITH_RETURN_LIST_INITIALIZER(register_vcpufd_changed_notifiers);
+
 static int map_kvm_run(KVMState *s, CPUState *cpu, Error **errp);
 static int map_kvm_dirty_gfns(KVMState *s, CPUState *cpu, Error **errp);
 static int vcpu_unmap_regions(KVMState *s, CPUState *cpu);
@@ -2314,6 +2317,22 @@ static int kvm_vmfd_change_notify(Error **errp)
                                             &vmfd_notifier, errp);
 }
 
+void kvm_vcpufd_add_change_notifier(NotifierWithReturn *n)
+{
+    notifier_with_return_list_add(&register_vcpufd_changed_notifiers, n);
+}
+
+void kvm_vcpufd_remove_change_notifier(NotifierWithReturn *n)
+{
+    notifier_with_return_remove(n);
+}
+
+static int kvm_vcpufd_change_notify(Error **errp)
+{
+    return notifier_with_return_list_notify(&register_vcpufd_changed_notifiers,
+                                            &vmfd_notifier, errp);
+}
+
 int kvm_irqchip_get_virq(KVMState *s)
 {
     int next_virq;
@@ -2841,6 +2860,13 @@ static int kvm_reset_vmfd(MachineState *ms)
     }
     assert(!err);
 
+    /* notify everyone that vcpu fd has changed. */
+    ret = kvm_vcpufd_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

@@ -87,6 +87,16 @@ void kvm_vmfd_remove_change_notifier(NotifierWithReturn *n)
 {
 }
 
+void kvm_vcpufd_add_change_notifier(NotifierWithReturn *n)
+{
+    return;
+}
+
+void kvm_vcpufd_remove_change_notifier(NotifierWithReturn *n)
+{
+    return;
+}
+
 int kvm_irqchip_add_irqfd_notifier_gsi(KVMState *s, EventNotifier *n,
                                        EventNotifier *rn, int virq)
 {

include/system/kvm.h

@@ -590,4 +590,21 @@ void kvm_vmfd_add_change_notifier(NotifierWithReturn *n);
  */
 void kvm_vmfd_remove_change_notifier(NotifierWithReturn *n);
 
+/**
+ * kvm_vcpufd_add_change_notifier - register a notifier to get notified when
+ * a KVM vcpu file descriptors changes as a part of the confidential guest
+ * "reset" process. Various subsystems should use this mechanism to take
+ * actions such as re-issuing vcpu ioctls as a part of setting up vcpu
+ * features.
+ * @n: notifier with return value.
+ */
+void kvm_vcpufd_add_change_notifier(NotifierWithReturn *n);
+
+/**
+ * kvm_vcpufd_remove_change_notifier - de-register a notifer previously
+ * registered with kvm_vcpufd_add_change_notifier call.
+ * @n: notifier that was previously registered.
+ */
+void kvm_vcpufd_remove_change_notifier(NotifierWithReturn *n);
+
 #endif