KVM: Ensure lockdep knows about kvm->lock vs. vcpu->mutex ordering rule

Documentation/virt/kvm/locking.rst tells us that kvm->lock is taken outside
vcpu->mutex. But that doesn't actually happen very often; it's only in
some esoteric cases like migration with AMD SEV. This means that lockdep
usually doesn't notice, and doesn't do its job of keeping us honest.

Ensure that lockdep *always* knows about the ordering of these two locks,
by briefly taking vcpu->mutex in kvm_vm_ioctl_create_vcpu() while kvm->lock
is held.
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Message-Id: <20230111180651.14394-3-dwmw2@infradead.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

virt/kvm/kvm_main.c

@@ -3954,6 +3954,13 @@ static int kvm_vm_ioctl_create_vcpu(struct kvm *kvm, u32 id)
 	}
 
 	mutex_lock(&kvm->lock);
+
+#ifdef CONFIG_LOCKDEP
+	/* Ensure that lockdep knows vcpu->mutex is taken *inside* kvm->lock */
+	mutex_lock(&vcpu->mutex);
+	mutex_unlock(&vcpu->mutex);
+#endif
+
 	if (kvm_get_vcpu_by_id(kvm, id)) {
 		r = -EEXIST;
 		goto unlock_vcpu_destroy;