virtio_balloon: disable VIOMMU support

virtio balloon bypasses the DMA API entirely so does not support the
VIOMMU right now.  It's not clear we need that support, for now let's
just make sure we don't pretend to support it.

Cc: stable@vger.kernel.org
Cc: Wei Wang <wei.w.wang@intel.com>
Fixes: 1a937693993f ("virtio: new feature to detect IOMMU device quirk")
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>

drivers/virtio/virtio_balloon.c

@@ -663,6 +663,12 @@ static int virtballoon_restore(struct virtio_device *vdev)
 }
 #endif
 
+static int virtballoon_validate(struct virtio_device *vdev)
+{
+	__virtio_clear_bit(vdev, VIRTIO_F_IOMMU_PLATFORM);
+	return 0;
+}
+
 static unsigned int features[] = {
 	VIRTIO_BALLOON_F_MUST_TELL_HOST,
 	VIRTIO_BALLOON_F_STATS_VQ,
@@ -675,6 +681,7 @@ static struct virtio_driver virtio_balloon_driver = {
 	.driver.name =	KBUILD_MODNAME,
 	.driver.owner =	THIS_MODULE,
 	.id_table =	id_table,
+	.validate =	virtballoon_validate,
 	.probe =	virtballoon_probe,
 	.remove =	virtballoon_remove,
 	.config_changed = virtballoon_changed,