CONFIG_VIRTIO_PCI_LEGACY: Virtio PCI Card 0.9.X Draft (circa 2014) and older device support. This option enables building a transitional driver, supporting both devices conforming to Virtio 1 specification, and legacy devices. If disabled, you get a slightly smaller, non-transitional driver, with no legacy compatibility. So look out into your driveway. Do you have a flying car? If so, you can happily disable this option and virtio will not break. Otherwise, leave it set. Unless you're testing what life will be like in The Future. If unsure, say Y.