Love the funny description of the kernel option for disabling legacy virtio PCI suppport (VIRTIO_PCI_LEGACY)... when to disable ... "look out into your driveway. Do you have a flying car?"
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.
Looks like the indirect work of +Rusty Russell