Towards Virtual Passthrough I/O on Commodity Devices

A commodity I/O device has no support for virtualization. A VMM can assign such a device to a single guest with direct, fast, but insecure access by the guest’s native device driver. Alternatively, the VMM can build virtual devices on top of the physical device, allowing it to be multiplexed across VMs, but with lower performance. We propose a technique that provides an intermediate option. In virtual passthrough I/O (VPIO), the guest interacts directly with the physical device most of the time, achieving high performance, as in passthrough I/O. Additionally, the guest/device interactions drive a model that in turn identifies (1) when the physical device can be handed off to another VM, and (2) if the guest programs the device to behave illegitimately. In this paper, we describe the VPIO model, and present preliminary results in using it to support a commodity network card within the Palacios VMM we are building. We believe that an appropriate model for an I/O device could be produced by the hardware vendor as part of the design, implementation, and testing process.