The recent emergence of network interface cards (NICs) with diverse hardware features for I/O virtualization poses an important challenge for virtual machine environments, particularly in the area of system management. In this paper, we make the case for developing a high level network I/O virtualization management system that can translate user-relevant policy specifications into the hardware and software- specific configurations that are needed on each particular hardware platform. As a first step toward this goal, we describe and classify configuration options that are presented by a wide variety of mechanisms for NIC hardware support for virtualization, and discuss workload and policy considerations that should be factored into configuration decisions. In addition, we propose new mechanisms for intra-node guest-to-guest networking that leverage NIC hardware switching support, and we present a unified system architecture for network I/O virtualization that exposes the configuration options that we identified to high-level management layers.