In order to address the high performance I/O needs of HPC and enterprise applications, modern interconnection fabrics, such as InfiniBand and more recently, 10GigE, rely on network adapters with RDMA capabilities. In virtualized environments, these types of adapters are configured in a manner that bypasses the hypervisor and allows virtual machines (VMs) direct device access, so that they deliver near-native low-latency/highbandwidth I/O. One challenge with the bypass approach is that it causes the hypervisor to lose control over VM-device interactions, including the ability to monitor such interactions and to ensure fair resource usage by VMs. Fairness violations, however, permit lowpriority VMs to affect the I/O allocations of other higher priority VMs and more generally, lack of supervision can lead to inefficiencies in the usage of platform resources. This paper describes the FaReS system-level mechanisms for monitoring VMs' usage of bypass I/O devices. Monitoring information ...