I/O intensive workloads running in virtual machines can suffer massive performance degradation. Direct assignment of I/O devices to virtual machines is the best performing I/O virtualization mechanism, but its performance still remains far from the bare-metal (non-virtualized) case. The primary gap between direct assignment I/O performance and bare-metal I/O performance is the overhead of mapping the VM's memory pages for DMA in IOMMU translation tables. One could avoid this overhead by mapping all of the VM's pages for the lifetime of the VM, but this leads to memory consumption which is unacceptable in many scenarios.