Efficient Memory Overcommitment for I/O Passthrough Enabled VMs via Fine-grained Page Meta-data Management

In virtualization systems, guest memory overcommitment helps to improve the utilization of the host memory resource. However, the widely adopted I/O passthrough technique makes this task not intuitive since the hypervisor must avoid DMA (Direct Memory Access) failures when the I/O device accesses the guest memory. There already exist several solutions, for example, IOPF (I/O Page Fault) can fix DMA failures by allowing page fault triggered from the I/O device side, vIOMMU and coIOMMU avoid DMA failures by monitoring the DMA buffers in the guest. However, these solutions all face the performance concerns introduced by the memory backup/restore mechanism, i.e., memory swapping. Some free page based methods (e.g., Ballooning, Free Page Reporting, Hyperupcall) are free from memory swapping, but they either are not DMA-safe or introduce high guest communication overhead. In this paper, we propose V-Probe, a high-efficiency approach to achieve memory overcommitment for I/O passthrough enabled VMs. Using fine-grained page meta-data management, V-Probe allows the hypervisor to inspect and reclaim guest free pages actively and efficiently while guaranteeing DMA-safety. Experiments show that, for both memory reclamation and reallocation, the overhead of V-Probe is in the scale of microseconds, which is faster than Ballooning and IOPF based methods by two orders of magnitude. And our micro-benchmark and macro-benchmark show that V-Probe limits the performance impact of memory overcommitment to a low level.