vIOMMU: Efficient IOMMU Emulation

Direct device assignment, where a guest virtual machine directly interacts with an I/O device without host intervention, is appealing, because it allows an unmodified (non-hypervisor-aware) guest to achieve near-native performance. But device assignment for unmodified guests suffers from two serious deficiencies: (1) it requires pinning all of the guest's pages, thereby disallowing memory overcommitment, and (2) it exposes the guest's memory to buggy device drivers. We solve these problems by designing, implementing, and exposing an emulated IOMMU (vIOMMU) to the unmodified guest. We employ two novel optimizations to make vIOMMU perform well: (1) waiting a few milliseconds before tearing down an IOMMU mapping in the hope it will be immediately reused ("optimistic teardown"), and (2) running the vIOMMU on a sidecore, and thereby enabling for the first time the use of a sidecore by unmodified guests. Both optimizations are highly effective in isolation. The former allows bare-metal to achieve 100% of a 10Gbps line rate. The combination of the two allows an unmodified guest to do the same.