My colleague Nadav Amit will be presenting his M.Sc. research, which I had the pleasure of helping with, this upcoming Sunday. The summer before last Nadav did a summer internship with my group at the Haifa Research Lab. Nadav’s internship was dedicated to analyzing the IOTLB behavior of ontemporary IOMMUs, and resulted in this WIOSCA paper. In order to analyze IOTLB behavior, we had to first collect traces of how modern operating systems set-up their DMA buffers, and to do that, Nadav developed IOMMU emulation in KVM.
For his M.Sc., Nadav researched how to emulate IOMMUs efficiently, leading to two primary contributions: first, that waiting just a few milliseconds before tearing down an IOMMU mapping can boost performance substantially due to high temporal reuse. Second, that is possible to emulate a hardware device without trapping to the hypervisor on every device interaction, by using a separate core (a sidecore) to run the device emulation code. The full abstract is below, and everyone is invited to the talk.
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 of all 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.