At design time, modern operating systems are locked in a specific safety and isolation strategy that mixes one or more hardware/software protection mechanisms (e.g. user/kernel separation); revisiting these choices after deployment requires a major refactoring effort. This rigid approach shows its limits given the wide variety of modern applications' safety/performance requirements, when new hardware isolation mechanisms are rolled out, or when existing ones break.

Unikernels are famous for providing excellent performance in terms of boot times, throughput and memory consumption, to name a few metrics. However, they are infamous for making it hard and extremely time consuming to extract such performance, and for needing significant engineering effort in order to port applications to them. We introduce Unikraft, a novel micro-library OS that (1) fully modularizes OS primitives so that it is easy to customize the unikernel and include only relevant components and (2) exposes a set of composable, performance-oriented APIs in order to make it easy for developers to obtain high performance.

OS design is traditionally heavily intertwined with protection mechanisms. OSes statically commit to one or a combination of (1) hardware isolation, (2) runtime checking, and (3) software verification early at design time. Changes after deployment require major refactoring; as such, they are rare and costly. In this paper, we argue that this strategy is at odds with recent hardware and software trends: protections break (Meltdown), hardware becomes heterogeneous (Memory Protection Keys, CHERI), and multiple mechanisms can now be used for the same task (software hardening, verification, HW isolation, etc). In short, the choice of isolation strategy and primitives should be postponed to deployment time.