Software-Defined CPU Modes

Abstract

Our CPUs contain a compute instruction set, which regular applications use. But they also feature an intricate underworld of different CPU modes, combined with trap and exception handling to transition between these modes. These mechanisms are manifold and complex, yet the layering and functionality offered by the CPU modes is fixed. We have to take what CPU vendors provide, including potential security problems from unneeded modes. This paper explores the question, whether CPU modes could instead be defined entirely by software. We show how such a design would function and explore the advantages it enables. We believe that pushing all existing modes under a common design umbrella would enforce a cleaner structure and more control over exposed functionality. At the same time, the flexibility of software-defined modes enables interesting new use cases.