Virtual Processors as Kernel Interface

After virtualization has gained traction in a variety of fields ranging from the desktop computer to datacenter servers, it is likely to make inroads into embedded systems as well. The complexity of a VM implementation depends on the virtualization abilities of the processor used. Unfortunately, the instruction set architecture of many popular embedded CPUs is not virtualizable, which precludes efficient pure or faithful virtualization. In this paper, we make the case for operating system (OS) rehosting, a flavor of virtualization that lends itself to implementations of low complexity and does not rely an CPU virtualization extensions. The feasibility of OS rehosting crucially depends on the traits of the interface of the underlying kernel. Our observation was that the ubiquitously used thread model is rather poorly suited to run an OS on top. As a solution, we propose the currently often employed threads be supplemented with virtual processors (vCPUs), an abstraction that is more aligned with the underlying hardware. To evaluate our proposition, we ported the Linux kernel to a vCPU enhanced version of the Fiasco microkernel. Compared to a previous thread-based version, the vCPU version required much less development effort. The performance gains range from slight to well-pronounced depending on the workload.