Usermode Linux (UML) has been developed and maintained in linus tree for decades and well used by kernel developers as an instant way of virtualization within userspace processes, without relying on hypervisor (i.e., qemu/kvm) or software partition (i.e., namespace). Recently unit testing framework for kernel tree, KUnit, bases UML as an underlying infrastructure for the framework, which brings us more opportunities to use UML. However, this testing capability of KUnit+UML is currently limited to MMU-full codebase where there are certain portion of code with ifndef CONFIG_MMU in the kernel tree. As a result, nommu code lacks the chance of testability and often introduces regressions in the rapid development cycle of linux kernel.
This talk introduces yet-another extension to UML, based on the architecture without MMU emulation, in order to exercise nommu code with a plenty of testing framework implemented on KUnit+UML . The kernel is configured with the option CONFIG_MMU=n, and we've implemented a different syscall hook and handling mechanisms with the different interactions to the host processes. With that, existing userspace programs (we've used Alpine Linux image with patched busybox/musl-libc) can run over this UML instance under nommu environment.
As a bonus using different implementation to the host interactions, we got speedups in several workloads we've tested including lmbench and netperf/iperf3 benchmarks.
I will briefly overview its implementation, the comparison with the original UML architecture, and share several measurement results obtained during the development. We will also share the upstreaming status which we have been proposed [*1].
*1: https://lore.kernel.org/all/cover.1762588860.git.thehajime@gmail.com/