Puss In Boots: on formalizing Arm’s Virtual Memory System Architecture

We present our formalization of Arm’s Virtual Memory System Architecture (VMSA). This work has been developed with, and ratified by, Arm and its partners, and is now part of the Arm Architecture Reference Manual. Additionally, we present our experimental validation methodology, which required extending KVM-unit-tests, a test harness for the Kernel Virtual Machine (KVM). We used this infrastructure to run around 1300 VMSA litmus tests on a variety of Arm machines, thereby validating our model w.r.t. existing hardware. Our testing uncovered infidelities to the definition of a feature called Translation Table Hardware Management, which led Arm to relax its architecture to accommodate those cases. Finally, as part of this work, we uncovered subtleties in the definition of a feature of the VMSA called Enhanced Translation Synchronization (ETS), which led Arm to deprecate ETS and replace it with a stronger feature called ETS2.