In the presentation about Flight Grade Xen I suggested that it is hard work, when doing Reverse Engineering for aircraft certifications, to know why Xen set a specific value into a specific register or state variable. “Knowing Why is Hard” – but is required by DO-178C (as noted in TC-15⁄27). This talk will suggest that Xen certifications could leverage a Device Tree provided to it at boot time on Arm, etc., as the first step in tracing up to system requirements. If we try to do this, what additional challenges remain, and what should we try to do next as an OSS community?