In my presentation about Flight Grade Xen I proposed that we should “Let LLR emphasize external assignments (sets) done within a tracer’s design scope (HLR).” I would like to use a design session to introduce the challenges that arise whenever we associate LLR with a line of Xen code that performs an assignment (either to a state variable or to a registers). Then I’d like to discuss ways to solve this problem and test it using real-world examples in a 5+ year history of Xen commits.