Declaring WARL on the ISA String - RISC-V Virtualization Challenges

Explore the challenges of RISC-V ISA string limitations and WARL (Write Any, Read Legal) behavior in this conference talk from the Linux Plumbers Conference. Examine how current ISA string descriptions cover only a subset of RISC-V variables, forcing discovery of remaining features through direct interaction with ISA implementations via trial and error methods. Understand how WARL behavior creates significant obstacles for virtualization by requiring register access that performance considerations make undesirable to trap and emulate, effectively undermining hardware-accelerated virtualization benefits. Learn about the lack of architectural control over WARL behavior and why eliminating it from the ISA at this stage presents practical challenges. Discover proposed solutions including tactfully ignoring WARL existence by writing only known legal values, which necessitates more comprehensive ISA descriptions. Investigate the Unified Database project's potential role in describing complete ISA specifications and deriving new device tree structures from YAML descriptions. Participate in discussions about optimal device tree formats and explore alternative approaches such as SBI-based ISA discovery using cpuid-style methods or other innovative techniques for addressing these fundamental RISC-V virtualization challenges.