RISC-V Summit

The RISC-V custom instruction encoding space provides vendors a rich opportunity to innovate. Now, the pending Composable Extensions (CX) Task Group is planning to develop ISA and non-ISA specifications to avoid collisions in opcodes and provide uniform naming, discovery, error handling, context management, and other features that will enable vendors to create a marketplace for composition and reuse of their independently authored custom (instruction) extensions and their software libraries. Instruction set switching is a way to manage this problem. A critical missing piece of the puzzle is the end user perspective: what is the application API, how will the OS manage requests, and what are the implications on the ABI? This presentation outlines a simple plan to address these issues through an API that allows users to allocate and virtualize state context in a uniform manner. We will also discuss the presently manually enforced ABI that keeps the active selector correct at all times while providing backwards compatibility with legacy custom instructions.