About

• Specification Name: Composable Custom Extensions

• Task Group: Composable Custom Extensions Task Group

• Task Group Charter: Charter

• Spec Jira: https://lf-riscv.atlassian.net/browse/RVS-2611

Background

Reuse of multiple custom extensions is rare, in part because custom extensions may conflict in use of custom instructions and CSRs, and because there is no common programming model or API for custom extensions. This can lead to disjoint solution silos and ecosystem fragmentation. To help address this gap, the TG will advance interop standards that make it easier to reuse certain kinds of custom extensions, enabling a plug-and-play ecosystem of such extensions.

Overview

The TG will define a framework of ISA and non-ISA specifications that together facilitate the decentralized, cooperative reuse of the custom instruction and custom CSR space, enabling practical reuse, within a system, of multiple, independently authored composable custom extensions (CXs), CX libraries, and CX unit (CXU) logic modules, while also remaining backwards compatible with legacy custom extensions.

Stakeholders Identification

• Privileged Architecture HC - ISA changes

• SoftCPU SIG - Use case

• Unified Discovery TG - Discovery

• Platform Runtime Services TG - Discovery, configuration

• Toolchains SIG - ABI support

• psABI - ABI

References: Active Groups and Specifications Under Development

Design Considerations

• Specifications are being designed to be useful to a variety of design point: FPGA and ASIC, microcontroller through HPC.

• Proof of concept will demonstrate supervisor operating system, but specifications are intended to be useful for M and M/U only systems as well.

Proof-of-Concept and Tests

Proof-of Concept

• Reference designs for all components that are specified, demonstrating interoperation where deemed necessary; an implementation of the API and ABI as part of implementing an end-to-end software PoC, including all discovery and configuration components.

Software Support (OS, RTOS, Hypervisor, etc.)

• Linux

Simulator Support

• QEMU - CX framework support; example composable custom extension

• Spike - Not planned

• SAIL - CX framework support

Tests (ACT for ISA or Software)

•  Planned

Software Ecosystem Impacts

Operating systems

• Linux - Discovery and configuration, context management, access control

Hypervisors

• Not planned

Libraries

• glibc - Discovery and configuration, user context management

Other run-times/languages

• Not planned

Applications (direct use)

• Not planned

Freeze Checklists

Key Milestones

Additional Notes