2025-03-28 Ordinary Meeting Notes

Date

Mar 28, 2025

Disclosures

RISC-V Disclosure Video

Open

Participants

Agenda

• Composability Criteria Discussion Wrap Up

• Future Topics of Discussion

Presentations

Composability Criteria Discussion Wrap Up

• Major points and topics of discussion has been added to preliminary draft specification

• Feel free to continue discussion on list

State of CX Criteria Discussion (talking points)

• Draft draft spec https://github.com/riscv/composable-custom-extensions/blob/main/src/criteria.adoc proposes

  • “Bottom up” approach from zero

  • Explicit definition of “composable”

    • Atomic (one at a time) instructions

    • Accessing only “composable state”

      • Custom extension’s state

      • Certain standard extensions’ hart state

      • Shared memory

  • Implicit definition of “portable”

• Discussions, Disagreements

  • One set of CX criteria vs. separate composable, portable, etc. criteria

  • “Composable state”

    • Custom extension’s state

      • Strict isolation vs. sharing

      • Single instance per hart only vs. option of multiple instances per hart

    • Certain standard extensions’ hart state

      • Explicitly enumerate elements of “standard extensions’ hart state” vs. implicitly include all such state

    • Shared memory accesses

      • RVWMO as-if-by-hart access vs. anything looser

    • Custom instructions' exception model

      • Atomic instructions – all or nothing behavior

      • Non-atomic, idempotent instructions – idempotently resumable behavior

Future Topics of Discussion

• ISA requirements, unprivileged and privileged

• State management

• Discovery (device tree, ACPI, user APIs, etc.)

• User space programming model

• Other topics?

Notes & Action Items

• Jan proposed considering the ISA requirements as a future topic of discussion.

• Jan revisited the topic of one or multiple state contexts per composable custom extension per hart, and expressed a preference for the latter.