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CMO-Phase-1-Scope.md

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CMO Phase 1 Scope

Introduction

This document summarizes the current plan-of-record for Phase 1 of the CMO extension. Additional details will be provided by various extension proposal documents.

Sub-extensions

Phase 1 is divided into three sub-extensions that add the following instructions and features:

  • Cache Block Management Operations (Zicbom)
    • CBO.INVAL - Invalidate Cache Block (at effective address)
    • CBO.CLEAN - Clean Cache Block (at effective address)
    • CBO.FLUSH - Flush Cache Block (at effective address)
    • Memory ordering with respect to other memory accesses
  • Cache Block Zero Operations (Zicboz)
    • CBO.ZERO - Zero Cache Block (at effective address)
    • Memory ordering with respect to other memory accesses
  • Cache Block Prefetch Operations (Zicbop)
    • PREFETCH.R - Prefetch Cache Block for Read (at effective address)
    • PREFETCH.W - Prefetch Cache Block for Write (at effective address)
    • PREFETCH.I - Prefetch Cache Block for Instruction Fetch (at effective address)

Instructions in the Zicbom and Zicboz sub-extensions support a [rs1] addressing mode. Instructions in the Zicbop sub-extension may support a modified form of a [rs1+imm12] addressing mode.

For Phase 1, Zicbom instructions operate to the copy of data in memory, while Zicboz updates the values of memory corresponding to a memory location like stores. Zicbop instructions may allocated in any cache as well as none.

Closed Issues

Note: "Closed" implies that a given issue has been documented in the specification

  • Permissions, protection, and access control (i.e. relationship to translation, PMAs, PMPs, and privilege level)
  • Safe transformations of operations
    • HW: safe transformations enable implementation choices
    • SW: safe transformations allow less privileged software to use ops
  • Memory ordering model (i.e. relationship to loads, stores, FENCEs, SFENCEs, etc.)
  • Discovery of block sizes and types
  • Final instruction encodings

Open Issues

Roadmap

These topics are deferred from Phase 1 and may be considered in Phase 2 and onward:

  • Effects of mismatched PMA/VA attributes and changing attributes
  • Additional levels or points of convergence for system optimization
  • Levels (Points of Persistence) beyond the POC for all harts and devices
  • Temporality (reuse) hints for PREFETCH operations
  • Cache Block Operations by Index
    • CBO.INVAL.IX - Invalidate Cache Block at Index
    • CBO.CLEAN.IX - Clean Cache Block at Index
    • CBO.FLUSH.IX - Flush Cache Block at Index
  • CMO.ALL
  • DEMOTE
  • Completion semantics
  • Security-related CMOs, e.g. CMO.ALL.SEC
  • Non-uniform (i.e. mixed) block sizes
  • Operations that return values, i.e. ranges, e.g. CMO.op.AR and CMO.op.UR
  • Others?