Zihintpause Extension Sail Support

[jjscheel]

Technical Group

Golden Model SIG

ratification-pkg

Zihintpause

Technical Liaison

Prashanth Mundkur

Task Category

SAIL model

Task Sub Category

• gcc
• binutils
• gdb
• intrinsics
• Java
• KVM
• ld
• llvm
• Linux kernel
• QEMU
• Spike

Ratification Target

Already Ratified

spec-link

https://drive.google.com/file/d/1uviu1nH-tScFfgrovvFCrj7Omv8tFtkp/view

Statement of Work (SOW)

Component names:
RISC-V Sail Model

Requirements:
The PAUSE instruction is a HINT that indicates the current hart’s rate of instruction retirement
should be temporarily reduced or paused. The duration of its effect must be bounded and may be
zero.

Sail support for this hint has not yet been created. This SOW delivers extension support for the Zihintpause in the Sail model, including command line configuration and build verification tests.

See Chapter 10 "'Zihintpause' Extensions for Pause Hint, Version 2.0" in the The RISC-V Instruction Set: Manual Volume I specification for details about the extension.

NOTE: Per the comment from Jordan Carlin below, this work should wait until the new module system (riscv/sail-riscv#572) is merged to that it will simplify the overriding of existing encoding. It should also wait for the Zicbop Sail PR #538 to demonstrate the enablement technique.

Unprivileged Architecture
Deliverables:

• RISC-V Sail model changes accepted into community for extension
• RISC-V Sail model build verification tests written for extension and accepted into community

Acceptance Criteria:

• PR(s) reviewed and accepted
• Build verification tests passing
• Passes any tests created in Architecture Compatibility Tests

SOW Signoffs: (delete those not needed)

• Task group liaison sign-off
• Development partner sign-off
• Golden Model SIG sign-off (if SAIL work)

Waiver

• Freeze
• Ratification

Pull Request Details

No response

[jordancarlin]

We've discussed hints from Zihintpause, Zihintntl, and Zicbop a few times in the Golden Model Meeting, and the consensus was to wait until the new module system (riscv/sail-riscv#572) is merged. See https://github.com/riscv/sail-riscv/wiki/Extension-Roadmap.

I believe they would all function as nops in the model, so it is really just the assembly that would need to be implemented. The module system will make overriding existing encodings easier, which all of these extensions do.

[jjscheel]

@jordancarlin, thanks for the update. I agree this is a good pre-requisite and I'll make updates to the SOW content to reference the plan.

@pmundkur, @gsterlin, please review for any additional gaps.

[jordancarlin]

The module system (riscv/sail-riscv#572) has been merged, so it should be possible to start on this now.

[pmundkur]

PR submitted: riscv/sail-riscv#1208

[pmundkur]

The PR got merged, and this task is now complete.