CORE-V Verification Environment

A previously stated goal of the core-v-verif project is to produce a unified verification environment for all CORE-V cores supported by the OpenHW. While several of the chapters of this document focus on the specifics of a single core, such as the CV32E40P, this chapter details how the components, structure and organization of core-v-verif are used to implement and deploy a complete simulation verification environment for any core in the CORE-V family.

Note

This chapter of the Verification Strategy is a work-in-progress. It contains several forward looking statements about verification components that are defined, but yet to be implemented.

The core-v-verif project is not a single verification environment that is capbable of supporting any-and-all CORE-V cores. Rather, core-v-verif supports the verification of multiple cores by enablng the rapid creation of core-specific verification environments. There is no attempt to define a one-size-fits-all environment as these inevidibly lead to either bloated code, needless complexity or both. Instead, the idea is to create a toolkit that allows for the rapid development of core-specific environments using a set of high-level reusable components and a standard UVM framework.

UVM environments are often described as a heirarchy with the device-under-test (the core) at the bottom and testcases at the top. In between are various components with increasing degress of abstraction as we go from the bottom levels (the register-transfer level) to the middle layers (transaction-level) to the top (tests). The lower layers of the environment see the least amount of re-use owing to the need to deal with core-specific issues. Components at this level are core-specific. At the transaction level there can be considerable amounts of re-use. For example, it is eary to imagine a single UVM Debug Agent serving the needs of any and all CORE-V cores. The test level sees a mix of re-usable tests (e.g. RV32IMAC compliance) and core-specific tests (e.g. hardward loops in CV32E40P).

The core-v-verif project expliots this idea to maximize re-use across multiple cores by striving to keep as much of the environment as possible independant of the core’s implementation. Components such as the instruction generator, reference model, CSR checkers can be made almost entirely independent of a specific core because they can be based on the ISA alone. Other components such as the functional coverage model, debug and interrupt Agents and the test-program environment can be implemented as a mix of re-usable components and core-specific components.

Depending on the details of the top-level interfaces of individual cores, the lowest layers of the core-v-verif environment may not be re-usable at all.

Environment Structure

A CORE-V verification environment, built from the resources provided by core-v-verif can be conceptually divided into four levels: Testbench Layer, Translation Layer, Abstraction Layer and Test Layer. Each of these will be discussed in turn.

Testbench Layer

A CORE-V testbench layer is comprised of two SystemVerilog modules and a number of SystemVerilog interfaces. We will discuss the SystemVerilog interfaces first, as this will make it easier to understand the structure and purpose of the modules.

SystemVerilog Interfaces

On any given CORE-V core, the top-level ports of the core can be categorized as follows:

  • Instruction and Data memory interface(s)
  • Clocks and Resets
  • Configuration
  • Interrupts
  • Trace
  • Debug
  • Special Status and Control

The Instruction and Data memory interface is listed first for a reason. This interface is generally the most core-specific. For example, CV32E supports I&D interfaces that are AHB-like while CVA6 supports AXI-like interfaces. These are significant difference and so the Testbench Layer deliberately hides this interface from the higher-level layers. This is done in the “DUT Wrapper” module, see below.

The remaining interface categories can be defined as generic collections of input or output signals whose operation can be defined by higher layers. A few examples should illustrate this point:

  • Clocks and resets can be parameterized arrays of clock and reset signals. The upper layers of the environment will define the number of clocks and implement the appropriate frequency and phase relationships. Resets are managed in the same manner.
  • The Interrupts interface can also be implemented as parameterized array of bits. The upper layers of the environment are responsible for asserting and deasserting these signals under direction of a UVM sequence and/or test.

Testbench Modules

The two modules of the Testbench Layer are the “DUT Wrapper” and the “Testbench”. The purpose of the wrapper is to conceal as many core-specific physical attributes as possible. As hinted at abovem this is done by keeping control of the core’s memory interface(s) and mapping all other ports to one of the non-memory interface types.

The wrapper instaniates a memory model that connects directly to the core’s instruction and data interface(s). This memory model also supports a number of memory mapped virtual peripherals. The core’s memory interface is not “seen” by any other part of the environment, so this interface (or these interfaces, as the case may be) can be completely different from other cores and the only part of the environment affected is the DUT wrapper, and its memory model. The address map of the modeled memory and peripherals is implemented to ensure compatibility with the test-program environment (described later in this chapter).

The Testbench module is mostly boiler-plate code that does the following: - instaniates the wrapper - push handles of the SV interfaces to the UVM configuration database - invoke run_test() - Implement a final code-block to display test pass/fail

The expection is that the DUT Wrapper module will be core-specific and will need to be coded from scratch for each CORE-V core. The Testbench module is also expected to be core-specific, but can be easily created by copying and modifying a Testbench module from a previous generation. The SystemVerilog interfaces for Clocks and Resets, Configuration, Interrupts, Trace, Debug, plus Special Status and Control are generic enough to be fully re-used.

Abstraction Layer

Instruction Set Generators

Reference Models

Functional Coverage Models

CORE-V UVM Agents

Test Program Environment

Test Layer

TODO

Verification Plans

TODO

Repository Structure

The top-level of the core-v-verif repository is specifically organized to support multiple verification environments. The directory structure below shows a version of core-v-verif that supports multiple CORE-V cores. What follows is a brief discription of the purpose of each top-level directory. Refer to the README files at each of these locations for additional information. If you read nothing else, please read $PROJ_ROOT/<core>/sim/README.md.

  • core-v-cores: the the Makefiles in the <core>/sim directory will clone the RTL for <core> into core-v-cores/<core>/rtl. This structure allows for the simulataneous verification of multiple cores from the same core-v-verif repostory.
  • <core>: this directory contains the <core> specific environment, testbench, tests and simulation directories.
  • ci: This directory supports common and core-specific scripts and configuration filesto support user-level regressions and the Metrics continuous integration flow.
  • lib: This is where the bulk of the re-usable components and tests are maintained. This is where you will find the instruction generator, reference model, common functional coverage models, UVM Agents for clocks-and-resets, interrupts, status, etc.
$PROJ\_ROOT/
├── core-v-cores
│   ├── <core1>
│   ├── <core2>
│   └── ...
├── <core>
│   ├── env
│   ├── sim
│   ├── tb
│   └── tests
├── ci
│   ├── common
│   ├── <core1>
│   ├── <core1>
│   └── ...
└── lib
    ├── sim_libs
    ├── riscv_tests
    ├── uvm_tests
    ├── uvm_agents
    └── uvm_libs

Getting to There from Here