Verification

Ibex Core

Overview

This is a SV/UVM testbench for verification of the Ibex core, located in dv/uvm/core_ibex. At a high level, this testbench uses the open source RISCV-DV random instruction generator to generate compiled instruction binaries, loads them into a simple memory model, stimulates the Ibex core to run this program in memory, and then compares the core trace log against a golden model ISS trace log to check for correctness of execution.

Testbench Architecture

As previously mentioned, this testbench has been constructed based on its usage of the RISCV-DV random instruction generator developed by Google. A block diagram of the testbench is below.

Testbench Architecture

Figure 11 Architecture of the UVM testbench for Ibex core

Memory Interface Agents

The code can be found in the dv/uvm/core_ibex/common/ibex_mem_intf_agent directory. Two of these agents are instantiated within the testbench, one for the instruction fetch interface, and the second for the LSU interface. These agents run slave sequences that wait for memory requests from the core, and then grant the requests for instructions or for data.

Interrupt Interface Agent

The code can be found in the dv/uvm/core_ibex/common/irq_agent directory. This agent is used to drive stimulus onto the Ibex core’s interrupt pins randomly during test execution.

Memory Model

The code can be found in the dv/uvm/core_ibex/common/mem_model directory. The testbench instantiates a single instance of this memory model that it loads the compiled assembly test program into at the beginning of each test. This acts as a unified instruction/data memory that serves all requests from both of the memory interface agents.

Test and Sequence Library

The code can be found in the dv/uvm/core_ibex/tests directory. The tests here are the main sources of external stimulus generation and checking for this testbench, as the memory interface slave sequences simply serve the core’s memory requests. The tests here are all extended from core_ibex_base_test, and coordinate the entire flow for a single test, from loading the compiled assembly binary program into the testbench memory model, to checking the Ibex core status during the test and dealing with test timeouts. The sequences here are used to drive interrupt and debug stimulus into the core.

Testplan

The goal of this bench is to fully verify the Ibex core with 100% coverage. This includes testing all RV32IMC instructions, privileged spec compliance, exception and interrupt testing, Debug Mode operation etc. The complete test list can be found in the file dv/uvm/core_ibex/riscv_dv_extension/testlist.yaml.

Please note that verification is still a work in progress.

Getting Started

Prerequisites & Environment Setup

In order to run the co-simulation flow, you’ll need:

  • A SystemVerilog simulator that supports UVM. The flow is currently tested with VCS.

  • A RISC-V instruction set simulator. For example, Spike or OVPsim. Note that Spike must be configured with --enable-commitlog and --enable-misaligned. The commit log is needed to track the instructions that were executed and --enable-misaligned tells Spike to simulate a core that handles misaligned accesses in hardware (rather than jumping to a trap handler). In addition, Spike does not support the RISC-V Bit Manipulation Extension (Bitmanip) by default. To support this draft extension implemented in Ibex, the riscv-bitmanip branch of Spike needs to be used.

  • A working RISC-V toolchain (to compile / assemble the generated programs before simulating them). Either download a pre-built toolchain (quicker) or download and build the RISC-V GNU compiler toolchain. For the latter, the Bitmanip patches have to be manually installed to enable support for the Bitmanip draft extension. For further information, checkout the Bitmanip Extension on GitHub and how we create the pre-built toolchains.

Once these are installed, you need to set some environment variables to tell the RISCV-DV code where to find them:

export RISCV_TOOLCHAIN=/path/to/riscv
export RISCV_GCC="$RISCV_TOOLCHAIN/bin/riscv32-unknown-elf-gcc"
export RISCV_OBJCOPY="$RISCV_TOOLCHAIN/bin/riscv32-unknown-elf-objcopy"
export SPIKE_PATH=/path/to/spike/bin
export OVPSIM_PATH=/path/to/ovpsim/bin

(Obviously, you only need to set SPIKE_PATH or OVPSIM_PATH if you have installed the corresponding instruction set simulator)

End-to-end RTL/ISS co-simulation flow

RTL/ISS co-simulation flow chart

Figure 12 RTL/ISS co-simulation flow chart

The last stage in this flow handles log comparisons to determine correctness of a given simulation. To do this, both the trace log produced by the core and the trace log produced by the chosen golden model ISS are parsed to collect information about all register writebacks that occur. These two sets of register writeback data are then compared to verify that the core is writing the correct data to the correct registers in the correct order.

However, this checking model quickly falls apart once situations involving external stimulus (such as interrupts and debug requests) start being tested, as while ISS models can simulate traps due to exceptions, they cannot model traps due to external stimulus. In order to provide support for these sorts of scenarios to verify if the core has entered the proper interrupt handler, entered Debug Mode properly, updated any CSRs correctly, and so on, the handshaking mechanism provided by the RISCV-DV instruction generator is heavily used, which effectively allows the core to send status information to the testbench during program execution for any analysis that is required to increase verification effectiveness. This mechanism is explained in detail at https://github.com/google/riscv-dv/blob/master/HANDSHAKE.md. As a sidenote, the signature address that this testbench uses for the handshaking is 0x8ffffffc. Additionally, as is mentioned in the RISCV-DV documentation of this handshake, a small set of API tasks are provided in dv/uvm/core_ibex/tests/core_ibex_base_test.sv to enable easy and efficient integration and usage of this mechanism in this test environment. To see how this handshake is used during real simulations, look in dv/uvm/core_ibex/tests/core_ibex_test_lib.sv. As can be seen, this mechanism is extensively used to provide runtime verification for situations involving external debug requests, interrupt assertions, and memory faults. To add another layer of correctness checking to the checking already provided by the handshake mechanism, a modified version of the trace log comparison is used, as comparing every register write performed during the entire simulation will lead to an incorrect result since the ISS trace log will not contain any execution information in the debug ROM or in any interrupt handler code. As a result, only the final values contained in every register at the end of the test are compared against each other, since any code executed in the debug ROM and trap handlers should not corrupt register state in the rest of the program.

The entirety of this flow is controlled by the Makefile found at dv/uvm/core_ibex/Makefile; here is a list of frequently used commands:

cd dv/uvm/core_ibex

# Run a full regression
make

# Run a full regression, redirect the output directory
make OUT=xxx

# Run a single test
make TEST=riscv_machine_mode_rand_test ITERATIONS=1

# Run a test with a specific seed, dump waveform
make TEST=riscv_machine_mode_rand_test ITERATIONS=1 SEED=123 WAVES=1

# Verbose logging
make ... VERBOSE=1

# Run multiple tests in parallel through LSF
make ... LSF_CMD="bsub -Is"

# Get command reference of the simulation script
python3 sim.py --help

# Generate the assembly tests only
make gen

# Pass addtional options to the generator
make GEN_OPTS="xxxx"  ...

# Compile and run RTL simulation
make TEST=xxx compile,rtl_sim

# Use a different ISS (default is spike)
make ... ISS=ovpsim

# Run a full regression with coverage
make COV=1

Run with a different RTL simulator

You can add any compile/runtime options in dv/uvm/core_ibex/yaml/simulator.yaml.

# Use the new RTL simulator to run
make ... SIMULATOR=xxx

Instruction Cache

Overview

NOTE: Icache verification, as well as documentation, is still in very early stages.

Due to the complexity of the instruction cache, a separate testbench is used to ensure that full verification and coverage closure is performed on this module. This testbench is located at dv/uvm/icache/dv.

As Icache verification is being carried out as part of the OpenTitan open-source project, the testbench derives from the dv_lib UVM class library, which is a set of extended UVM classes that provides basic UVM testbench functionality and components.

This DV environment will be compiled and simulated using the dvsim simulation tool. The master .hjson file that controls simulation with dvsim can be found at dv/uvm/icache/dv/ibex_icache_sim_cfg.hjson. The associated testplan .hjson file is located at dv/uvm/icache/data/ibex_icache_testplan.hjson. As this testbench is still in its infancy, it is currently only able to be compiled, as no tests or sequences are implemented, nor are there any entries in the testplan file. To build the testbench locally using the VCS simulator, run the following command from the root of the Ibex repository:

./vendor/lowrisc_ip/dvsim/dvsim.py dv/uvm/icache/dv/ibex_icache_sim_cfg.hjson --build-only
--skip-ral --purge --sr sim_out

Specify the intended output directory using either the --sr or -scratch-root option. The --skip-ral option is mandatory for building/simulating the Icache testbench, as it does not have any CSRs, excluding this option will lead to build errors. --purge directs the tool to rm -rf the output directory before running the tool, this can be removed if not desired.