DiffTest

DiffTest (差分测试): a modern co-simulation framework for RISC-V processors.

Example: Generate Verilog

DiffTest interfaces are provided in Chisel bundles and expected to be integrated into Chisel designs with auto-generated C++ interfaces.

We strongly recommend using Chisel as the design description language when using DiffTest. It will greatly benefit the verification setup since we are providing some advanced features only in Chisel, such as datapath optimizations for higher simulation speed on emulation platforms.

If you are using DiffTest in a non-Chisel environment, we still provide examples of the generated Verilog modules. You may configure the test interfaces in src/test/scala/DifftestMain.scala based on your design details. The generated Verilog and C++ files will match (in type and count) what you have described about your use case. After running the following command, files will be generated at build.

make

Example Chisel Usage

We are supporting Chisel 3.6.1 (the last version supporting Scala FIRRTL Compiler) as well as 6.5.0 (the latest stable version supporting MLIR FIRRTL Compiler).

Here are the detail instructions on integrating DiffTest to your own project.

1. Add this submodule to your design.

In your Git project:

git submodule add https://github.com/OpenXiangShan/difftest.git

In Mill build.sc:

import $file.difftest.build

// We recommend using a fixed Chisel version.
object difftest extends millbuild.difftest.build.CommonDiffTest {
  def crossValue: String = "3.6.1"

  override def millSourcePath = os.pwd / "difftest"
}

// This is for advanced users only.
// All supported Chisel versions are listed in `build.sc`.
// To pass a cross value to difftest:
object difftest extends Cross[millbuild.difftest.build.CommonDiffTest](chiselVersions) {
  override def millSourcePath = os.pwd / "difftest"
}

In Makefile:

emu: sim-verilog
	@$(MAKE) -C difftest emu WITH_CHISELDB=0 WITH_CONSTANTIN=0

2. Add difftest modules (in Chisel or Verilog) to your design. All modules have been listed in the APIs chapter. Some of them are optional.

import difftest._

val difftest = DifftestModule(new DiffInstrCommit, delay = 1, dontCare = true)
difftest.valid  := io.in.valid
difftest.pc     := SignExt(io.in.bits.decode.cf.pc, AddrBits)
difftest.instr  := io.in.bits.decode.cf.instr
difftest.skip   := io.in.bits.isMMIO
difftest.isRVC  := io.in.bits.decode.cf.instr(1, 0)=/="b11".U
difftest.rfwen  := io.wb.rfWen && io.wb.rfDest =/= 0.U
difftest.wdest  := io.wb.rfDest
difftest.wpdest := io.wb.rfDest

3. Call val difftesst = DifftestModule.finish(cpu: String) at the top module whose module name should be SimTop. The variable name difftest must be used to ensure DiffTest could capture the input signals.

An optional UART input/output can be connected to DiffTest. DiffTest will automatically DontCare it internally.

val difftest = DifftestModule.finish("Demo")

// Optional UART connections. Remove this line if UART is not needed.
difftest.uart <> mmio.io.uart

Alternatively, you can skip the optional UART connections by using an overloaded version of DifftestModule.finish(cpu: String, createTopIO: Boolean) with the 2nd parameter createTopIO set to false. This overloaded version can be used in non-module context (e.g. in App class) as following.

object Main extends App {
  // ...
  DifftestModule.finish("Demo", false)
}

4. Generate verilog files for simulation.

5. make emu and start simulating & debugging!

We provide example designs, including:

• XiangShan
• NutShell
• Rocket

If you encountered any issues when integrating DiffTest to your own design, feel free to let us know with necessary information on how you have modified your design. We will try our best to assist you.

APIs

Currently we are supporting the RISC-V base ISA as well as some extensions, including Float/Double, Debug, and Vector. We also support checking the cache coherence via RefillTest.

Probe Name	Descriptions	Mandatory
DiffArchEvent	Exceptions and interrupts	Yes
DiffInstrCommit	Executed instructions	Yes
DiffTrapEvent	Simulation environment call	Yes
DiffArchIntRegState	General-purpose registers	Yes
DiffArchFpRegState	Floating-point registers	No
DiffArchVecRegState	Vector registers	No
DiffCSRState	Control and status registers (CSRs)	Yes
DiffVecCSRState	CSRs for the Vector extension	No
DiffHCSRState	CSRs for the Hypervisor extension	No
DiffDebugMode	Debug mode registers	No
DiffIntWriteback	General-purpose writeback operations	No
DiffFpWriteback	Floating-point writeback operations	No
DiffVecWriteback	Vector writeback operations	No
DiffArchIntDelayedUpdate	Delayed general-purpose writeback	No
DiffArchFpDelayedUpdate	Delayed floating-point writeback	No
DiffStoreEvent	Store operations	No
DiffSbufferEvent	Store buffer operations	No
DiffLoadEvent	Load operations	No
DiffAtomicEvent	Atomic operations	No
DiffL1TLBEvent	L1 TLB operations	No
DiffL2TLBEvent	L2 TLB operations	No
DiffRefillEvent	Cache refill operations	No
DiffLrScEvent	Executed LR/SC instructions	No
DiffNonRegInterruptPengingEvent	Non-register interrupts pending	No
DiffMhpmeventOverflowEvent	Mhpmevent-register overflow	No
DiffDiffCriticalErrorEvent	Raise critical-error	No

The DiffTest framework comes with a simulation framework with some top-level IOs. They will be automatically created when calling DifftestModule.finish(cpu: String).

• LogCtrlIO
• PerfCtrlIO
• UARTIO

These IOs can be used along with the controller wrapper at src/main/scala/common/LogPerfControl.scala.

For compatibility on different platforms, the CPU should access a C++ memory via DPI-C interfaces. This memory will be initialized in C++.

You may also use macro DISABLE_DIFFTEST_RAM_DPIC to remove memory DPI-Cs and use Verilog arrays instead.

val mem = DifftestMem(memByte, 8)
when (wen) {
    mem.write(
    addr = wIdx,
    data = in.w.bits.data.asTypeOf(Vec(DataBytes, UInt(8.W))),
    mask = in.w.bits.strb.asBools
    )
}
val rdata = mem.readAndHold(rIdx, ren).asUInt

To use DiffTest, please include all necessary modules and top-level IOs in your design. It's worth noting the Chisel Bundles may have arguments with default values. Please set the correct parameters for the interfaces.

Plugins

There are several plugins to improve the RTL-simulation and debugging process.

See the Makefiles for the compilation time arguments. Run emu --help to see the full list of supported run-time arguments.

LightSSS: a lightweight simulation snapshot mechanism

After the simulation aborts, we require some debugging information to assist locating the root cause, such as waveform and DUT/REF logs. Traditionally, this requires a second run for the simulation with debugging enabled for the last period of simulation (region of interest, ROI). To avoid such tedious stage, we propose a snapshot mechanism to periodically take snapshots for the simulation process with minor performance overhead. A recent snapshot will be restored after the simulation aborts to reproduce the abortion with debugging information enabled. To understand the technical details of this mechanism, please refer to our MICRO'22 paper.

The plugin LightSSS is by default included at compilation time and should be manually enabled during simulation time using --enable-fork. You may configure the snapshot period using --fork-interval. A typical period is 1 (for small designs) to 30 (for super large designs) seconds. After the simulation aborts, DiffTest automatically re-runs the simulation from a recent snapshot and enables debugging information, including waveform and DUT/REF logs. You may want to redirect the stderr to a file to capture the REF logs output by NEMU and Spike. Please avoid using --enable-fork together with other debugging options, such as -b, -e, --dump-wave, --dump-ref-trace, etc. The behavior when they are enabled simultaneously is undefined.

spike-dasm: a disassembly engine for RISC-V instructions

When the simulation aborts, DiffTest gives a report on the current architectural states and a list of recently commited instructions. To simplify the debugging process, we may want the disassembly of the executed instructions. DiffTest is currently using the spike-dasm command provided by the riscv-isa-sim project for RISC-V instruction disassembly.

To use this plugin, you are required to build it from source and install the tool to somewhere in your PATH. DiffTest will automatically detect its existence by searching the PATH. Please refer to the original README for detailed installation instructions. Feel free to change the --prefix= argument to where you have access to, such as ~/.cache, so that you won't need the sudo privilege for the installation.

References

• Theories of DiffTest / DiffTest 的基本原理
  • 一生一芯计划讲义
• Advanced theories of DiffTest / DiffTest 原理的进阶问题讨论
  • Paper 1, Paper 2
  • SMP-DiffTest 支持多处理器的差分测试方法: PPT, 视频
• Next-generation DiffTest / DiffTest 的下一步演进
  • DiffTest on Cadence Palladium: to be released soon