Kuree / hgdb

hgdb is a flexible hardware debugging framework. It offers runtime APIs to interact with the simulator.

Core features

hgdb is designed to be versatile and provides an abstraction to facilitate hardware debugging. It offers the following features:

• Breakpoints, including step-over and conditional breakpoint.
• Frame/context reconstruction with complex data types.
• Full reverse debugging in replay mode, and limited capability in interactive debugging.
• Set signal values in interactive debugging
• Symbol table and query. No RTL modification required.

Supported Simulators

The simulators listed below have been tested in regression tests. Theoretically hgdb can run on any Verilog/SystemVerilog specification compliant simulator.

• Cadence® Xcelium™
• Synopsys VCS®
• Mentor Questa®
• Verilator
• Icarus Verilog

Supported Generator Frameworks

We are working on passes to extract symbol tables from different generator frameworks. The list below will be growing!

• Chisel/Firrtl, via hgdb-firrtl.
• CIRCT, native support. Current requires patch here.
• Kratos, native support.
• Hand-written Verilog/SystemVerilog, via hgdb-rtl (work in progress).

Usage

The easiest way to get started is to install the compiled shared object via pip. To do so, simply type

pip install libhgdb

You can find the download shared library using the following one-liner

python -c "import pkgutil; print(pkgutil.get_loader('libhgdb').path)"

You can copy it ot symbolic link to places you want to use.

Compile from source

To compile it from source, you need a C++20 compatible compiler, such as gcc-10 or clang-10. Make sure that git submodules are properly cloned.

git clone --recurse-submodules -j8 https://github.com/Kuree/hgdb
cd hgdb
mkdir build && cd build && cmake ..
make hgdb -j

You should see the compiled shared library in build/src/

How to use it with simulators

You need to provide specific flags to the simulator in order to load the runtime. Notice that in most cases you need to make sure that the simulator can find libhgdb.so. The easiest way is to invoke commands with LD_LIBRARY_PATH=${hgdb_lib_path}$, where ${hgdb_lib_path} is the directory containing libhgdb.so. Here are some examples on how to use it with different simulators.

• Cadence® Xcelium™

  xrun [commands] -access +rw -loadvpi libhgdb.so:initialize_hgdb_runtime

• Synopsys VCS®

  vcs [commands] -debug_acc+all -load libhgdb.so

• Mentor Questa®

  vsim [flags] -pli libghdb.so

• Verilator

  Verilator is a little bit tedious since it is not specification-compliant.

  First, we need to generate the verilator files with extra VPI flags

  verilator [flags] --vpi ${path_to_libhgdb.so}``

  In addition, most signals should be labeled as public, otherwise breakpoints and frame inspection will not work. An easy way is to use --public-flat-rw flag when invoking verilator. In addition to the flags, we need add following code to the test bench:

  • Forward declare the runtime call:

    namespace hgdb {
    void initialize_hgdb_runtime_cxx();
    }

  • At the beginning of the test bench code:

    hgdb::initialize_hgdb_runtime_cxx();

    Also make sure argc and argv are properly passed to verilator:

    Verilated::commandArgs(argc, argv);

  • At each posedge of the clock, we need to call specific callback:

    VerilatedVpi::callCbs(cbNextSimTime);

    You can check out this example test bench for more details.

• Icarus Verilog

  Icarus Verilog only takes shared library with .vpi extension. As a result, it is a good idea to simply symbolic link libhgdb.so to libhgdb.vpi in the current working directory. When you run the compiled circuit with vvp, add the following command:

  vvp -M. -mlibhgdb [commands]

Runtime command-line arguments

You can change the runtime settings using plus-args when invoking the simulator. Here is a short list of options you can change:

• +DEBUG_PORT=num, where num is the port number. By default this is 888`
• +DEBUG_LOG=1, enable the debugging log. Useful when debugging the behavior of the runtime

There are several predefined environment variables one can use to debug the runtime. It is not recommended for production usage:

• DEBUG_DISABLE_BLOCKING: when present, will disable the initial blocking. As a result, the simulator will starts execution without user's explicit "start" or "continue" command.
• DEBUG_DATABASE_FILENAME#=filename:line_num: where # counts from 0. The runtime will query the predefined breakpoints starting from 0 and stops if corresponding environment variable name not found.

Which debugger to use

hgdb offers several open-sourced debuggers:

• Visual Studio Code Debugger Extension
• gdb-style debugger

You can check out the debuggers here.

Reverse-debugging

hgdb supports full reverse-debugging via trace file. Users can forward and backward any time, with breakpoint support. This is achieved by a trace replay tool that implements hgdb's compatibility layer. The tool, hgdb-replay, is shipped with libhgdb package. To use it, simply do

hgdb-replay waveform.vcd [args]

where [args] are optional arguments passed to the debug runtime.

Source-level waveform

hgdb also supports source-level waveform by rewriting existing waveform against the symbol table. The rewritten waveform will produce source-level constructs, such as Bundle and arrays. Currently only VCD format is supported. The rewrite tool hgdb-rewrite-vcd is shipped with libhgdb package.

$ hgdb-rewrite-vcd <original.vcd> <debug.db> <new.vcd>

Symbol table generation

The symbol table used by hgdb is designed to be compiler-friendly and language-independent. Hardware generator framework developers should check this document out to see more details.

Available language bindings

Below shows a list of language bindings offered by hgdb and their implementation status

• C/C++: creation query runtime
• Python: creation query
• SystemVerilog: runtime
• tcl: query