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hBPF = eBPF in hardware

An extended Berkley Packet Filter (eBPF) CPU written entirely in Python3 for PC and FPGA.


Back in 1992 the original Berkeley Packet Filter (BPF) was designed for capturing and filtering network packets that matched specific rules. Filters are implemented as programs to be run on a register-based virtual RISC machine providing a small number of instructions inside the Linux Kernel.

Soon it became clear that extending the Kernel by user-supplied programs proves to be useful. But the design of the virtual machine e.g instruction set (ISA) or register width (32-bit vs. 64-bit) couldn't keep up with the demands.

At some point in 2014, work to extend the existing BPF virtual machine started to make it useful in other parts of the Linux Kernel. More, wider registers, additional instructions and a JIT eventually resulted in extended BPF. The original and now obsolete BPF version has been retroactively renamed to classic BPF (cBPF). Nowadays, the Linux Kernel runs eBPF only and loaded cBPF bytecode is transparently translated into eBPF before execution.

Further information can be found in the eBPF specification or in the Linux Kernel documentation.

The hBPF project now implements most of eBPF features in hardware (FPGA).


This project was started beginning of 2021 as an experiment of how fast and how far you can get, with using alternate hardware description languages (compared to the classic 'V' languages VHDL and Verilog), most of the time open-source tools (compared to expensive, commercial toolchains) and cheap development boards (e.g Arty-S7).

It is not meant to compete with multi-core accelerator cards like Xilinx Alveo, instead its used to gain insights and perform experiments.

Current State

  • All unit-tests pass

  • Can process binaries created with assembler or C (LLVM) without any changes

  • Partially optimized

    • This is a constant work in progress ...
    • Statistic information per test case is collected as described here.
    • Clock cycles per op-code are available here.

    Many op-codes require 1 clock cycle, jumps (conditional and unconditional) require 2 clock cycles and math op-codes like mod or div require the most clock cycles to complete.

  • no stack

Additional infos can be found in the Blog.

Tested with:

Migen Git commit: 9a0be7a4210ff96043412539eb5388659b81831d

LiteX Git commit: 85d6cb4b8df98730bd83b412dff6108df188bbb4

LiteETH Git commit: 7acb2a8c1c1abda63496d422d093dc6bbfe135a8

Note: Ensure that you build hBPF with above Git commits of LiteX/Migen dependencies.


The following instructions gets you going, assuming no requirements are installed on your system. If some of the required components are already available then skip the corresponding steps or packages.

# Install system dependencies (assuming Debian based distro)
$ sudo apt install git curl clang llvm python3-pip make xxd \
                   libpcap0.8-dev libbpf-dev openocd
# If you prefer 'hexdump' over of 'xxd'...
$ sudo apt install bsdmainutils
# For ubpf which uses python2
$ sudo apt install python-pip
$ pip install --user -r tools/ubpf/requirements.txt
# Python3 dependencies
$ pip3 install --user colour_runner
$ pip3 install --user git+

Note: Included submodule ubpf requires Python Parcon module. Dont install Parcon via pip3 install --user parcon but install directly from git via pip3 install --user git+ as shown above.

Note: Depending on your installed Python version (e.g. > 3.7) there might be an issue with Parcon module from git repo. In this case you will get dummy_threading errors when building. There is a pull request #22 which fixes this. Alternatively you can install Parcon with:

$ pip3 install --user git+
# Install rust - needed to compile tools
# See additional infos at:
$ curl --proto '=https' --tlsv1.2 -sSf | sh
$ source $HOME/.cargo/env
# Create project root folder (e.g. projects)
$ mkdir -p projects
$ cd projects
# Install Litex
# See additional infos at:
$ mkdir litex
$ cd litex
$ curl -o
$ chmod +x
$ ./ init install --user
# Install hBPF
$ cd ..
$ git clone
$ cd hbpf
$ git submodule init
$ git submodule update
# Compile Tools
$ cd tools/rust-cbpf
$ cargo build
$ cd ../wishbone-utils/wishbone-tool
$ cargo build
# run Tests in tests folder
$ cd ../../../tests

# Test CPU emulator:
# This completes fast
$ ./

# Test FPGA simulation:
# This takes time ......
$ ./

# Test hardware:
# This needs an FPGA board ....
# (ensure that you change the configuration at the top of this
# file to match your target hardware)
$ ./

For working with hardware you need in addition to the above, a toolchain which supports your board/target. This project includes two boards/targets from Xilinx so to build the FPGA bitstreams yourself you have to install Xilinx Vivado.

Note: Prebuilt bitstreams for included targets are provided.


The main purpose of implementing an eBPF CPU in hardware is the same as that of the original cBPF: processing network packets.

By attaching a hBPF CPU directly to a network PHY/MAC a form of a smart NIC could be created. Such a NIC is capable to perform tasks on packets offloaded by the host CPU for performance reasons.

But a hBPF CPU has most necessary features so it can even be used standalone as a simple microcontroller.

The following picture shows an overview of how hBPF can be used.

hBPF overview

hBPF is implemented using the LiteX SoC builder framework. LiteX itself is based on Migen, a toolbox to create FPGA designs in Python3.


The hBPF CPU has access to separated program- and data-memory (Harvard architecture). Data-memory (8-Bit) holds network packets (defaults to 2048 bytes) which are processed based on the instructions in program memory (64-Bit, defaults to 1024 bytes).

At the moment the /reset signals goes HIGH, the CPU starts processing instructions read from program memory. When processing the exit instruction, the halt signal goes high and the CPU stops processing instructions. Whenever an error occurs (e.g division by 0, invalid instruction etc.) the error signal is set HIGH and the CPU stops processing instructions setting signal halt HIGH as well.

Registers R1 - R5 are not cleared on reset and can be used as input arguments to a program, register R0 is used to return something. The same mechanism is also used when calling FPGA extensions by using call-handlers.

When CPU starts, internal ticks register is cleared and incremented every clock tick until CPU halts which can be used to measure execution speed.

The math.divider submodule contains decimal divider and the math.shifter a left/right shifter including arithmetic shifts.

The project includes the following components:


To get a better understanding of how eBPF works an emulator was developed in Python3. It implements a complete eBPF CPU in Python and was a playground to try out concepts and experiment with opcodes. The emulator code itself is not used in the FPGA implementations.


The FPGA implementation of hBPF was done using LiteX and Migen tools. The same Python3 code which later is converted to Verilog can be tested with the simulation capabilities from LiteX.

FPGA Implementations

The LiteX hBPF implementation can be converted to Verilog and processed by toolchains for various target devices and boards. Included are three real hardware implementations for Xilinx devices (Spartan7, Artix7 and Zynq) on Arty-S7, Arty-A7 and Zybo-Z7 boards.

Based on the following overview, they can be used to test and debug hBPF and also to run the HW unit-tests as described under testing further down. They are not connected to an Ethernet PHY/MAC at the moment. Next samples will show how to use LiteETH to process real network packets.


Each implementation requires about 10500 LUTs including Wishbone Bridge and LiteScope Debugger. The CPU core alone requires about 8000 LUTs. Both implementations were tested with 100MHz.

Deviations from eBPF

In contrast to the eBPF implementation used in the Linux Kernel, hBPF has some deviations and differences.


In eBPF, the call instruction is used to call selected subroutines in the Linux Kernel (so called Helper Functions). In the hBPF implementation, the call opcode can be used to extend the CPU by means of call-handlers.

Register R1

In the Linux eBPF implementation register R1 is used as base pointer to the data-memory. This is not necessary for hBPF so register R1 is used as input argument just as R2 - R5.


Not supported at the moment.


This section is divided in two parts:

  • Developing hBPF

    describes how to develop hBPF itself, modifying or extending it.

  • Developing for hBPF

    describes how to develop software which runs on a hBPF (or eBPF) CPU.

Developing hBPF


hBPF is completely written in Python3 and uses the following tools:


Assuming you have all prerequisites installed and their binaries are available via search path, perform the following steps:

  1. check out hBPFs Git repository

  2. open hBPF repository folder in VS-Code

    • start developing
  3. run unit-tests either inside VS-Code or direct from the command line.

  4. to build an FPGA bitstream for a real hardware target, select a target in source/fpga/hw folder and run


All three hBPF implementations can be tested using comprehensive unit-tests. All based on the same set of test cases. To accomplish this, the Python based eBPF assembler of the userspace BPF (uBPF) project is used.

Unit-tests can either be run from inside VS-Code or direct from command line in tests folder.

  • - run test cases against emulated CPU (fast)
  • - runs tests against simulated FPGA design (takes long, be patient)
  • - runs test-cases against hBPF CPU running on real hardware via a serial Wishbone bridge (medium fast)

Each test-case consists of a file in the test (or one of its sub-folders) folder. Test files are text files with a .test file extension. It consists of sections starting with -- describing the test. The following test description shows a Fibonacci test with an asm section which is compiled on the fly before the test is executed, an args section which defines the input values, a result section defining whats expected in register R0 after test completes and an expected section which defines additional criteria that must be met like the number of clock cycles the test may take.

# Example test case description. Calculating the Fibonacci
# for provided input in register R1
-- asm
mov r0, r1
jeq r1, 0, +11
jeq r1, 1, +9
mov r3, 0
mov r0, 1
mov r2, 2
mov r4, r0
add r0, r3
add r2, 1
mov r3, r4
jgt r2, r1, +2
ja -6
mov r0, 1
-- args
r1 = 9
-- result
-- expected
clocks = 324

Also available are a raw section which describes a test program as hex dump and a mem section which describes the data-memory as hex dump.

Before executing a test the asm section is compiled to binary and sent to the device under test (either the emulator, simulator or hardware). For real hardware this is done using the Wishbone serial bridge. If a raw section is found in the test description it is sent after converting to binary from hex dump. Also the data-memory is loaded from the mem section contents if available.

Next input registers R1-R5 are set according to args section. If section is not available or some registers are not set in section then they are set to 0. To start the test the hBPF CPU is brought out of reset by setting RESET_N signal to HIGH and monitoring hBPF CPU signals until HALT signal goes high or expected clocks (when not specified default clocks = 1000) are reached. Then control signals and output register R0 are compared against the test definition.

In addition, statistic information per test case is collected as described here.

Developing for hBPF

The existing eBPF development tools (compilers, assemblers, filters) can be used to develop for hBPF. Samples can be found in the development directory.

Hardware targets and examples

Some examples on the included targets are provided:

hBPF development targets

Packet processing with LiteETH and hBPF


This project is by no means complete or error free or production ready. Feel free to play with it and contribute if you find a bug or thing which could be made better.


This project is licensed under BSD 3-Clause License. See LICENSE file.


If you would like to contribute to hBPF (which we would greatly appreciate) please read the Contributing Guide beforehand.

Code of conduct

Contributor Covenant

Also please read the Code of Conduct to keep the hBPF community approachable and respectable!

Logo / Icon

hBPF Icon

The PCbee (Printed Circuit Board Bee) was created based on's bee icon to visualize the hardware aspect of hBPF.