vrtd

vrtd is a systemd-managed daemon written in C (gnu11) that multiplexes access to AMD Alveo V80 FPGA devices managed by the SLASH kernel module. It communicates with client applications over an AF_UNIX socket using a binary request/response protocol, passing device file descriptors out-of-band via SCM_RIGHTS.

Architecture

vrtd sits between the client libraries and the low-level driver stack:

libvrtdpp  (C++ RAII wrapper)
libvrtd    (C wire-protocol client library)
──────── AF_UNIX / SOCK_SEQPACKET ────────
vrtd       (this daemon)
libslash   (driver wrapper)
Linux kernel module (slash)
AMD Alveo V80 hardware

Key responsibilities:

• Sysfs-based automatic device discovery at startup
• Per-client DMA buffer management (HBM and DDR) with automatic cleanup on disconnect
• FPGA bitstream programming via the design writer subsystem
• Clock frequency control via AXI clock wizard
• PCIe hotplug (secondary bus reset) support
• Role-based access control with per-device granularity
• Systemd integration: socket activation, watchdog, sd-event loop, journal logging

Directory layout

Path	Contents
src/	Daemon source code (C11)
include/vrtd/	Public wire-protocol headers shared with libvrtd
libvrtd/	C client library for the vrtd wire protocol
libvrtdpp/	C++ RAII wrapper around libvrtd
conf/	Default configuration file (vrtd.conf)
systemd/	systemd service and socket unit files
sysusers/	systemd-sysusers configuration (vrtd user/group)
udev/	udev rules for device permissions
cmake/	CMake config-file templates

Building

Prerequisites: libslash must be installed first (or built in-tree with -DVRTD_INCLUDE_LIBSLASH=ON). System dependencies:

sudo apt install cmake pkg-config libsystemd-dev libinih-dev

Build:

cd vrt/vrtd
cmake -B build -S . -G Ninja
cmake --build build
sudo cmake --install build

Running

# Manual
sudo vrtd

# Production (systemd)
sudo systemctl enable --now vrtd

The daemon reads its configuration from /etc/vrt/vrtd.conf (see conf/vrtd.conf for the default format). The configuration file uses an INI-style format to define roles, user permissions, and per-device access rules.

Minimum toolchain versions

The baseline is Ubuntu 22.04 LTS/RHEL 9. Lower versions may work but are untested and may break in any update.

Dependency	Minimum version
CMake	3.22.1
GCC	11.4.0
glibc	2.35
Linux	5.15.0
libsystemd	249.11

Developers contributing to vrtd are encouraged to make use of useful extensions and capabilities as long as they are supported by the versions listed above.

Coding conventions

Read this section before writing C code for this daemon.

This daemon is not written in standard/POSIX C, but instead leans heavily on C11, libsystemd, glibc features, Linux syscall features, and GNU compiler extensions (also supported by Clang). The goal is not to write a portable application, but a modern systemd daemon using all the tools at our disposal.

The language version is C11 with GNU extensions (-std=gnu11). C23 features should not be used unless they are available as GNU extensions.

All .c source files must start with #define _GNU_SOURCE after the copyright header and before including any other headers.

For the complete coding style guide — error handling, macros, ownership rules, RAII patterns, GNU extensions, naming, and formatting — see STYLE.md.

License

MIT — see LICENSE.