#unikernel #libos

nightly rusty-hermit

RustyHermit - A Rust-based, lightweight unikernel

11 releases

new 0.3.20 Mar 24, 2020
0.3.19 Mar 18, 2020
0.3.17 Feb 23, 2020
0.3.14 Dec 17, 2019
0.3.10 Oct 6, 2019

#16 in Operating systems

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RustyHermit - A Rust-based, lightweight unikernel

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HermitCore is a unikernel targeting a scalable and predictable runtime for high-performance and cloud computing. Unikernel means, you bundle your application directly with the kernel library, so that it can run without any installed operating system. This reduces overhead, therfore, interesting applications include virtual machines and high-performance computing.

The RustyHermit can run Rust applications, as well as C/C++/Go/Fortran applications. A tutorial on how to use these programming languages on top of RustyHermit is published at https://github.com/hermitcore/hermit-playground.


HermitCore is a research project at RWTH-Aachen and was originally written in C (libhermit). We decided to develop a new version of HermitCore in Rust and name it RustyHermit. The ownership model of Rust guarantees memory/thread-safety and enables us to eliminate many classes of bugs at compile-time. Consequently, the use of Rust for kernel development promises less vulnerabilities in comparsion to common programming languages.

The kernel and the integration into the Rust runtime is entirely written in Rust and does not use any C/C++ Code. We extend the Rust toolchain so that the build process is similar to Rust's usual workflow. Rust applications that do not bypass the Rust runtime and directly use OS services are able to run on RustyHermit without modifications.

Building RusytHermit

It is required to install the Rust toolchain. Please visit the Rust website and follow the installation instructions for your operating system. It is important that the nightly channel is used to install the toolchain.

rustup default nightly

After the installation of the toolchain, the source code of the Rust runtime, the cargo subcommand cargo-download, and llvm-tools have to be installed as follow:

cargo install cargo-download
rustup component add rust-src
rustup component add llvm-tools-preview

As an example, the following commands create a template for the test application Hello World.

cargo new hello_world --bin
cd hello_world

To bind the library operating system to the application, add in the file Cargo.toml the crate hermit-sys to the list of dependency. In addition, it is important to use at least the optimization level 1. Consequently, it is required to extend Cargo.toml with following lines.

[target.'cfg(target_os = "hermit")'.dependencies]
hermit-sys = "0.1.*"

opt-level = 3
debug = false
rpath = false
lto = true
debug-assertions = false

opt-level = 1
debug = true
rpath = false
lto = false
debug-assertions = true

Finally, import the crate in the main file of your application.


#[cfg(target_os = "hermit")]
extern crate hermit_sys;

fn main() {
        println!("Hello World!");

The final step is building the application as follows:

cargo build -Z build-std=std,core,alloc,panic_abort --target x86_64-unknown-hermit

If the command failed with the error message

linker `rust-lld` not found

the path to the llvm-tools is not set. On Linux, it is typically installed at ${HOME}/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/bin.

PATH=${HOME}/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/bin:$PATH cargo build -Z build-std=std,core,alloc,panic_abort --target x86_64-unknown-hermit

Otherwise, the linker can be replaced by lld as follows:

RUSTFLAGS="-C linker=lld" cargo build -Z build-std=std,core,alloc,panic_abort --target x86_64-unknown-hermit

Running RustyHermit

Using uhyve as hypervisor

RustyHermit can run within our own hypervisor uhyve , which requires KVM to create a virtual machine. Please install the hypervisor as follows:

cargo install uhyve

Afterwards, your are able to start RustyHermit applications within our hypervisor:

uhyve target/x86_64-unknown-hermit/debug/hello_world

The maximum amount of memory can be configured via environment variables like in the following example

HERMIT_CPUS=4 HERMIT_MEM=8G uhyve target/x86_64-unknown-hermit/debug/hello_world

The virtual machine is configured using the following environment variables

Variable Default Description
HERMIT_CPUS 1 Number of cores the virtual machine may use
HERMIT_MEM 512M Memory size of the virtual machine. The suffixes M and G can be used to specify a value in megabytes or gigabytes
HERMIT_VERBOSE 0 Hypervisor prints kernel log messages stdout. ("1" enables log)

For instance, the following command starts the demo application in a virtual machine, which has 4 cores and 8GiB memory:

$ HERMIT_CPUS=4 HERMIT_MEM=8G uhyve target/x86_64-unknown-hermit/debug/hello_world

More details can be found in the uhyve README.

Using Qemu as hypervisor

It is also possible to run RustyHermit within Qemu. In this case, the loader rusty-loader is required to boot the application. To build the loader, the cargo subcommand xbuild and the assembler nasm are required. After the installation of xbuild, the loader can be build as follows.

$ git clone https://github.com/hermitcore/rusty-loader.git
$ cd rusty-loader
$ cargo xbuild --target x86_64-unknown-hermit-loader.json

Afterwards, the loader is stored in target/x86_64-unknown-hermit-loader/debug/ as rusty-loader. As final step, the unikernel application app can be booted with following command:

$ qemu-system-x86_64 -display none -smp 1 -m 64M -serial stdio  -kernel path_to_loader/rusty-loader -initrd path_to_app/app -cpu qemu64,apic,fsgsbase,rdtscp,xsave,fxsr

It is important to enable the processor features fsgsbase and rdtscp because it is a prerequisite to boot RustyHermit.

Extending RustyHermit

The best way to extend the kernel is to work with the branch devel of the repository rusty-hermit. It includes this repository as submodule and link the unikernel directly to the test application.

According to the following instructions, the test application can be found under target/x86_64-unknown-hermit/debug/rusty_demo.

git clone https://github.com/hermitcore/rusty-hermit.git
cd rusty-hermit
git submodule init
git submodule update
cargo build -Z build-std=std,core,alloc,panic_abort --target x86_64-unknown-hermit

Use RustyHermit for C/C++, Go, and Fortran applications

This kernel can still be used with C/C++, Go, and Fortran applications. A tutorial on how to do this is available at https://github.com/hermitcore/hermit-playground.

Missing features

  • Multikernel support (might be comming)
  • Virtio support (comming soon)
  • Network support (comming soon)


RustyHermit is derived from following tutorials and software distributions:

  1. Philipp Oppermann's excellent series of blog posts.
  2. Erik Kidd's toyos-rs, which is an extension of Philipp Opermann's kernel.
  3. The Rust-based teaching operating system eduOS-rs.

HermitCore's Emoji is provided for free by EmojiOne.


Licensed under either of

at your option.


Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.

RustyHermit is being developed on GitHub. Create your own fork, send us a pull request, and chat with us on Slack