created_at2019-08-21 10:50:23.524709
updated_at2024-04-11 13:53:08.195001
descriptionA minimal hypervisor for Hermit
Martin Kröning (mkroening)



uhyve - A minimal hypervisor for Hermit

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uhyve is small hypervisor to boot the Hermit kernel, which is a unikernel operating system targeting a scalable and predictable runtime behavior for HPC and cloud environments.

Warning: At the moment uhyve grants full host file system access from within the unikernel with the permissions of the user running uhyve. Thus, it should not be used for applications which require isolation from the host system.


An installation of the Rust toolchain is required. Please visit the Rust website and follow the installation instructions.

Install uhyve with

$ cargo install --locked uhyve



To check if your system supports virtualization, you can use the following command:

if egrep -c '(vmx|svm)' /proc/cpuinfo > /dev/null; then echo "Virtualization support found"; fi

On Linux, uhyve depends on the virtualization solution KVM (Kernel-based Virtual Machine). If the following command gives you some output, you are ready to go!

lsmod | grep kvm

NOTE: If in case the above steps don't work, make sure to check in your BIOS settings that virtualization is enabled there.


Disclaimer: Currently, uhyve is mainly developed for Linux. The macOS version has not been tested extensively and does not support all features of the Linux version.

Apple's Command Line Tools must be installed. The following terminal command installs these tools without Apple's IDE Xcode:

xcode-select --install

Additionally, the included hypervisor bases on the Hypervisor Framework depending on OS X Yosemite (10.10) or newer. To verify if your processor is able to support this framework, run the following in your Terminal:

sysctl kern.hv_support

The output kern.hv_support: 1 indicates virtualization support.

Starting with Big Sur, all processes using the Hypervisor API must have the com.apple.security.hypervisor entitlement and therefore must be signed.

Building from source

To build from source, simply checkout the code and use cargo build.

git clone https://github.com/hermitcore/uhyve.git
cd uhyve
cargo build --release

Signing uhyve to run on macOS Big Sur

uhyve can be self-signed with the following command.

codesign -s - --entitlements app.entitlements --force path_to_uhyve/uhyve

The file app.entitlements must have following content:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">

For further details have a look at Apple's documentation.

Running Hermit apps within uhyve

Use the hypervisor to start the unikernel.

uhyve /path/to/the/unikernel/binary


uhyve can be configured via environment variables. The following variables are supported.

  • HERMIT_CPUS: specifies the number of cores the virtual machine may use.
  • HERMIT_MEM: defines the memory size of the virtual machine. The suffixes M and G can be used to specify a value in megabytes or gigabytes, respectively.
  • setting HERMIT_VERBOSE to 1 makes the hypervisor print kernel log messages to the terminal.
  • HERMIT_GDB_PORT=port activate a gdb server for the application running inside uhyve. See below

By default, the loader initializes a system with one core and 512 MiB RAM.

Example: 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 /path/to/the/unikernel/binary

Debugging of Hermit apps (unstable)

Basic support of (single-core) applications is already integrated into uhyve. By specifying variable HERMIT_GDB_PORT=port, uhyve is working as gdbserver and is waiting on port port for a connection to a gdb. For instance, with the following command uhyve is waiting on port 6677 for a connection.

HERMIT_GDB_PORT=6677 uhyve /path_to_the_unikernel/hello_world

In principle, every gdb-capable IDE should be able to debug Hermit applications. (Eclipse, VSCode, ...)

The repository hermit-rs provides example configuration files to debug a Hermit application with Visual Code.

Debugging Hermit apps

Known issues

  • Uhyve isn't able to pass more than 128 environment variables to the unikernel.


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.

Commit count: 1223

cargo fmt