Unikraft
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Getting started with Unikraft

The easiest way to get started with Unikraft is to use the command-line utility kraft, which is a companion tool used for defining, configuring, building, and running Unikraft applications. With kraft you can seamlessly create a build environment for your unikernel and painlessly manage dependencies for its build.

As an alternative to kraft it is also possible to build Unikraft using a regular Makefile.

Installing the CLI companion tool kraft

The kraft tool and Unikraft build system have a number of package requirements; please run the following command (on apt-get-based systems) to install the requirements:

apt-get install -y --no-install-recommends \
  build-essential \
  libncurses-dev \
  libyaml-dev \
  flex \
  git \
  wget \
  socat \
  bison \
  unzip \
  uuid-runtime \
  python3 \
  python3-setuptools

To install kraft simply run:

pip3 install git+https://github.com/unikraft/kraft.git

You can then type kraft to see its help menu:

Usage: kraft [OPTIONS] COMMAND [ARGS]...

Options:
  --version         Show the version and exit.
  -C, --no-color    Do not use colour in output logs.
  -T, --timestamps  Show timestamps in output logs.
  -Y, --yes         Assume yes to any binary prompts.
  -v, --verbose     Enables verbose mode.
  -h, --help        Show this message and exit.

Commands:
  build       Build the application.
  clean       Clean the application.
  configure   Configure the application.
  fetch       Fetch library dependencies.
  init        Initialize a new unikraft application.
  lib         Unikraft library commands.
  list        List architectures, platforms, libraries or applications.
  menuconfig  Open the KConfig Menu editor
  prepare     Runs preparations steps on libraries.
  run         Run the application.
  up          Configure, build and run an application.

Influential Environmental Variables:
  UK_WORKDIR The working directory for all Unikraft
  source code [default: ~/.unikraft]
  UK_ROOT    The directory for Unikraft's core source
  code [default: $UK_WORKDIR/unikraft]
  UK_LIBS    The directory of all the external Unikraft
  libraries [default: $UK_WORKDIR/libs]
  UK_APPS    The directory of all the template applications
  [default: $UK_WORKDIR/apps]
  KRAFTRC  The location of kraft's preferences file
  [default: ~/.kraftrc]

Help:
  For help using this tool, please open an issue on Github:
  https://github.com/unikraft/kraft

Building an Application

The simplest way to get the sources for, build and run an application is by running the following commands:

kraft list
kraft up -p PLATFORM -m ARCHITECTURE APP

You can also build and run an applications from the existing templates by using the -t option. For example, building and running the helloworld app:

kraft up -p kvm -m x86_64 -t helloworld helloworld

For more information about all the steps behind kraft up, check Creating an App. For more information about the up command type kraft up -h. For more information about kraft type kraft -h or read the documentation at Unikraft’s website. If you find any problems please fill out an issue. Thank you!

Building Unikraft via Makefile

To manually set up the build environment for the helloworld app, we first have to create a specific directory structure:

 my-unikernel
      ├────apps
      │      └─app-helloworld     <- helloworld application
      ├─── libs                   <- additional libraries go here
      └─── unikraft               <- Unikraft core

We can do this by executing the following sequence of commands:

$ mkdir my-unikernel
$ cd my-unikernel
$ git clone https://github.com/unikraft/unikraft
$ mkdir libs
$ mkdir apps
$ cd apps
$ git clone https://github.com/unikraft/app-helloworld
$ cd app-helloworld

All following configuration and build operations are done from within the helloworld application folder. Notice that it already contains a Makefile that looks like this:

UK_ROOT ?= $(PWD)/../../unikraft
UK_LIBS ?= $(PWD)/../../libs
LIBS :=

all:
	@$(MAKE) -C $(UK_ROOT) A=$(PWD) L=$(LIBS)

$(MAKECMDGOALS):
	@$(MAKE) -C $(UK_ROOT) A=$(PWD) L=$(LIBS) $(MAKECMDGOALS)

In the first two lines, we provide the paths to the Unikraft core repository and the directory where we can put additional libraries. Since the helloworld application uses nolibc as C library, which comes with the Unikraft core, we do not have any external dependencies. We thus do not have to download or reference any libraries in the Makefile and can leave the LIBS variable empty.

With the Makefile present and the directory structure correctly set up, we are able to configure the unikernel as the next step. This will allow us to select the target architecture (e.g., aarch64 or x86-64) and platform (e.g., KVM or Xen).

$ make menuconfig

By default the build system selects x86-64 as target architecture, so we only have to choose a target platform. We will decide for KVM by navigating to the Platform Configuration | KVM guest option and selecting it. Afterwards, we save and exit the configuration menu.

You can then build the unikernel with:

$ make

Consider adding -jX with X being the number of CPUs in your machine to speed up the build process.

When the build completes you will have a Unikraft-based kernel image (build/app-helloworld_kvm-x86_64) that you can start with QEMU/KVM using the -kernel option. See here for further information on running the unikernel with KVM.

Adding external dependencies

When the application becomes more complex, it is likely that not all functionality is included in the Unikraft core repository. As you might have noticed already from the code organization in the core repository (lib/*), Unikraft puts an emphasis on encapsulating functionality in libraries that can be easily added, removed, or replaced to tailor the unikernel to the application’s needs.

To add a library you simply clone it into the libs directory created earlier. In this example, we add the lightweight IP (lwip) network stack lib-lwip:

$ cd my-unikernel
$ cd libs
$ git clone https://github.com/unikraft/lib-lwip

Next, we have reference the library in the Makefile of app-helloworld so that it becomes available for selection in the configuration:

UK_LIBS ?= $(PWD)/../../libs

LIBS := $(UK_LIBS)/lib-lwip
#LIBS := $(LIBS-y):$(UK_LIBS)/lib-X

We can now re-run

make menuconfig

and select lwip in the Library Configuration sub-menu. We can then rebuild the unikernel, by running:

$ make clean
$ make prepare
$ make

The prepare step gives the build system the chance to pull the actual source code for lwip and apply necessary patches to make it work with Unikraft. This has to be done only once after adding a new library.

Note that the build may fail because depending on the configuration, lwip requires a more extensive C library (e.g., newlib) and a POSIX threads implementation (e.g., pthread-embedded). You can add these additional dependencies just as described.