Kraft is the tool developed by the Unikraft team, to make application deployment easier. To automatically download, configure, build and run an application (e.g. helloworld), run

$ kraft list update
$ kraft up -t [email protected] ./my-first-unikernel

If you are already working with cloned / forked repositories from Unikraft, kraft can also help you configure, build and run you application. kraft up can be broken down into the following commands:

$ kraft configure
$ kraft build
$ kraft run

Required Tools and Resources

For this session, the following tools are needed: qemu-kvm, qemu-system-x86_64, qemu-system-aarch64, gcc-aarch64-linux-gnu, bridge-utils. To install on Debian/Ubuntu use the following command:

$ sudo apt -y install qemu-kvm qemu-system-x86 qemu-system-arm gcc-aarch64-linux-gnu bridge-utils

Support Files

Session support files are available in the repository. If you haven’t cloned the repository yet, clone it and enter the session directory:

$ git clone

$ cd docs/

$ cd content/en/community/hackathons/sessions/behind-scenes/

$ ls
demo  images  sol  work

Otherwise, if you already cloned the repository, update it and enter the session directory. Assuming the session directory is path/to/repository/clone, do the following:

$ cd path/to/repository/clone

$ git pull --rebase

$ cd content/en/community/hackathons/sessions/behind-scenes/

$ ls
demo  images  sol  work


01. Virtualization

Through virtualization, multiple operating systems (OS) are able to run on the same hardware, independently, thinking that each one of them controls the entire system. This can be done using a hypervisor, which is a low-level software that virtualizes the underlying hardware and manages access to the real hardware, either directly or through the host Operating System. There are 2 main virtualized environments: virtual machines and containers, each with pros and cons regarding complexity, size, performance and security. Unikernels come somewhere between those 2.

Virtual Machines

Virtual machines represent an abstraction of the hardware over which an operating system can run, thinking that it is alone on the system and that it controls the hardware below it. Virtual machines rely on hypervisors to run properly. Those hypervisors can be classified in 2 categories: Type 1 and Type 2. We won’t go in depth into them, but it is good to know how they are different:

  • The Type 1 hypervisor, also known as bare-metal hypervisor, has direct access to the hardware and controls all the operating systems that are running on the system. KVM, despite the appearances, is a Type 1 hypervisor.
  • The Type 2 hypervisor, also known as hosted hypervisor, has to go through the host operating system to reach the hardware. An example of Type 2 hypervisor is VirtualBox.

type 1 hypervisor os

type 2 hypervisor os

Operating systems over type 1 hypervisor Operating systems over type 2 hypervisor


Containers are environments designed to contain and run only one application and its dependencies. This leads to very small sizes. The containers are managed by a Container Management Engine, like Docker, and are dependent on the host OS, as they cannot run without it.




Unikraft has a size comparable with that of a container, while it retains the power of a virtual machine, meaning it can directly control the hardware components (virtualized, or not, if running bare-metal). This gives it an advantage over classical Operating Systems. Being a special type of operating system, Unikraft can run bare-metal or over a hypervisor.

type 1 hypervisor uk

type 2 hypervisor uk

Unikraft over Type 1 hypervisor Unikraft over Type 2 hypervisor

The following table makes a comparison between regular Virtual Machines (think of an Ubuntu VM), Containers and Unikernels, represented by Unikraft:

Virtual Machines Containers Unikernels
Time performance Slowest of the 3 Fast Fast
Memory footprint Heavy Depends on the number of features Light
Security Very secure Least secure of the 3 Very secure
Features Everything you would think of Depends on the needs Only the absolute necessary

02. linuxu and KVM

Unikraft can be run in 2 ways:

  • As a virtual machine, using QEMU/KVM or Xen. It acts as an operating system, having the responsibility to configure the hardware components that it needs (clocks, additional processors, etc). This mode gives Unikraft direct and total control over hardware components, allowing advanced functionalities.
  • As a linuxu build, in which it behaves as a Linux user-space application. This severely limits its performance, as everything Unikraft does must go through the Linux kernel, via system calls. This mode should be used only for development and debugging.

When Unikraft is running using QEMU / KVM, it can either be run on an emulated system or a (para)virtualized one. Technically, KVM means virtualization support is enabled. If using QEMU in emulated mode, KVM is not used. To keep things simple, we will use interchangeably the terms QEMU, KVM or QEMU / KVM to refer to this use (either virtualized, or emulated).

Emulation is slower, but it allows using CPU architectures different from the local one (you can run ARM code on a x86 machine). Using (para)virtualisation, aka hardware acceleration, greater speed is achieved and more hardware components are visible to Unikraft.

03. Unikraft Core

The Unikraft core is comprised of several components:

  • the architecture code: This defines behaviours and hardware interactions specific to the target architecture (x86_64, ARM, RISC-V). For example, for the x86_64 architecture, this component defines the usable registers, data types sizes and how Thread-Local Storage should happen.
  • the platform code: This defines interaction with the underlying hardware, depending on whether a hypervisor is present or not, and which hypervisor is present. For example, if the KVM hypervisor is present, Unikraft will behave almost as if it runs bare-metal, needing to initialize the hardware components according to the manufacturer specifications. The difference from bare-metal is made only at the entry, where some information, like the memory layout, the available console, are supplied by the bootloader (Multiboot), and there’s no need to interact with the BIOS or UEFI. In the case of Xen, many of the hardware-related operations must be done through hypercalls, thus reducing the direct interaction of Unikraft with the hardware.
  • internal libraries: These define behaviour independent of the hardware, like scheduling, networking, memory allocation, basic file systems. These libraries are the same for every platform or architecture, and rely on the platform code and the architecture code to perform the needed actions. The internal libraries differ from the external ones in the implemented functionalities. The internal ones define parts of the kernel, while the external ones define user-space level functionalities. For example, uknetdev and lwip are 2 libraries that define networking components. Uknetdev is an internal library that interacts with the network card and defines how packages are sent using it. Lwip is an external library that defines networking protocols, like IP, TCP, UDP. This library knows that the packages are somehow sent over the NIC, but it is not concerned how. That is the job of the kernel.

04. libc in Unikraft

The Unikraft core provides only the bare minimum components to interact with the hardware and manage resources. A software layer, similar to the standard C library in a general-purpose OS, is required to make it easy to run applications on top of Unikraft.

Unikraft has multiple variants of a libc-like component:

  • nolibc is a minimalistic libc, part of the core Unikraft code, that contains only the functionality needed for the core (strings, qsort, etc).
  • isrlib is the interrupt-context safe variant of nolibc. It is used for interrupt handling code.
  • newlibc is the most complete libc currently available for Unikraft, but it still lacks some functionalities, like multithreading. Newlibc was designed for embedded environments.
  • musl is, theoretically, the best libc that will be used by Unikraft, but it’s currently in testing.

Nolibc and isrlib are part of the Unikraft core. Newlibc and musl are external libraries, from the point of view of Unikraft, and they must be included to the build, as shown in Session 01: Baby Steps.

05. Configuring Unikraft -

Unikraft is a configurable operating system, where each component can be modified / configured, according to the user’s needs. This configuration is done using a version of Kconfig (used in the Linux kernel), through the files. In these files, options are added to enable libraries, applications and different components of the Unikraft core. The user can then apply those configuration options, using make menuconfig, which generates an internal configuration file, .config, that can be understood by the build system. Once configured, the Unikraft image can be built, using make, and run, using the appropriate method (Linux ELF loader, qemu-kvm, xen, others).

Configuration can be done in a few ways:

  • Manually, using

    $ make menuconfig
  • Adding a dependency in for a component, so that the dependency gets automatically selected when the component is enabled. This is done using the depends on and select keywords in The configuration gets loaded and the .config file is generated by running

    $ make menuconfig

    This type of configuration removes some configuration steps, but not all of them.

In this session, we will use the first configuration options.

06. The Build System - basics

Once the application is configured, in .config, symbols are defined (e.g. CONFIG_ARCH_X86_64). Those symbols are usable both in the C code, to include certain functionalities only if they were selected in the configuring process, and in the actual building process, to include / exclude source files, or whole libraries. This last step is done in, where source code files are added to libraries. During the build process, all the files (from the Unikraft core and external libraries) are evaluated, and the selected files are compiled and linked, to form the Unikraft image.

unikraft build

The build process of Unikraft


  • Unikraft is a special type of operating system, that can be configured to match the needs of a specific application.
  • This configuration is made possible by a system based on Kconfig, that uses files to add possible configurations, and .config files to store the specific configuration for a build.
  • The configuration step creates symbols that are visible in both Makefiles and source code.
  • Each component has its own, where source files can be added, removed, or be made dependent on the configuration.
  • Unikraft has an internal libc, but it can use others, more complex and complete, like musl.
  • Being an operating system, it needs to be run by a hypervisor, like KVM or xen, to work at full capacity. It can also be run as an ELF, in Linux, but in this way the true power of Unikraft is not achieved.

Work Items

Support Files

Session support files are available in the repository. If you already cloned the repository, update it and enter the session directory:

$ cd path/to/repository/clone

$ git pull --rebase

$ cd content/en/community/hackathons/sessions/behind-scenes/

$ ls
demo  images  sol  work

If you haven’t cloned the repository yet, clone it and enter the session directory:

$ git clone

$ cd docs/

$ cd content/en/community/hackathons/sessions/behind-scenes/

$ ls
demo  images  sol  work

01. Tutorial / Reminder: Building and Running Unikraft

We want to build the helloworld application, using the Kconfig-based system, for the linuxu and KVM platforms, for the ARM and x86 architectures, and then run them.

02. Tutorial: Make It Speak

The goal of this exercise is to enable the internal debugging library for Unikraft (ukdebug) and make it display messages up to the info level. We also want to identify which hardware components are initialized and where.

03. More Messages

Sometimes we need a more detailed output. For this, ukdebug has the option to show debug level messages. Enable them and run Unikraft, for either ARM or x86 architectures, or both.

04. Going through the Code

Having the output of ukdebug, go through the Unikraft code, in the unikraft folder. Find the components that you have seen in the outputs, in the platform library, and where the kernel messages are sent. The platform library, even though is called a library, is not in the lib subfolder. It is placed in the plat folder. Explore the code, at your own pace. Can you also find where the main function is called?

05. I Have an Important Message

Send an important kernel message, that everyone needs to see, right before the main function is called. Try different message levels (critical, error, warning, info, debug), to see how they differ.

Note: sending a critical kernel message will not affect how Unikraft runs after the message.

06. Tutorial / Reminder: Adding Filesystems to an Application

For this tutorial, the aim is to create a simple QEMU / KVM application that reads from a file and displays the contents to standard output. A local directory is to be mounted as the root directory (/) inside the QEMU / KVM virtual machine.

We will use both the manual approach (make and qemu-system-x86_64 / qemu-guest) and kraft to configure, build and run the application.


The basic setup is in the work/06-adding-filesystems/ folder in the session directory. Enter that folder:

$ cd work/06-adding-filesystems/

$ ls -F
guest_fs/  kraft.yaml*  main.c  Makefile  qemu-guest*

The guest_fs/ local directory is to be mounted as the root directory (/) inside the QEMU / KVM virtual machine. It contains the grass file. The program (main.c) reads the contents of the /grass file and prints it to standard output. lists the main.c file as the application source file to be compiled and linked with Unikraft.

Makefile is used by the manual configuration and build system. kraft.yaml is used by kraft to configure, build and run the application. is a wrapper script around qemu-system-x86_64 used to manually run the application. Similarly, qemu-guest is a wrapper script used internally by kraft. We’ll use it as well to run the application.

Important: This setup belongs as an application folder in the apps/ folder in your working directory as discussed in the 1st tutorial of this session. Your best approach would be to copy this folder (work/06-adding-filesystems/) to the apps/ folder in your working directory. You will then get a hierarchy such as:

|-- apps/
|   |-- 06-adding-filesystems/
|   `-- helloworld/
|-- libs/
`-- unikraft/

If, at any point of this tutorial, something doesn’t work, or you want a quick check, see the reference solution in sol/06-adding-filesystems/ folder in the session directory.

Using the Manual Approach

Firstly, we will use the manual approach to configure, build and run the application.


For filesystem functionalities (opening, reading, writing files) we require a more powerful libc. Musl is already ported in Unikraft and will do nicely. For this, we update the LIBS line in the Makefile:

LIBS := $(UK_LIBS)/lib-musl

Update the UK_ROOT and UK_LIBS variables in the Makefile to point to the folders storing the Unikraft and libraries repositories.

Make sure that both unikraft and musl repositories are on the staging branch. Go to each of the two repository folders (unikraft and musl) and check the current branch:

$ git checkout

Now we need to enable 9pfs and musl in Unikraft. To do this, we run:

$ make menuconfig

We need to select the following options, from the Library Configuration menu:

  • libmusl
  • vfscore: VFS Core Interface
  • vfscore: VFS Configuration -> Automatically mount a root filesystem -> Default root filesystem -> 9pfs
    • For the Default root device option fill the fs0 string (instead of the default rootfs string).

These configurations will also mark as required 9pfs and uk9p in the menu.

We want to run Unikraft with QEMU / KVM, so we must select KVM guest in the Platform Configuration menu. For 9pfs we also need to enable, in the KVM guest options menu, Virtio -> Virtio PCI device support.

Save the configuration and exit.

Do a quick check of the configuration in .config by pitting it against the config.sol file in the reference solution:

$ diff -u .config ../../sol/06-adding-filesytstems/config.sol

Differences should be minimal, such as the application identifier.


Build the Unikraft image:


Building the Unikraft image will take a while. It has to pull musl source code, patch it and then build it, together with the Unikraft source code.

Run with qemu-system-x86_64

To run the Unikraft image with QEMU / KVM, we use the wrapper script, that calls qemu-system-x86_64 command with the proper arguments:

$ ./ ./build/unikraft-kraft-9pfs-issue_kvm-x86_64
o.   .o       _ _               __ _
Oo   Oo  ___ (_) | __ __  __ _ ' _) :_
oO   oO ' _ `| | |/ /  _)' _` | |_|  _)
oOo oOO| | | | |   (| | | (_) |  _) :_
 OoOoO ._, ._:_:_,\_._,  .__,_:_, \___)
                   Tethys 0.5.0~825b115
Hello, world!
File contents: The grass is green!
Bye, world!

A completely manual run would use the command:

$ qemu-system-x86_64 -fsdev local,id=myid,path=guest_fs,security_model=none -device virtio-9p-pci,fsdev=myid,mount_tag=fs0 -kernel build/06-adding-filesystems_kvm-x86_64 -nographic
Powered by
o.   .o       _ _               __ _
Oo   Oo  ___ (_) | __ __  __ _ ' _) :_
oO   oO ' _ `| | |/ /  _)' _` | |_|  _)
oOo oOO| | | | |   (| | | (_) |  _) :_
 OoOoO ._, ._:_:_,\_._,  .__,_:_, \___)
                   Tethys 0.5.0~825b115
Hello, world!
File contents: The grass is green!
Bye, world!

Lets break it down:

  • -fsdev local,id=myid,path=guest_fs,security_model=none - assign an id (myid) to the guest_fs/ local folder
  • -device virtio-9p-pci,fsdev=myid,mount_tag=fs0 - create a device with the 9pfs type, assign the myid for the -fsdev option and also assign the mount tag that we configured above (fs0) Unikraft will look after that mount tag when trying to mount the filesystem, so it is important that the mount tag from the configuration is the same as the one given as argument to qemu.
  • -kernel build/06-adding-filesystems_kvm-x86_64 - tells QEMU that it will run a kernel; if this parameter is omitted, QEMU will think it runs a raw file
  • -nographic - prints the output of QEMU to the standard output, it doesn’t open a graphical window

Run with qemu-guest

qemu-guest is the script used by kraft to run its QEMU / KVM images. Before looking at the command, take some time to look through the script, and maybe figure out the arguments needed for our task.

To run a QEMU / KVM application using qemu-guest, we use:

$ ./qemu-guest -e guest_fs/ -k build/06-adding-filesystems_kvm-x86_64

If we add the -D option, we can see the qemu-system command generated.

You may get the following error:

[    0.100664] CRIT: [libvfscore] <rootfs.c @  122> Failed to mount /: 22

If you do, check that the mount tag in the configuration is the same as the one used by qemu-guest. qemu-guest will use the tag fs0.

The fs0 tag is hardcoded for qemu-guest (and, thus, for kraft). This is why we used the fs0 tag when configuring the application with make menuconfig. Another tag could be used but then we couldn’t run the application with qemu-guest or kraft. It could only be run by manually using qemu-system-x86_64 with the corresponding arguments.

07. Tutorial: Give the User a Choice

The goal of this exercise is to modify, for the Helloworld app, so that the user can choose if the app will display Hello world, or what it reads from the file from the previous exercise.

First of all, we need to add a new configuration in We will do it like this:

	bool "Read my file"
	default n
	  Reads the file in guest_fs/ and prints its contents,
	  instead of printing helloworld

After this, we need to modify our code in main.c, to use this configuration option.

	printf("Hello world!\n");
	FILE *in = fopen("file", "r");
	char buffer[100];

	fread(buffer, 1, 100, in);
	printf("File contents: %s\n", buffer);

Note that, for our configuration option APPHELLOWORLD_READFILE, a symbol, CONFIG_APPHELLOWORLD_READFILE, was defined. We tell GCC that, if that symbol was not defined, it should use the printf("Hello world!\n"). Otherwise, it should use the code written by us.

The last step is to configure the application. We do this by running make menuconfig, then going to the Application Options and enabling our configuration option.

Now we can build and run the new Unikraft image.

08. Tutorial: Arguments from Command Line

We want to configure the helloworld app to receive command line arguments and then print them.

For this, the helloworld application already has a configuration option. Configure the application by running

$ make menuconfig

In the configuration menu, go to Application Options and enable Print arguments. If we build and run the image now, using qemu-guest, we will see that two arguments are passed to Unikraft: the kernel argument, and a console. We want to pass it an additional argument, "foo=bar".

Before this, make sure to reset your configuration, so Unikraft won’t use 9pfs for this task:

$ make clean

Raw qemu-system command

To send an argument with qemu-system, we use the -append option, like this:

$ qemu-system-x86_64 -kernel build/app-helloworld_kvm-x86_64 -append "console=ttyS0 foo=bar" -nographic

qemu-guest script

To send an argument with the qemu-guest script, we use the -a option, like this:

$ ./qemu-guest -k build/app-helloworld_kvm-x86_64 -a "foo=bar"


To send an argument while using kraft, run it like this:

$ kraft run "foo=bar"

09. Adding a new source file

Create a new source file for your application and implement a function that sorts a given integer array, by calling qsort(), in turn, from different libc variants, and then prints that array. For each library, check the size of the Unikraft image. Enable nolibc and then, as a separate config / build, newlibc, both by using make menuconfig and modifying kraft.yaml. You will have four different configurations and builds:

  • nolibc + kraft
  • nolibc + make
  • newlibc + kraft
  • newlibc + make

10. More Power to the User

Add the possibility to include the new source file only if a configuration option is selected. Make sure that after this change, the application can still be built and run.

11. Less Power to the User

Delete and reconfigure / rebuild the app. What happens when you run the app?

12. Give Us Feedback

We want to know how to make the next sessions better. For this we need your feedback.