01. 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.

02. 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.

03. Configuring Unikraft - Config.uk

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 Config.uk 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 Config.uk 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 Config.uk. 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.

04. 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 Makefile.uk, where source code files are added to libraries. During the build process, all the Makefile.uk 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 Config.uk 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 Makefile.uk, 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  index.md  sol  work

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

$ git clone https://github.com/unikraft/docs.git

$ cd docs/

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

$ ls
demo  images  index.md  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. 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  launch.sh*  main.c  Makefile  Makefile.uk  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. Makefile.uk 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.

launch.sh 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 launch.sh script, that calls qemu-system-x86_64 command with the proper arguments:

$ ./launch.sh ./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.