---
# System prepended metadata

title: Release Notes - v0.20.0

---

:::info
To generate the feature list:

```bash
gh pr list -s closed -L 100 --search "is:pr is:closed closed:>2025-07-16" --json author,title,number --template '{{range .}}{{ printf "* %s (#%v) by @%s\n" .title .number .author.login }}{{end}}'
```

To generate the list of reviewers:

```bash
git --no-pager log --format="* %(trailers:key=Reviewed-by,valueonly,separator=%x2C )" --abbrev-commit HEAD...RELEASE-0.19.1 | sed 's/, /\n* /g' | sort -u
```
:::

:::danger
before submitting, please search the document for "TODO", "commit"
:::

[toc]

## GitHub Release Notes

We are proud to announce the latest version of Unikraft, `v0.20.0`!
This release comes with the much anticipated redesign and implementation of VFS, which brings enhanced performance, better maintainability, and improved compatibility with modern applications.
For more info refer to [the accompanying blog post](https://unikraft.org/blog/2025-09-08-unikraft-releases-v0.20.0).

> [!IMPORTANT]
> **DEPRECATION NOTICE:** The old VFS, vfscore, will be deprecated in Unikraft 0.22.0 (early 2026).
> For help on transitioning, consult the migration guide or join us on Discord.

> [!IMPORTANT]
> **COMPATIBLITY NOTICE:** The implementation of `arch_prctl` system call has been migrated from `app-elfloader` to `libposix-process`.
> Configurations using `CONFIG_APPELFLOADER_ARCH_PRCTL` should now use `CONFIG_LIBPOSIX_PROCESS_ARCH_PRCTL`.

A summary of the most important changes follows.
For the full list of changes see the [changelog](https://github.com/unikraft/unikraft/compare/RELEASE-0.19.1...RELEASE-0.20.0).

### 🚀 New Features

### Filesystem & Storage

- `libukfs`: New library for the filesystem interface, driver templates and utilities
- `libukfs-ramfs`: `ukfs` memory-resident volatile filesystem
- `libukfs-devfs`: `ukfs` pseudo-filesystem that provides files under /dev
- `libukfs-pseudo`: New library home to pseudo-files previously provided by `posix-tty`; also provides pseudo-files under devfs
- `posix-vfs`: New library for POSIX-compliant virtual filesystem layer on top of `ukfs`
- `posix-vfs-fstab`: New library for mounting filesystems at boot according to predefined tables (fstabs)
- `libukpod`: Utility library for managing pages-on-demand - used by filesystem drivers
- `libuksparsebuf`: Utility library for efficient management of sparse buffers - used by filesystem drivers
- `libukfile`: Add interface to expose & manage file backing memory

#### Context Management

- `lcpu`: Enhanced exception handling with context escape trampolines

#### System Call and Process

- `libposix-fdio`: Support the `sendfile()` syscall
- `libposix-process`: Signal the application on system error (`SIGSEGV`, `SIGILL` etc.)

#### Networking

- `virtio-net`, `liblwip`: Support Mergeable RX buffers (`VIRTIO_NET_F_MRG_RXBUF`)
- `virtio-net`, `liblwip`: Support Large Receive Offload (LRO)

### 🐛 Bug Fixes & Improvements

#### Memory Management

- `libukallocbuddy`: Refactor to support asymmetric `palloc()` / `pfree()` operations
- `ukallocpool`: Various improvements to enhance performance
- `libukvmem`: Fix VMA stack population logic
- `libukalloc`: Ensure sufficient memory allocation for region metadata
- `libukallocregion`: Various enhancements and improvements

#### Context Management

- `libuksched`: Remove unnecessary ECTX saving/restoration for cooperative thread switching

#### System Call and Process

- `libposix-process`: Refactor the kernel event API
- `libposix-process`: Fix exit code propagation to termination context

#### Architecture and Platform

- `lcpu`: Align exception handling stacks across architectures

#### Synchronization & Atomics

- `libuklock`: Optimize `ukmutex` and allow opt-out of atomics in the fast patch.
- `libuklock`: Introduce semaphore operation to consume all tokens at once

#### Network Stack

- `virtio-net`: Use single descriptor for packets
- `virtio-net`: Keep event suppression mechanism internal
- `virtio-net`: Split feature probing and negotiation

#### Development & Debugging

- `libsyscall-shim`: Enhance syscall tracing with pretty-print decoding for socket, filesystem, and process syscalls

---

## Twitter

Unikraft v0.20.0 is here! 🚀

Check out the blog post for details on the latest changes: https://unikraft.org/blog/2025-09-08-unikraft-releases-v0.20.0

---

## LinkedIn

Unikraft v0.20.0 is here! 🚀

It improves application support and core components, particularly the redesign of the filesystem implementation. 💯

Check out the blog post for details on the latest changes (https://unikraft.org/blog/2025-09-08-unikraft-releases-v0.20.0) and join us on GitHub (https://github.com/unikraft/) and on Discord (https://bit.ly/UnikraftDiscord).

---

## Blog Post

We are proud to announce the latest release of Unikraft, v0.20.0.

This release comes with the much anticipated redesign and implementation of VFS, which brings enhanced performance, better maintainability, and improved compatibility with modern applications.
It also contains plenty of fixes that improve the stability and application support of Unikraft.

In this blog post, we present the new features available in Unikraft.
For a full breakdown, please check out the [changelog](https://github.com/unikraft/unikraft/compare/RELEASE-0.19.1...RELEASE-0.20.0).

> [!IMPORTANT]
> **DEPRECATION NOTICE:** The old VFS, vfscore, will be deprecated in Unikraft 0.22.0 (early 2026).
> For help on transitioning, consult the migration guide or join us on Discord.

> [!IMPORTANT]
> **COMPATIBLITY NOTICE:** The implementation of `arch_prctl` system call has been migrated from `app-elfloader` to `libposix-process`.
> Configurations using `CONFIG_APPELFLOADER_ARCH_PRCTL` should now use `CONFIG_LIBPOSIX_PROCESS_ARCH_PRCTL`.

### New VFS

This release marks a milestone in the ongoing effort to replace `vfscore` and modernize the Unikraft file and filesystem stack with the introduction of `ukfs` -- the Unikraft filesystem interface -- as well as a new full-stack VFS implementation.

Building upon the `ukfile` interface introduced in 0.16, `ukfs` exists as a separation layer between filesystem drivers and higher level consumers.
Filesystem nodes exist as special instances of ukfiles, integrating seamlessly with existing filesystem-agnostic code, such as `posix-fd*`, while providing the `ukfs` API for interested consumers.

A key such consumer is `posix-vfs`, a core library providing the familiar *nix virtual filesystem (VFS) anchored at `/`, along with all related state, operations, and syscalls.

On the other end of the stack this release also introduces `ukfs-ramfs`, a memory-resident volatile filesystem, replacing legacy vfscore `ramfs`, along with `uksparsebuf` and `ukpod`, two internal utility libraries supporting file drivers.

Among the many performance and compatibility benefits of the new VFS stack and ramfs, a few notable ones are: sparse files, multiple hardlinks, bind mounts, and mounting on top of a non-empty directory (shadowing its previous contents until unmount).

As a consequence of the extensive rewrite done across the past four releases, the responsibilities of `vfscore` are now handled by dedicated libraries in the `ukfile`/`ukfs` stack:

- `ukfile` -- core abstraction for "a file"
- `ukfs` -- core abstraction for "a filesystem node"
- `posix-fd` -- open file descriptions, state held by open files
- `posix-fdio` -- POSIXey I/O & control operations on open files
- `posix-fdtab` -- file descriptors & their management
- `posix-vfs` -- *the* filesystem (VFS) anchored at root, everything to do with paths
- `posix-vfs-fstab` -- constructing the VFS at boot

All had their `README.md` files updated to better describe their design and interactions.
Consult these for a more in-depth overview of the new file(system) stack.

#### Migrating

All changes relating to the new VFS stack are opt-in, with existing configs that use vfscore continuing to work.

We encourage users to migrate their configurations as soon as practical, as vfscore is likely to be deprecated in the near future.
This can be easily done following these steps:

1. Replace all hard dependencies to `vfscore` with soft dependencies in your application’s [`Config.uk`](http://Config.uk) (`select LIBVFSCORE` → `imply LIBVFSCORE`)
   - This will maintain compatibility while allowing you to test migrating
   - Do this for all vfscore drivers as well (`LIB9PFS`, `LIBDEVFS`, `LIBRAMFS`)

   From interactive config prompt (e.g., `make menuconfig`):

1. Disable `vfscore` 

1. Enable `posix-vfs`; under its menu:
   - Enable "Provide filesystem syscalls" (`CONFIG_LIBPOSIX_VFS_SYSCALLS`)

1. Enable `posix-vfs-fstab`; under its menu:
   - Configure the builtin / user / fallback fstabs similar to vfscore
   - (OPTIONAL) If using devfs, enable it as a mount for `/dev`

**NOTE**: Filesystems other than ramfs & devfs are not yet supported.
Continue using the vfscore stack for now.

### Updates on Application Support

Application support has improved based on the updates of the Unikraft core.
And together with additions and improvements to the [`catalog`](https://github.com/unikraft/catalog) and [`catalog-core`](https://github.com/unikraft/catalog-core) repositories.

An important milestone is the now functional support of the [Java runtime](https://github.com/unikraft/catalog/tree/main/library/java/17).

### Community Activities

#### Unikraft Summer Workshop 2025

[Unikraft Summer Workshop 2025 (*USW'25*)](https://unikraft.org/hackathons/usw25), a **free and virtual** workshop held by members of the Unikraft community, took part for 3 weeks, between June 23 and July 4, 2025.
It consisted of 6 sessions filled with tutorials and workshops on how to configure, build, run and debug applications using Unikraft.

The final hackathon took place on Saturday, 12 July 2025, 9am-5pm CEST.
22 participants, split into 6 teams, contributed to the [`catalog-core`](https://github.com/unikraft/catalog-core) and [`catalog`](https://github.com/unikraft/catalog) repositories, and to various libraries and other components of the Unikraft ecosystem.

#### GSoC'25 Projects

GSoC'25 projects are almost done.
We are very happy with the results of our GSoC participants.
Check out their work products:

**[Prasoon Kumar](https://github.com/prasoon054/) from [Indian Institute of Technology Bombay](https://www.iitb.ac.in/) in Mumbai, India**

* Project: Expanding the Unikraft Software Support Ecosystem
* Mentors: [Răzvan Vîrtan](https://github.com/razvanvirtan), [Răzvan Deaconescu](https://github.com/razvand)
* [Work product](https://github.com/unikraft/gsoc/tree/staging/gsoc-2025/work-product/Prasoon-Kumar-Expanding-Software-Support-Ecosystem)

**[Abolfazl Soltani](https://github.com/SalarSamani) from [Vrije Universiteit Amsterdam](https://vu.nl/nl) in Amsterdam, The Netherlands**

* Project: Update Newlib and Pthread-embedded Libraries
* Mentors: [Ștefan Jumărea](https://github.com/StefanJum), [Cezar Crăciunoiu](https://github.com/craciunoiuc)
* [Work product](https://github.com/unikraft/gsoc/blob/staging/gsoc-2025/work-product/Salar-Samani-Newlib-Pthread-Unikraft.md)

**[Shashank Srivastava](https://github.com/shank250) from [GL Bajaj Institute of Technology and Management](https://www.glbimr.ac.in/) in Greater Noida, India**

* Project: Testing Framework for Unikraft Builds
* Mentors: [Răzvan Deaconescu](https://github.com/razvand), [Răzvan Vîrtan](https://github.com/razvanvirtan)
* Work product under way

Thanks so much for your involvement!
And keep contributing! 🤝
