https://github.com/arighi/virtme-ng/assets/423281/485608ee-0c82-46d1-b311-e1b7af0a4e44

What is virtme-ng?

virtme-ng is a tool that allows to easily and quickly recompile and test a Linux kernel, starting from the source code.

It allows recompiling the kernel in few minutes (rather than hours), then the kernel is automatically started in a virtualized environment that is an exact copy-on-write copy of your live system, which means that any changes made to the virtualized environment do not affect the host system.

In order to do this a minimal config is produced (with the bare minimum support to test the kernel inside qemu), then the selected kernel is automatically built and started inside qemu, using the filesystem of the host as a copy-on-write snapshot.

This means that you can safely destroy the entire filesystem, crash the kernel, etc. without affecting the host.

Kernels produced with virtme-ng are lacking lots of features, in order to reduce the build time to the minimum and still provide you a usable kernel capable of running your tests and experiments.

virtme-ng is based on virtme, written by Andy Lutomirski [email protected] (web | git).

Quick start

$ uname -r
5.19.0-23-generic
$ git clone git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
$ cd linux
$ vng --build --commit v6.2-rc4
...
$ vng
          _      _
   __   _(_)_ __| |_ _ __ ___   ___       _ __   __ _
   \ \ / / |  __| __|  _   _ \ / _ \_____|  _ \ / _  |
    \ V /| | |  | |_| | | | | |  __/_____| | | | (_| |
     \_/ |_|_|   \__|_| |_| |_|\___|     |_| |_|\__  |
                                                |___/
   kernel version: 6.2.0-rc4-virtme x86_64

$ uname -r
6.2.0-rc4-virtme
^
|___ Now you have a shell inside a virtualized copy of your entire system,
     that is running the new kernel! \o/

Then simply type "exit" to return back to the real system.

Installation

• virtme-ng is packaged in most major distributions:

  

  Note: it might not be the latest stable version containing new features and bug-fixes. Do not hesitate to help with the packaging!

• The latest stable version is also published on pypi:

  $ pip install virtme-ng
  

  You will need to install the dependences manually, see the Requirements section.

• Install from source

  To install virtme-ng from source, you can clone this git repository and build a standalone virtme-ng running the following commands:

  $ git clone https://github.com/arighi/virtme-ng.git
  $ cd virtme-ng
  $ BUILD_VIRTME_NG_INIT=1 pip3 install .
  

  There are some extra dependences on top of the ones mentioned in the Requirements section. If you are on Debian/Ubuntu, you may need to install the following packages to build virtme-ng from source properly:

  $ sudo apt install python3-pip flake8 pylint cargo rustc qemu-system-x86
  

  If you'd prefer to use uv:

  $ BUILD_VIRTME_NG_INIT=1 uv tool install .
  

• Run from source

  You can also run virtme-ng directly from source, make sure you have all the requirements installed (optionally you can build virtme-ng-init for a faster boot, by running make), then from the source directory simply run any virtme-ng command, such as:

  $ ./vng --help
  

Requirements

• You need Python 3.8 or higher

• QEMU 1.6 or higher is recommended (QEMU 1.4 and 1.5 are partially supported using a rather ugly kludge)

  • You will have a much better experience if KVM is enabled. That means that you should be on bare metal with hardware virtualization (VT-x or SVM) enabled or in a VM that supports nested virtualization. On some Linux distributions, you may need to be a member of the "kvm" group. Using VirtualBox or most VPS providers will fall back to emulation. If you are using GitHub Actions, KVM support is supported on "larger Linux runners" -- which is now the default runner -- but it has to be manually enabled, see how it is used in our tests or here with Docker.

• Depending on the options you use, you may need a statically linked busybox binary somewhere in your path.

• Optionally, you may need virtiofsd 1.7.0 (or higher) for better filesystem performance inside the virtme-ng guests.

• Optionally, you may need socat for the --console and --console-client options, and the host's kernel should support VSOCK (CONFIG_VHOST_VSOCK).

• Optionally, you may need sshd installed for the --ssh and --ssh-client options.

• Optionally, if the shell completion is not available (e.g. when installed from pip or from source), you can install shtab and run:

  # Bash
  $ mkdir -p ~/.local/share/bash-completion/completions/
  $ shtab --shell=bash -u virtme_ng.run.make_parser > ~/.local/share/bash-completion/completions/vng
  
  # ZSH
  $ shtab --shell=zsh  -u virtme_ng.run.make_parser | sudo tee /usr/local/share/zsh/site-functions/_vng >/dev/null
  

Configuration

• You may customize the default configuration by providing one of the following, by order of preference: $HOME/.config/virtme-ng/virtme-ng.conf, $HOME/.virtme-ng.conf or /etc/virtme-ng.conf. As a fallback for any missing values, the default ones will be used.

• The format of the file is JSON. Default values:

  {
      "default_opts": {},
      "clean_exclude": [],
      "systemd": {
          "masks": ["getty@"],
          "fstab": "/dev/null"
      }
  }
  

• clean_exclude is a list of patterns to exclude from vng --clean. Each pattern is passed as a -e <pattern> argument to git clean. For example, to preserve a .semcode.db file when running vng --clean:

  {
      "clean_exclude": [".semcode.db"]
  }
  

Examples

• Build a kernel from a clean local kernel source directory (if a .config is not available virtme-ng will automatically create a minimum .config with all the required feature to boot the instance):

  $ vng -b
  

• Build tag v6.1-rc3 from a local kernel git repository:

  $ vng -b -c v6.1-rc3
  

• Generate a minimal kernel .config in the current kernel build directory:

  $ vng --kconfig
  

• Print to stdout the kernel config flags set by virtme-ng:

  $ vng --kconfig --dry-run
  

• Run a kernel previously compiled from a local git repository in the current working directory:

  $ vng
  

• Use kexec inside an x86 virtme-ng guest to reboot into the same kernel. Start the guest with an initramfs and retain it after boot:

  $ vng --force-initramfs --append retain_initrd
  

  Then, inside the guest, load and execute the kernel using the retained initramfs:

  $ sudo kexec --load arch/x86/boot/bzImage \
      --initrd=/sys/firmware/initrd --reuse-cmdline
  $ sudo kexec --exec
  

  The retained initramfs must be compatible with the kernel being loaded. This is the case here because vng boots arch/x86/boot/bzImage by default. This requires CONFIG_KEXEC=y and CONFIG_BLK_DEV_INITRD=y, which are both enabled in the default virtme-ng kernel configuration.

• Run an interactive virtme-ng session using the same kernel as the host:

  $ vng -r
  

• Test installed kernel 6.2.0-21-generic kernel (NOTE: /boot/vmlinuz-6.2.0-21-generic needs to be accessible):

  $ vng -r 6.2.0-21-generic
  

• Run a pre-compiled vanilla v6.6 kernel fetched from the Ubuntu mainline builds repository (useful to test a specific kernel version directly and save a lot of build time):

  $ vng -r v6.6
  

• Download and test kernel 6.2.0-1003-lowlatency from deb packages:

  $ mkdir test
  $ cd test
  $ apt download linux-image-6.2.0-1003-lowlatency linux-modules-6.2.0-1003-lowlatency
  $ for d in *.deb; do dpkg -x $d .; done
  $ vng -r ./boot/vmlinuz-6.2.0-1003-lowlatency
  

• Build the tip of the latest kernel on a remote build host called "builder", running make inside a specific build chroot (managed remotely by schroot):

  $ vng --build --build-host builder \
    --build-host-exec-prefix "schroot -c chroot:kinetic-amd64 -- "
  

• Run the previously compiled kernel from the current working directory and enable networking:

  $ vng --net user
  

• Run the previously compiled kernel adding an additional virtio-blk device:

  $ qemu-img create -f qcow2 /tmp/disk.img 8G
  $ vng --disk /tmp/disk.img
  

• Use a disk VM built using Nix as a package manager (example here) and containing tools, e.g. to run the tests in a more reproducible way:

  # mkdir -p /nix/store
  $ vng -r --disk nix-store.erofs.disk
  > sudo mount /dev/vda /nix/store
  > PATH="/nix/store/tools/bin:${PATH}"
  > hello
  Hello, world!
  

• Recompile the kernel passing some env variables to enable Rust support (using specific versions of the Rust toolchain binaries):

  $ vng --build RUSTC=rustc-1.62 BINDGEN=bindgen-0.56 RUSTFMT=rustfmt-1.62
  

• Build the arm64 kernel (using a separate chroot in /opt/chroot/arm64 as the main filesystem):

  $ vng --build --arch arm64 --root /opt/chroot/arm64/
  

• Build the kernel using a separate build directory, and run it, in verbose:

  $ export KBUILD_OUTPUT=.virtme/build
  $ vng --build --verbose
  $ vng --verbose
  

• Same example, but using O=:

  $ vng --build --verbose -- O=.virtme/build
  $ vng --verbose -- O=.virtme/build
  

• Accelerate the kernel rebuilds using CCache (if installed):

  $ PATH="/usr/lib/ccache:${PATH}" \
      KBUILD_BUILD_TIMESTAMP=0 \
      vng --build
  # or export the two variables before, see 'man ccache' for more details
  

• Execute uname -r inside a kernel recompiled in the current directory and send the output to cowsay on the host:

  $ vng -- uname -r | cowsay
   __________________
  < 6.1.0-rc6-virtme >
   ------------------
          \   ^__^
           \  (oo)\_______
              (__)\       )\/\
                  ||----w |
                  ||     ||
  

• Run a bunch of parallel virtme-ng instances in a pipeline, with different kernels installed in the system, passing each other their stdout/stdin and return all the generated output back to the host (also measure the total elapsed time):

  $ time true | \
  > vng -r 5.19.0-38-generic -e "cat && uname -r" | \
  > vng -r 6.2.0-19-generic  -e "cat && uname -r" | \
  > vng -r 6.2.0-20-generic  -e "cat && uname -r" | \
  > vng -r 6.3.0-2-generic   -e "cat && uname -r" | \
  > cowsay -n
   ___________________
  / 5.19.0-38-generic \
  | 6.2.0-19-generic  |
  | 6.2.0-20-generic  |
  \ 6.3.0-2-generic   /
   -------------------
          \   ^__^
           \  (oo)\_______
              (__)\       )\/\
                  ||----w |
                  ||     ||
  
  real    0m2.737s
  user    0m8.425s
  sys     0m8.806s
  

• Run the vanilla v6.7-rc5 kernel with an Ubuntu 22.04 rootfs:

  $ vng -r v6.7-rc5 --user root --root ./rootfs/22.04 --root-release jammy -- cat /etc/lsb-release /proc/version
  ...
  DISTRIB_ID=Ubuntu
  DISTRIB_RELEASE=22.04
  DISTRIB_CODENAME=jammy
  DISTRIB_DESCRIPTION="Ubuntu 22.04.3 LTS"
  Linux version 6.7.0-060700rc5-generic (kernel@kathleen) (x86_64-linux-gnu-gcc-13 (Ubuntu 13.2.0-7ubuntu1) 13.2.0, GNU ld (GNU Binutils for Ubuntu) 2.41) #202312102332 SMP PREEMPT_DYNAMIC Sun Dec 10 23:41:31 UTC 2023
  

• Run with systemd as init:

  $ sudo vng -r --systemd --exec "systemctl status | head"
  ● virtme-ng
     State: starting
     Units: 392 loaded (incl. loaded aliases)
     Jobs: 4 queued
     Failed: 3 units
     Since: Mon 2025-05-26 11:00:47 -03; 4s ago
  systemd: 257.5+suse.8.gc10a66fb4d
  Tainted: unmerged-bin
     CGroup: /
           ├─init.scope
  

• Run with systemd as init in an external rootfs:

  $ vng -r --systemd --user root --root ./rootfs/sid --exec "systemctl status | head"
  ● virtme-ng
     State: degraded
     Units: 273 loaded (incl. loaded aliases)
     Jobs: 0 queued
     Failed: 4 units
     Since: Mon 2025-05-26 14:01:06 UTC; 2s ago
  systemd: 257.5-2
  Tainted: unmerged-bin
     CGroup: /
           ├─init.scope
  

• Run a kernel against a disk image instead of a host directory:

  $ vng --run ./bzImage --root-disk ./tumbleweed.raw -- findmnt /
  TARGET SOURCE   FSTYPE OPTIONS
  /      /dev/vda ext4  ro,relatime
  

  --root-disk attaches the image to the guest over virtio-blk and lets the kernel mount it as the root filesystem, the same way it would on real hardware, instead of exporting a host directory over virtiofs or 9p. This gives the guest a real block-backed root, which is what you want when the code under test cares about the block layer, when the filesystem semantics of virtiofs and 9p get in the way (mmap, hardlinks, file locking), or when testing a foreign architecture, where virtiofs is often unavailable.

  The kernel under test still comes from the host, and so do its modules and the virtme-ng guest tools: they are exported read-only on mounts of their own, so the image never needs to know anything about the kernel it boots.

  The image is attached read-only unless --rw is given.

  The kernel (or the initramfs) has to be able to reach the disk, so it needs CONFIG_VIRTIO_BLK and the filesystem of the image enabled (either builtin or as a module), along with the virtio transports virtme-ng exports the guest tools over. Filesystems are not part of the generic virtme-ng kernel configuration, so pick the one matching your image:

  $ vng --kconfig --configitem CONFIG_EXT4_FS=y
  $ vng --build
  

  If the filesystem is only available as a module, pass --root-fstype so the initramfs loads it instead of requiring it built in:

  $ vng --run ./bzImage --root-disk ./tumbleweed.raw --root-fstype ext4
  

• Boot a partitioned disk image, such as a cloud image, by naming the partition holding the root filesystem:

  $ vng --run ./bzImage --root-disk ./cloud.qcow2 --root-dev /dev/vda3
  

  --root-disk always attaches the image first, so it is reliably /dev/vda and its partitions /dev/vda1, /dev/vda2, and so on, regardless of what else is attached to the guest.

• Boot a foreign riscv64 Debian kernel using a Debian sid image as root filesystem:

  $ wget https://cloud.debian.org/images/cloud/sid/daily/latest/debian-sid-nocloud-riscv64-daily.qcow2
  $ wget http://security.debian.org/debian-security/pool/updates/main/l/linux/linux-image-6.12.107+deb13-riscv64_6.12.107-1_riscv64.deb
  $ dpkg -x linux-image-6.12.107+deb13-riscv64_6.12.107-1_riscv64.deb debian
  $ vng --arch riscv64 \
  >     --run ./debian/boot/vmlinux-6.12.107+deb13-riscv64 \
  >     --root-disk ./debian-sid-nocloud-riscv64-daily.qcow2 \
  >     --root-dev /dev/vda1 \
  >     --root-fstype ext4 \
  >     --busybox /path/to/static/riscv64-busybox
  

• Mirror the available host CPU socket/core/thread layout in the guest and pin each guest vCPU to the corresponding host CPU:

  $ vng -r --cpus host
  

  The CPUs available to vng are determined by its CPU affinity, so this also works inside a cpuset. QEMU can only describe uniform topologies: if the available CPUs have unequal numbers of cores per socket or threads per core (e.g., hybrid CPUs with SMT performance cores and non-SMT efficiency cores), vng picks a uniform layout that keeps SMT siblings together, using as many host CPUs as possible (leaving out cores with fewer threads or merging sockets if needed), and prints a warning. This option does not copy host NUMA memory placement or cache topology into the guest.

• Run the current kernel creating a 1GB NUMA node with CPUs 0,1,3 assigned and a 3GB NUMA node with CPUs 2,4,5,6,7 assigned:

  $ vng -r -m 4G --numa 1G,cpus=0-1,cpus=3 --numa 3G,cpus=2,cpus=4-7 -- numactl -H
  available: 2 nodes (0-1)
  node 0 cpus: 0 1 3
  node 0 size: 1005 MB
  node 0 free: 914 MB
  node 1 cpus: 2 4 5 6 7
  node 1 size: 2916 MB
  node 1 free: 2797 MB
  node distances:
  node   0   1
    0:  10  20
    1:  20  10
  

• Run the current kernel creating 4 NUMA nodes of 1GB each and assign different distance costs between the NUMA nodes to simulate non-uniform memory access:

  $ vng -r --cpu 8 -m 4G \
  >     --numa 1G,cpus=0-1 --numa 1G,cpus=2-3 \
  >     --numa 1G,cpus=4-5 --numa 1G,cpus=6-7 \
  >     --numa-distance 0,1=51 --numa-distance 0,2=31 --numa-distance 0,3=41 \
  >     --numa-distance 1,2=21 --numa-distance 1,3=61 \
  >     --numa-distance 2,3=11 -- numactl -H
  available: 4 nodes (0-3)
  node 0 cpus: 0 1
  node 0 size: 1006 MB
  node 0 free: 974 MB
  node 1 cpus: 2 3
  node 1 size: 953 MB
  node 1 free: 919 MB
  node 2 cpus: 4 5
  node 2 size: 943 MB
  node 2 free: 894 MB
  node 3 cpus: 6 7
  node 3 size: 1006 MB
  node 3 free: 965 MB
  node distances:
  node     0    1    2    3
     0:   10   51   31   41
     1:   51   10   21   61
     2:   31   21   10   11
     3:   41   61   11   10
  

• Run glxgears inside a kernel recompiled in the current directory:

  $ vng -g -- glxgears
  
  (virtme-ng is started in graphical mode)
  

• Execute an awesome window manager session with kernel 6.2.0-1003-lowlatency (installed in the system):

  $ vng -r 6.2.0-1003-lowlatency -g -- awesome
  
  (virtme-ng is started in graphical mode)
  

• Run the steam snap (tested in Ubuntu) inside a virtme-ng instance using the 6.2.0-1003-lowlatency kernel:

  $ vng -r 6.2.0-1003-lowlatency --snaps --net user -g -- /snap/bin/steam
  
  (virtme-ng is started in graphical mode)
  

• Generate a memory dump of a running instance and read 'jiffies' from the memory dump using the drgn debugger:

  # Start the vng instance in debug mode
  $ vng --debug
  
  # In a separate shell session trigger the memory dump to /tmp/vmcore.img
  $ vng --dump /tmp/vmcore.img
  
  # Use drgn to read 'jiffies' from the memory dump:
  $ echo "print(prog['jiffies'])" | drgn -q -s vmlinux -c /tmp/vmcore.img
  drgn 0.0.23 (using Python 3.11.6, elfutils 0.189, with libkdumpfile)
  For help, type help(drgn).
  >>> import drgn
  >>> from drgn import NULL, Object, cast, container_of, execscript, offsetof, reinterpret, sizeof
  >>> from drgn.helpers.common import *
  >>> from drgn.helpers.linux import *
  >>> (volatile unsigned long)4294675464
  

• Attach a GDB session to a running instance started with --debug:

  # Start the vng instance in debug mode
  $ vng --debug
  
  # In a separate terminal run the following command to attach the gdb session:
  $ vng --gdb
  kernel version = 6.9.0-virtme
  Reading symbols from vmlinux...
  Remote debugging using localhost:1234
  native_irq_disable () at ./arch/x86/include/asm/irqflags.h:37
  37		asm volatile("cli": : :"memory");
  (gdb)
  
  # NOTE: a vmlinux must be present in the current working directory in order
  # to resolve symbols, otherwise vng # will automatically search for a
  # vmlinux available in the system.
  

• Connect to a simple remote shell (socat is required, VSOCK will be used):

  # Start the vng instance with server support:
  $ vng --console
  
  # In a separate terminal run the following command to connect to a remote shell:
  $ vng --console-client
  

• Enable SSH in the vng guest:

  # Start the vng instance with ssh server support:
  $ vng --ssh
  
  # Connect to the vng guest from the host via ssh:
  $ vng --ssh-client
  

• Generate some results inside the vng guest and copy them back to the host using SCP:

  # Start the vng instance with SSH server support:
  arighi@host~> vng --ssh
  ...
  arighi@virtme-ng~> ./run.sh > result.txt
  
  # In another terminal, copy result.txt from the guest to the host using scp:
  arighi@host~> scp -F ~/.cache/virtme-ng/.ssh/virtme-ng-ssh.conf virtme-ng%2222:~/result.txt .
  
  # The SSH command can be printed using this command, and easily adapted later:
  arighi@host~> vng --ssh-client --dry-run
  ssh -F /home/arighi/.cache/virtme-ng/.ssh/virtme-ng-ssh.conf virtme-ng%2222
  
  # With systemd >= 256, it is possible to use the 'vsock/<CID>' hostname directly:
  arighi@host~> ssh vsock/2222
  arighi@virtme-ng~>
  

• Run virtme-ng inside a docker container:

  $ docker run -it --privileged ubuntu:latest /bin/bash
  # apt update
  # echo 'debconf debconf/frontend select Noninteractive' | debconf-set-selections
  # apt install --yes git cargo qemu-kvm udev iproute2 busybox-static \
    coreutils python3-requests python3-argcomplete libvirt-clients kbd kmod file rsync zstd virtiofsd
  # git clone https://github.com/arighi/virtme-ng.git
  # cd virtme-ng
  # make
  # ./vng -r v6.6 -- uname -r
  6.6.0-060600-generic
  

  See also: .github/workflows/run.yml as a practical example on how to use virtme-ng inside docker.

• Run virtme-ng with GPU passthrough:

  # Confirm host kernel has VFIO and IOMMU support
  # Check if NVIDIA module is installed on the host
  $ modinfo nvidia
  # If the nvidia module is installed, blacklist the nvidia modules
  $ sudo bash -c 'echo -e "blacklist nvidia\nblacklist nvidia-drm\nblacklist nvidia-modeset\nblacklist nvidia-peermem\nblacklist nvidia-uvm" > /etc/modprobe.d/blacklist-nvidia.conf'
  # Host will need to be rebooted for blacklist to take effect.
  # Get GPU device ID
  $ lspci -nn | grep NVIDIA
    0000:01:00.0 VGA compatible controller [0300]: NVIDIA Corporation AD104GLM [RTX 3500 Ada Generation Laptop GPU] [10de:27bb] (rev a1)
    0000:01:00.1 Audio device [0403]: NVIDIA Corporation Device [10de:22bc] (rev a1))
  # Configure VFIO for device passthrough
  $ sudo bash -c 'options vfio-pci ids=10de:27bb,10de:22bc' > /etc/modprobe.d/vfio.conf
  # Load VFIO module
  $ sudo modprobe vfio-pci
  # Pass PCI address to virtme-ng
  $ sudo vng --nvgpu "01:00.0" -r linux
  

Implementation details

virtme-ng allows to automatically configure, build and run kernels using the main command-line interface called vng.

A minimal custom .config is automatically generated if not already present when --build is specified.

It is possible to specify a set of custom configs (.config chunk) in ~/.config/virtme-ng/kernel.config, or using --config chunk-file's or --configitem CONFIG_FOO=bar's. These user-specific settings will successively override the default settings. The final overrides are the mandatory config items that are required to boot and test the kernel inside QEmu, using virtme-run.

Then the kernel is compiled either locally or on an external build host (if the --build-host option is used); once the build is done only the required files needed to test the kernel are copied from the remote host if an external build host is used.

When a remote build host is used (--build-host) the target branch is force pushed to the remote host inside the ~/.virtme directory.

Then the kernel is executed using the virtme module. This allows to test the kernel using a safe copy-on-write snapshot of the entire host filesystem.

All the kernels compiled with virtme-ng have a -virtme suffix to their kernel version, this allows to easily determine if you're inside a virtme-ng kernel or if you're using the real host kernel (simply by checking uname -r).

External kernel modules

It is possible to recompile and test out-of-tree kernel modules inside the virtme-ng kernel, simply by building them against the local directory of the kernel git repository that was used to build and run the kernel.

Default options

Typically, if you always use virtme-ng with an external build server (e.g., vng --build --build-host REMOTE_SERVER --build-host-exec-prefix CMD) you don't always want to specify these options, so instead, you can simply define them in your configuration file (refer to the Configuration section) under default_opts and then simply run vng --build.

Example (always use an external build server called kathleen and run make inside a build chroot called chroot:lunar-amd64). To do so, add the default_opts section in your configuration file as following:

{
    "default_opts": {
        "build_host": "kathleen",
        "build_host_exec_prefix": "schroot -c chroot:lunar-amd64 --"
    }
}

Now you can simply run vng --build to build your kernel from the current working directory using the external build host, prepending the exec prefix command when running make.

AI Agent Integration (MCP Server)

virtme-ng includes built-in support for the Model Context Protocol (MCP), which allows AI assistants (like Claude, Cursor, or other MCP-compatible tools) to interact with virtme-ng programmatically.

What is MCP?

The Model Context Protocol is an open protocol that enables AI assistants to securely access tools and data sources. The virtme-ng MCP server exposes kernel development operations as tools that AI agents can use.

Starting the MCP Server

The MCP server can be started in two ways:

1. Using the vng-mcp binary directly:

   $ vng-mcp
   

2. Using the vng command:

   $ vng --mcp
   

Both methods are equivalent - vng --mcp simply executes vng-mcp internally. The server will start in stdio mode and communicate using the MCP protocol.

Configuration with AI Assistants

Add the following to your AI assistant configuration file:

• Cursor: ~/.config/cursor/mcp.json
• Claude Code (CLI): ~/.config/claude/config.json
• Claude Desktop: ~/.config/Claude/claude_desktop_config.json

{
  "mcpServers": {
    "virtme-ng": {
      "command": "vng",
      "args": ["--mcp"]
    }
  }
}

Example Workflows

Once configured, you can ask your AI assistant:

• "Build this kernel and check dmesg for errors/warnings"
• "Build the kernel in /path/to/linux then test it by running 'uname -r'"
• "Recompile and test the kernel with KASAN enabled"
• "Test this kernel by running my test script at /path/to/test.sh"
• "Compare the behavior of this kernel with the mainline kernel v6.12"
• "Validate that each commit in this patch series builds and boots correctly"
• "Apply the patch from lore.kernel.org with message ID xyz and test it"
• "Verify commit HEAD~3 builds and boots correctly"
• "Validate commits with KASAN enabled using the build server 'builder'"

The AI will use the appropriate MCP tools to accomplish these tasks.

Troubleshooting

• If you experience poor performance, check the Requirements section, maybe some optional tools or services are not available, e.g. KVM support and virtiofsd. Please note that some Linux distributions are currently not shipping virtme-ng-init tool, which runs faster than the Bash init script, used as fallback.

• The guest helper scripts use a #!/usr/bin/env bash shebang, so the guest root filesystem needs /usr/bin/env. This is present on virtually every distribution, including minimal ones such as Alpine, but may be missing from a hand-built --root chroot.

• Some guest kernels may fail to mount an external virtiofs root filesystem when POSIX ACL support is enabled. This can show up as the root filesystem mount apparently succeeding, followed by switch_root failing with Connection refused and the guest panicking. In this case, try disabling ACLs on the root virtiofs export with --no-root-posix-acl.

• If you get permission denied when starting qemu, make sure that your username is assigned to the group kvm or libvirt:

  $ groups | grep "kvm\|libvirt"
  

• When using --network bridge to create a bridged network in the guest you may get the following error:

  ...
  failed to create tun device: Operation not permitted
  

  This is because qemu-bridge-helper requires CAP_NET_ADMIN permissions.

  To fix this you need to add allow all to /etc/qemu/bridge.conf and set the CAP_NET_ADMIN capability to qemu-bridge-helper, as following:

  $ sudo filecap /usr/lib/qemu/qemu-bridge-helper net_admin
  

• If the guest fails to start because the host doesn't have enough memory available you can specify a different amount of memory using --memory MB, (this option is passed directly to qemu via -m, default is 1G).

• If you're testing a kernel for an architecture different from the host, keep in mind that you need to use also --root DIR to use a specific chroot with the binaries compatible with the architecture that you're testing, or --root-disk IMAGE to boot a disk image for that architecture.

  If the chroot doesn't exist in your system virtme-ng will automatically create it using the latest daily build Ubuntu cloud image:

  $ vng --build --arch riscv64 --root ./tmproot
  

• If the build on a remote build host is failing unexpectedly you may want to try cleaning up the remote git repository, running:

  $ vng --clean --build-host HOSTNAME
  

• Snap support is still experimental and something may not work as expected (keep in mind that, by default, virtme-ng will try to run snapd in a bare minimum system environment without systemd), if some snaps are not running try to disable apparmor, adding --append="apparmor=0" to the virtme-ng command line.

• Systemd support (--systemd) is still experimental. If something does not work for you, try masking the unit that is freezing, e.g. --append "systemd.mask=$PROBLEMATIC_UNIT" (refer to the Configuration section for a more permanent setup). Be aware that you might also need --user root, or if you're using your own / as ROOTFS, you may need to run vng itself as root.

• Running virtme-ng instances inside docker: in case of failures/issues, especially with stdin/stdout/stderr redirections, make sure that you have udev installed in your docker image and run the following command before using vng:

  $ udevadm trigger --subsystem-match --action=change
  

• To mount the legacy CGroup filesystem (v1) layout, add SYSTEMD_CGROUP_ENABLE_LEGACY_FORCE=1 to the kernel boot options:

  $ vng -r --append "SYSTEMD_CGROUP_ENABLE_LEGACY_FORCE=1" -- 'df -T /sys/fs/cgroup/*'
  Filesystem     Type   1K-blocks  Used Available Use% Mounted on
  blkio          cgroup         0     0         0    - /sys/fs/cgroup/blkio
  cpu            cgroup         0     0         0    - /sys/fs/cgroup/cpu
  cpuacct        cgroup         0     0         0    - /sys/fs/cgroup/cpuacct
  devices        cgroup         0     0         0    - /sys/fs/cgroup/devices
  memory         cgroup         0     0         0    - /sys/fs/cgroup/memory
  pids           cgroup         0     0         0    - /sys/fs/cgroup/pids
  

• If you experience stale file handle errors (ESTALE) in the guest, this may be due to an inode revalidation issue with VirtioFS.

  Applying #340 prevents these errors, but host-side file changes may not be immediately visible in the guest. If you observe out-of-sync filesystem issues, you can simply force a resync by invalidating the page cache in the guest:

  # echo 3 > /proc/sys/vm/drop_caches
  

• Running virtme-ng using the same kernel as the host might fail:

  $ vng -r
  unable to access /boot/vmlinuz-(...) (check for read permissions)
  

  Read access to the built kernel is required:

  $ sudo chmod +r /boot/vmlinuz-*
  

• A kernel panic can be seen on custom built kernels, e.g.

  $ vng -vr (...)/arch/x86_64/boot/bzImage  ## use -v to show all the errors
  VFS: Cannot open root device "ROOTFS" or unknown-block(0,0): error -19
  Please append a correct "root=" boot option; here are the available partitions:
  (...)
  Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(0,0)
  

  That's probably because some kernel config's are missing: the kernel should be built with vng --build or virtme-configkernel --update should be used.

• Using virtme-ng with the shell init script on Ubuntu using Rust CoreUtils (UUtils) might fail:

  $ vng -r --no-virtme-ng-init -v
  (...)
  Security violation: Requested utility `env` does not match executable name:
    /usr/libexec/virtiofsd
  [ 1.344090] Kernel panic - not syncing: Attempted to kill init! exitcode=0x00000100
  [ 1.344221] CPU: 0 UID: 0 PID: 1 Comm: virtme-init Not tainted 7.0.0-22-generic #22-Ubuntu PREEMPT(lazy)
  

  That's because of a security patch only applied in Ubuntu, see this bug report. A workaround:

  $ sudo ln -sf gnuenv /usr/bin/env
  ## revert with: sudo ln -sf ../lib/cargo/bin/coreutils/env /usr/bin/env
  

Contributing

Please see DCO-1.1.txt.

virtme-ng uses pre-commit to perform some checks, e.g. code formatting and linting. Therefore it is recommended to set up pre-commit for development:

$ cd "$VIRTME_NG"
$ # Activate pre-commit hooks for virtme-ng
$ pre-commit install
pre-commit installed at .git/hooks/pre-commit

Additional resources

• LWN: Faster kernel testing with virtme-ng (November, 2023)
• LPC 2023: Speeding up Kernel Testing and Debugging with virtme-ng
• Kernel Recipes 2024: virtme-ng
• Linux Foundation Mentorship Session: Speeding Up Kernel Development With virtme-ng

Credits

virtme-ng is written by Andrea Righi [email protected]

virtme-ng is based on virtme, written by Andy Lutomirski [email protected] (web | git).