erplibre/script/qemu

QEMU/KVM — Linux VM deployment (Ubuntu / Debian / Fedora)

deploy_qemu.py deploys a Linux VM (libvirt/KVM) from an official cloud image, using qemu-img + cloud-init + virt-install. Pick the distribution with --distro (ubuntu default, debian, fedora) and the release with --version; run --list-images to see the full catalogue with minimum specs. It:

1. Downloads the cloud image by itself (cached, no double download).
2. Converts it to a dedicated qcow2 working disk and resizes it.
3. Generates user-data / meta-data and builds the seed.iso (cloud-init).
4. Runs virt-install importing the disk + the seed as a CD-ROM.
5. Waits for the DHCP lease and prints the SSH command.

Prerequisites

• A host with KVM available (bare-metal or nested virtualization enabled).
• sudo rights (the deployment writes to /var/lib/libvirt/images and drives libvirt).

Installation

The script auto-installs the missing pieces it needs: on first run it detects your package manager (apt / dnf / pacman / zypper / brew), lists the missing components (the client tools, plus the libvirt daemon and the QEMU system emulator), asks for confirmation, installs them with sudo, then enables and starts libvirtd. Use -y to accept automatically or --no-install-deps to disable this behaviour.

To install everything manually on Ubuntu/Debian (recommended full KVM stack):

sudo apt install qemu-utils virtinst libvirt-clients cloud-image-utils \
    libvirt-daemon-system qemu-system-x86
sudo systemctl enable --now libvirtd
sudo usermod -aG libvirt,kvm "$USER"   # re-login / reconnectez-vous

libvirt-daemon-system provides the libvirtd daemon (and the /var/run/libvirt/libvirt-sock socket) and qemu-system-x86 the emulator — without them virt-install fails with "Failed to connect socket to '/var/run/libvirt/libvirt-sock'". The script installs and starts them for you; this manual command is only needed if you prefer to prepare the host yourself or run with --no-install-deps.

Usage

Simplest form — the image is downloaded automatically (path derived from --version, cached in /var/lib/libvirt/images/iso):

sudo ./script/qemu/deploy_qemu.py --name test-vm --version 24.04 \
    --ssh-key ~/.ssh/id_ed25519.pub

Download (and verify) an image without creating a VM:

sudo ./script/qemu/deploy_qemu.py --download-only --version 24.04 --verify

Deploy with an interactive password instead of an SSH key:

sudo ./script/qemu/deploy_qemu.py --name test-vm --version 24.04 --ask-password

Larger VM (8 GB RAM, 8 vCPU, 120 GB disk), overwriting an existing disk:

sudo ./script/qemu/deploy_qemu.py --name test-vm --version 24.04 \
    --memory 8192 --vcpus 8 --disk-size 120G --ask-password --force

Preview what would happen, without doing anything (no sudo, no download):

./script/qemu/deploy_qemu.py --name test-vm --version 24.04 --dry-run

Non-interactive deployment (accept dependency install automatically):

sudo ./script/qemu/deploy_qemu.py --name test-vm --version 24.04 \
    --ssh-key ~/.ssh/id_ed25519.pub -y

Supported Ubuntu versions: 20.04, 22.04, 24.04 (default), 24.10, 25.04, 25.10. Provide an explicit image path as a positional argument to override the automatic download location.

After deployment

virsh list --all
virsh console test-vm                    # Ctrl+] to quit / pour quitter
virsh domifaddr test-vm --source lease   # find the IP / trouver l'IP
ssh erplibre@<IP>

The default user is erplibre (change it with --user).

Via the TODO menu

The script is integrated into the interactive assistant. Run make todo (or ./script/todo/todo.py), then go to Execute → Deploy → QEMU/KVM - Deploy an Ubuntu VM (libvirt). From there you can deploy a VM, preview a dry-run, download an image, list VMs and show a VM IP address — the menu asks for the parameters and builds the command for you.

Main options

• --distro — ubuntu (default), debian or fedora.
• --version — release for the distro (default: the distro's default).
• --list-images — print all distros/versions and their specs, then exit.
• --image-dir — image cache directory (default /var/lib/libvirt/images/iso).
• --download-only — download the image then exit (no VM).
• --name — VM name (required for deployment).
• --memory, --vcpus, --disk-size — VM sizing. When omitted, --memory and --disk-size default to the minimum required by the chosen version (libosinfo values, see --list-images: Ubuntu 24.04+ → 3072 MB/20G, Debian → 1024 MB/10G, Fedora → 2048 MB/15G); --vcpus defaults to 2.
• --ssh-key, --ask-password, --password-hash — authentication.
• -y / --assume-yes — auto-accept dependency installation.
• --no-install-deps — never auto-install dependencies.
• --dry-run — show the commands without executing anything.
• --force — overwrite the existing working qcow2 disk.

Run ./script/qemu/deploy_qemu.py --help for the full list.

Managing VMs

List, stop and remove VMs (the qcow2 disk under /var/lib/libvirt/images is kept unless you delete it):

sudo virsh list --all          # toutes les VM et leur état / all VMs and state
sudo virsh shutdown <nom-vm>   # arrêt propre ACPI / graceful shutdown
sudo virsh destroy <nom-vm>    # arrêt forcé / force off (pull the plug)
sudo virsh undefine <nom-vm>   # supprime la définition / remove definition
sudo virsh domifaddr <nom-vm>  # adresse IP de la VM / VM IP address

destroy only powers the VM off (disk kept); undefine removes its definition. To fully recreate a VM with the same name, destroy + undefine it first, or redeploy with --force.

SSH access from another machine (ProxyJump)

With the default NAT network the VM is reachable only from the KVM host. To reach it from another machine without changing the network, use the host as a jump host (it already reaches the VM). Get the VM IP with sudo virsh domifaddr <nom-vm>, then from the other machine:

# Rebond SSH vers la VM / jump through the KVM host
ssh -J user@<ip-hote> erplibre@<ip-vm>

# Tunnel d'un service, ex. Odoo 8069 / tunnel a service, then http://localhost:8069
ssh -L 8069:<ip-vm>:8069 user@<ip-hote>

To make it permanent, add this to ~/.ssh/config on the other machine (then just ssh myvm):

Host myvm
    HostName <ip-vm>            # ex. 192.168.122.50 (reseau NAT)
    User erplibre
    ProxyJump user@<ip-hote>    # IP LAN de l'hote KVM

This works over Wi-Fi and needs no VM shutdown — the simplest option for personal access. Prefer a bridge (below) if the VM must be a full server exposed on the LAN.

QEMU inside QEMU (nested) & exposing the VM via a bridge

If the KVM host is itself a VM (QEMU-in-QEMU), the deployment works only when nested virtualization is enabled on the outer/physical host and the middle VM uses CPU mode host-passthrough. Check from inside the KVM host (the first command must be non-empty):

grep -E -o '(vmx|svm)' /proc/cpuinfo | sort -u   # extensions visibles / visible
# Sur l'hote PHYSIQUE / on the PHYSICAL host:
cat /sys/module/kvm_intel/parameters/nested      # Intel -> Y/1
cat /sys/module/kvm_amd/parameters/nested        # AMD   -> Y/1

To enable nesting on the physical host (Intel shown; use kvm_amd on AMD), then recreate the middle VM with host-passthrough:

echo "options kvm_intel nested=1" | sudo tee /etc/modprobe.d/kvm-nested.conf
sudo modprobe -r kvm_intel && sudo modprobe kvm_intel   # ou / or reboot

On s390x and arm64 the parameter lives on the kvm module itself, not on kvm_intel / kvm_amd — and /sys/module/kvm/parameters/nested does not even exist on x86. Reading the wrong file returns a reassuring 0 that commands nothing:

echo "options kvm nested=1" | sudo tee /etc/modprobe.d/kvm-nested.conf
sudo modprobe -r kvm && sudo modprobe kvm               # ou / or reboot

nested on a machine means « let MY guests run VMs ». To accelerate a VM created on host H, the setting belongs to the hypervisor above H, not to H itself. The one command that settles it, run on H:

ls -l /dev/kvm     # absent -> pas d'imbrication, tout sera émulé

Measured on an s390x host that was itself a KVM guest without nesting: /dev/kvm absent, virsh dumpxml showing <domain type='qemu'>, and a 7 min 30 boot instead of well under a minute. systemd-detect-virt inside the VM is not proof of acceleration — on s390x, QEMU fabricates the STSI answer and reports kvm even under TCG. Only <domain type=…> on the host is conclusive.

If nesting is unavailable, QEMU still runs via software emulation (TCG) — it works but is slow.

# /etc/netplan/01-br0.yaml
network:
  version: 2
  renderer: networkd
  ethernets:
    enp3s0: {dhcp4: no, dhcp6: no}
  bridges:
    br0:
      interfaces: [enp3s0]
      dhcp4: yes
      parameters: {stp: false, forward-delay: 0}

Apply safely (auto-reverts if you lose the connection) and verify — or use NetworkManager (Ubuntu desktop):

# netplan
sudo netplan try && sudo netplan apply
ip addr show br0        # br0 porte l'IP du LAN / br0 holds the LAN IP

# NetworkManager (alternative)
nmcli con add type bridge ifname br0 con-name br0
nmcli con add type ethernet ifname enp3s0 master br0 con-name br0-port
nmcli con modify br0 ipv4.method auto
nmcli con down "Wired connection 1" ; nmcli con up br0

Then attach the VM to the bridge — either at creation:

sudo ./script/qemu/deploy_qemu.py --name <nom-vm> --version 24.04 \
    --ssh-key ~/.ssh/id_ed25519.pub --network bridge=br0,model=virtio -y --force

or by editing a VM already created: stop it, replace its <interface> block (type='network' / <source network='default'/> → type='bridge' / <source bridge='br0'/>), then start it again:

sudo virsh shutdown <nom-vm>
sudo virsh edit <nom-vm>        # mettre l'interface en bridge=br0
sudo virsh start <nom-vm>
sudo virsh domifaddr <nom-vm>   # nouvelle IP LAN / new LAN IP

The VM now gets a LAN IP from your router, reachable by other machines. From the Internet you additionally need a port-forward on your router (or a VPN); in a nested setup the outer host must also forward/expose the middle VM.