Debian VM build scripts

This is a collection of scripts that can be used to build a kernel, initial ram disk and root file system suitable for booting Debian in a virtual machine.

It is intended for use with toyvm on Apple Silicon (Arm) and Intel Macs, but should work with any aarch64 or x86_64 virtual machine that implements virtio devices and supports directly booting a Linux kernel.

See Releases for pre-built binaries for Apple Silicon (and other aarch64 platforms), created using these scripts.

Building

An existing Linux installation of the same architecture as the target is required. For Apple Silicon, this can be a Raspberry Pi 4 running the 64-bit version of Raspberry Pi OS.

Docker is used to create a consistent build environment. On a Debian-based distribution such as Raspberry Pi OS, install Docker with:

sudo apt install docker.io

Start the build with:

sudo ./build.sh

(Root privileges are required to set up a loopback device for populating the disk image, and to issue Docker commands by default).

⚠ This script tags the Docker images it creates with the names debian-vm-build-kernel and debian-vm-build-rootfs, overwriting any existing tags with these names.

Build times

The table below shows the approximate time taken for a complete build on the systems I've tested with:

Time	System
24 minutes	Mac mini 2020 (M1), Debian in toyvm, 8GB RAM, 8 CPU cores
47 minutes	MacBook Pro 2018 (2.3GHz quad core i5), Debian in toyvm, 8GB RAM, 8 CPU cores/threads
2 hours, 33 minutes	Raspberry Pi 4B, Raspberry Pi OS 64-bit, 4GB RAM, 4 CPU cores

This includes the time taken to download Docker images and Debian packages.

Output

Build products are placed in the output subdirectory of the current working directory. The output consists of the following files:

deb/ - kernel .deb packages
- deb/linux-headers-<version>_arm64.deb
- deb/linux-image-<version>_arm64.deb
- deb/linux-libc-dev_<version>_arm64.deb
debian-rootfs-aarch64.img - sparse file containing the root filesystem image
initrd.img-<version> - initial ram disk image
vmlinuz-<version> - kernel image

Configuration

🔑 The initial root password is password.

The kernel is built with the standard Debian configuration, adjusted to add built-in support for virtio devices (the drivers are normally built as modules).

The root filesystem is generated using debootstrap, and as such contains only the bare minimum required to install packages. For convenience, the build scripts configure the virtual network interface and add the standard Debian package sources.

There is no swap configured.

Apple's Virtualization.framework (and therefore toyvm) supports only raw disk image files. To keep disk usage to a minimum, the root filesystem disk image is written to a sparse file.

Booting in toyvm

tar xvjf debian-vm-aarch64_*.tar.bz2
toyvm -k vmlinuz-* -i initrd.img-* -d debian-rootfs-aarch64.img 'console=hvc0 root=/dev/vda1 nosplash'

Hints

Enlarging the root filesystem

With the VM stopped, first increase the size of the disk image file (20GB in this example):

$ dd if=/dev/zero of=debian-rootfs-aarch64.img bs=1g count=0 seek=20

Inside the VM, assuming the first virtual disk contains the root filesystem, resize the partition to fill the entire disk image:

# apt install parted
# parted /dev/vda
GNU Parted 3.2
Using /dev/vda
Welcome to GNU Parted! Type 'help' to view a list of commands.
(parted) resizepart
Warning: Not all of the space available to /dev/vda appears to be used, you can
fix the GPT to use all of the space (an extra 20971520 blocks) or continue with
the current setting?
Fix/Ignore? Fix
Partition number? 1
Warning: Partition /dev/vda1 is being used. Are you sure you want to continue?
Yes/No? Yes
End?  [10.7GB]? 100%
(parted) quit
Information: You may need to update /etc/fstab.

Finally, resize the filesystem:

# resize2fs /dev/vda1

Snapshots

APFS supports file cloning, which is useful for creating disk image 'snapshots'. Unchanged blocks are shared between clones, making efficient use of storage.

$ df -h .
Filesystem     Size   Used  Avail Capacity iused      ifree %iused  Mounted on
/dev/disk3s5  460Gi  148Gi  293Gi    34% 1095372 4826881148    0%   /System/Volumes/Data
$ cp -c debian-rootfs-aarch64.img debian-rootfs-aarch64-snapshot.img
$ df -h .
Filesystem     Size   Used  Avail Capacity iused      ifree %iused  Mounted on
/dev/disk3s5  460Gi  148Gi  293Gi    34% 1095373 4826881147    0%   /System/Volumes/Data

danielrfry/debian-vm-build