Booting Cloud Images with Alioth

Introduction

Cloud images (e.g., Ubuntu, Fedora) typically ship a complete operating system as a single disk image file, containing both the Linux kernel and the initramfs. A bootloader is usually required to extract these components from the disk image to boot the system.

Instead of relying on traditional firmware like SeaBIOS or OVMF, Alioth takes a different approach: it first boots into a minimal Linux kernel (the "bootloader") and then uses kexec to jump into the Linux kernel contained within the cloud image. The idea is borrowed from LinuxBoot.

This guide demonstrates how to boot Fedora 43 on an x86_64 Linux machine.

Building Artifacts

First, clone the repository and build the Alioth binary and the bootloader kernel.

git clone https://github.com/google/alioth
cd alioth

# Build Alioth
cargo build --release

# Build the bootloader kernel
bootloader/build.sh

The Alioth binary will be located at target/release/alioth, and the bootloader kernel image at target/bootloader-x86_64/kernel-x86_64.

Preparing Disk Images

Download and Convert the Cloud Image

Download the Fedora 43 cloud image and convert it from QCOW2 to raw format, as Alioth currently requires raw disk images.

wget https://dl.fedoraproject.org/pub/fedora/linux/releases/43/Cloud/x86_64/images/Fedora-Cloud-Base-Generic-43-1.6.x86_64.qcow2

./alioth img convert \
    -f qcow2 -O raw \
    Fedora-Cloud-Base-Generic-43-1.6.x86_64.qcow2 \
    Fedora-Cloud-Base-Generic-43-1.6.x86_64.raw

Create a Cloud-init Disk

Create a cloud-init NoCloud disk to configure the VM user and password.

truncate --size 64K /tmp/cloud-init.raw
mkfs.vfat -n CIDATA /tmp/cloud-init.raw

# Create user-data
mcopy -oi /tmp/cloud-init.raw - ::user-data <<EOF
#cloud-config
password: password
chpasswd:
  expire: False
EOF

# Create meta-data
mcopy -oi /tmp/cloud-init.raw - ::meta-data <<EOF
local-hostname: vm
EOF

Booting the VM

Run the following command to start the VM.

./alioth -l info --log-to-file boot \
    -m size=4G -p count=4 \
    -k ./kernel-x86_64 \
    --entropy \
    --pvpanic \
    --blk file,path=Fedora-Cloud-Base-Generic-43-1.6.x86_64.raw \
    --blk file,path=/tmp/cloud-init.raw,readonly=true

Once the LinuxBoot splash screen appears, press 1 or Enter to select and boot the Fedora Linux kernel.

When the login screen appears, log in using the username fedora and the password password.