docs: instructions on booting cloud images

README.md

@@ -51,6 +51,8 @@ alioth -l info --log-to-file \
     --num-cpu 2
 ```
 
+For instructions on booting a cloud image, see [Booting Cloud Images](docs/cloud-image.md).
+
 ## Features
 
 - **Cross-Platform:** Runs on `x86_64` (Linux) and `aarch64` (Linux & macOS).

docs/cloud-image.md

@@ -0,0 +1,96 @@
+# 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](https://man7.org/linux/man-pages/man8/kexec.8.html) to jump into the
+Linux kernel contained within the cloud image. The idea is borrowed from
+[LinuxBoot](https://linuxboot.org/).
+
+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.
+
+```bash
+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/linux/arch/x86/boot/bzImage`.
+
+## 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.
+
+```bash
+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](https://cloudinit.readthedocs.io/en/latest/reference/datasources/nocloud.html#example-creating-a-disk)
+to configure the VM user and password.
+
+```bash
+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.
+
+```bash
+./alioth -l info --log-to-file boot \
+    -m size=4G -p count=4 \
+    -k ./bzImage \
+    --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`.