# Run Clang/LLVM Compiled Arm64 Linux Kernel on QEMU Virt with Debian Rootfs
[toc]

## Enviroment Setup
1. Initramfs, Linux Kernel, external modules and QEMU
Please see [Run Clang/LLVM Compiled Arm64 Linux Kernel on QEMU Virt with BusyBox Initramfs](https://hackmd.io/@Ming-Jun/SJTBZOA0A).
2. Install packages

$ sudo apt-get install binfmt-support debootstrap qemu-user-static

3. Install qemu-binfmt for WSL2 according to ref[1]

$ wget https://github.com/qemu/qemu/raw/master/scripts/qemu-binfmt-conf.sh
$ chmod +x qemu-binfmt-conf.sh
$ sudo ./qemu-binfmt-conf.sh --qemu-path /usr/bin --qemu-suffix -static --debian
$ sudo update-binfmts --import qemu-aarch64

$ cat /proc/sys/fs/binfmt_misc/qemu-aarch64
enabled
interpreter /usr/bin/qemu-aarch64-static
flags:
offset 0
magic 7f454c460201010000000000000000000200b700
mask ffffffffffffff00fffffffffffffffffeffffff

## Arm64 Debian Rootfs
1. Create rootfs

$ export MY_ROOTFS_DIR=/path/to/your/directory
$ echo ${MY_ROOTFS_DIR}
/path/to/your/directory

$ sudo debootstrap \
       --arch=arm64 \
       --foreign \
       bookworm \
       ${MY_ROOTFS_DIR} \
       http://ftp.debian.org/debian/
$ sudo cp /usr/bin/qemu-aarch64-static ${MY_ROOTFS_DIR}/usr/bin
$ sudo chroot ${MY_ROOTFS_DIR} ./debootstrap/debootstrap --second-stage
I: Installing core packages...
I: Unpacking required packages...
...
I: Configuring required packages...
...
I: Unpacking the base system...
...
I: Configuring the base system...
...
I: Base system installed successfully.

2. Post process

$ sudo chroot ${MY_ROOTFS_DIR} passwd -d root
$ sudo chroot ${MY_ROOTFS_DIR} apt clean
$ sudo rm -f ${MY_ROOTFS_DIR}/usr/bin/qemu-aarch64-static

3. Create rootfs image with `-L my_label`

$ sudo mke2fs -t ext4 -L my_label -d ${MY_ROOTFS_DIR} debian_rootfs.ext4 512M
$ sudo chmod 666 debian_rootfs.ext4

## Modify Busybox Initramfs
1. Create `/your/path/to/busybox-1.36.1/_install/init_switch_root` with the following content

#!/bin/sh
# Busybox init script for switching root to another rootfs

/bin/echo "Mounting /proc"
/bin/mkdir -p /proc
/bin/mount -t proc proc /proc

/bin/echo "Mounting /sys"
/bin/mkdir -p /sys
/bin/mount -t sysfs sysfs /sys

/bin/echo "Mounting /dev"
/bin/mkdir -p /dev
/bin/mount -t tmpfs tmpfs /dev

/bin/echo "Mounting /tmp"
/bin/mkdir -p /tmp
/bin/mount -t tmpfs tmpfs /tmp

# Follow busybox/doc/mdev.txt to set up mdev
/bin/echo /sbin/mdev > /proc/sys/kernel/hotplug
/sbin/mdev -s

# Search the first block device with LABEL="my_label"
NEW_ROOT_DEV=$(/sbin/blkid | /bin/grep 'LABEL="my_label"' | /bin/sed 's/:.*//')
if [ -n "${NEW_ROOT_DEV}" ]; then
    NEW_ROOT="/mnt/new_root"
    NEW_INIT="/sbin/init"

    /bin/echo "Mounting new rootfs"
    /bin/mkdir -p ${NEW_ROOT}
    /bin/mount "${NEW_ROOT_DEV}" "${NEW_ROOT}"

    /bin/echo "Switching root to ${NEW_ROOT_DEV}"
    /bin/echo "" > /proc/sys/kernel/hotplug
    exec /sbin/switch_root -c /dev/console "${NEW_ROOT}" "${NEW_INIT}" "$@"
fi

exec /bin/sh

2. Update initramfs image

$ cd /your/path/to/busybox-1.36.1/_install
$ rm -f init
$ ln -s init_switch_root init
$ find . | cpio --quiet -H newc -o | gzip -9 -n > ../initramfs.cpio.gz

## QEMU
1. Run with `-drive file` option

$ qemu-system-aarch64 \
      -machine virt \
      -m size=1024M \
      -cpu cortex-a57 \
      -smp 2 \
      -nographic \
      -initrd /your/path/to/busybox-1.36.1/initramfs.cpio.gz \
      -drive file=/your/path/to/debian_rootfs.ext4,format=raw,if=virtio \
      -kernel /your/path/to/linux/arch/arm64/boot/Image \
      --append "console=ttyAMA0"

2. Boot to debian rootfs
## Reference
1. [Install qemu-binfmt on WSL2](https://forums.developer.nvidia.com/t/installation-fails-on-wsl/274008/23?page=2)