Virtualizing Raspbian (or any ARM/Linux distro) headless using QEMU

Why not use QEMU's "raspi2" machine for emulation?

Spot something missing? It doesn't implement USB, which makes it useless for headless and graphical use as you can plug in neither a network connection nor a keyboard or mouse.

(Update 2021-01-08: QEMU has apparently added raspi USB support in versions 5.1.0 onward, so you could skip much of the setup detailed here if doing this from scratch.)

-M raspi2b -kernel kernel7l.img -dtb bcm2709-rpi-2-b.dtb -append "root=/dev/mmcblk0 rw console=ttyAMA0"

The plan

Instead of (poorly) emulating a real piece of hardware, QEMU also has a virt machine [1] that is designed for virtualization. It gives you a modern system with PCI and also works out-of-the-box with Linux without providing a Device Tree (QEMU generates one internally).

The most straightforward way of getting network and disk into such a VM is to use virtio-net and virtio-disk respectively, which is what we'll be doing.

I picked Arch Linux ARM's armv7 kernel from here, though any other should work just as well provided it comes with the appropriate modules. To load the virtio modules during boot we'll require an initramfs, but more on that later.

Extracting Raspbian's root filesystem into a virtual disk image

Start by downloading Raspbian from the Raspberry Pi website, then run the script below or follow the steps manually.

The script will create a copy of the image file, expand the image and its partition to 10 gigabytes, mount the partition using a loop device and make two adjustments:

• Remove both SD card partitions from /etc/fstab, these don't exist inside the VM and we will be mounting the rootfs ourselves

• Disable several startup services that do not work inside the VM

After unmounting the partition it will convert the filesystem into a qcow2 format image for use with QEMU.

#!/bin/bash -e
input=2020-02-13-raspbian-buster-lite.img
[ -f $input ]

mkdir mnt
cp --reflink=auto $input source.img
truncate -s 10G source.img
echo ", +" | sfdisk -N 2 source.img
dev=$(sudo losetup -fP --show source.img)
[ -n "$dev" ]
sudo resize2fs ${dev}p2
sudo mount ${dev}p2 ./mnt -o rw
sudo sed '/^PARTUUID/d' -i ./mnt/etc/fstab
sudo rm -f \
        ./mnt/etc/systemd/system/multi-user.target.wants/{hciuart,dphys-swapfile}.service \
        ./mnt/etc/rc?.d/?01{resize2fs_once,rng-tools,rng-tools-debian}
sudo umount ./mnt
sudo chmod a+r ${dev}p2
qemu-img convert -O qcow2 ${dev}p2 rootfs.qcow2
sudo losetup -d $dev
rm source.img; rmdir mnt

The kernel and initramfs

#!/bin/bash -e
pkg=$(echo linux-armv7-*.pkg.tar.xz)
[ -f "$pkg" ]

mkdir initrd; pushd initrd
mkdir bin dev mnt proc sys
tar -xaf "../$pkg" --strip-components=1 usr/lib/modules
rm -rf lib/modules/*/kernel/{sound,drivers/{net/{wireless,ethernet},media,gpu,iio,staging,scsi}}
wget https://www.busybox.net/downloads/binaries/1.31.0-defconfig-multiarch-musl/busybox-armv7l -O bin/busybox
cat >init <<"SCRIPT"
#!/bin/busybox sh
busybox mount -t proc none /proc
busybox mount -t sysfs none /sys
busybox mount -t devtmpfs none /dev

for mod in virtio-pci virtio-blk virtio-net; do
        busybox modprobe $mod
done

busybox mount -o rw /dev/vda /mnt || exit 1

busybox umount /proc
busybox umount /sys
busybox umount /dev

exec busybox switch_root /mnt /sbin/init
SCRIPT
chmod +x bin/busybox init
bsdtar --format newc --uid 0 --gid 0 -cf - -- * | gzip -9 >../initrd.gz
popd; rm -r initrd

Booting the virtual machine

With the initramfs built, we have all parts needed to actually run the VM: [2]

qemu-system-arm -M virt,highmem=off -m 2048 -smp 4 -kernel zImage -initrd initrd.gz \
        -drive file=rootfs.qcow2,if=virtio -nic user,model=virtio \
        -append "console=ttyAMA0" -nographic