Booting ubuntu 16.04 cloud images on Arm64

For testing kvm/qemu, prebaked images cloud images are nice. However, there is a few steps to get started. First we need a recent Qemu (2.5 is good enough). An efi firmware is needed, and cloud-utils, for customizing our VM.

sudo apt install -y qemu qemu-utils cloud-utils
wget https://releases.linaro.org/components/kernel/uefi-linaro/15.12/release/qemu64/QEMU_EFI.fd
wget https://cloud-images.ubuntu.com/xenial/current/xenial-server-cloudimg-arm64-uefi1.img

Cloud images are plain - there is no user setup, no default user/pw combo, so to log in to the image, we need to customize the image on first boot. The defacto tool for this is cloud-init. The simplest method for using cloud-init is passing a block media with a settings file - of course for real cloud deployment, you would use one of fancy network based initialization protocols cloud-init supports. Enter the following to a file, say cloud.txt:

#cloud-config

users:
  - name: you
    ssh-authorized-keys:
      - ssh-rsa AAAAB3Nz....
    sudo: ['ALL=(ALL) NOPASSWD:ALL']
    groups: sudo
    shell: /bin/bash

This minimal config will just set you a user with ssh key. A more complex setup can install packages, write files and run arbitrary commands on first boot. In professional setups, you would most likely end up using cloud-init only to start Ansible or another configuration management tool.

cloud-localds cloud.img cloud.txt
qemu-system-aarch64 -smp 2 -m 1024 -M virt -bios QEMU_EFI.fd -nographic \
       -device virtio-blk-device,drive=image \
       -drive if=none,id=image,file=xenial-server-cloudimg-arm64-uefi1.img \
       -device virtio-blk-device,drive=cloud \
       -drive if=none,id=cloud,file=cloud.img \
       -netdev user,id=user0 -device virtio-net-device,netdev=user0 -redir tcp:2222::22 \
       -enable-kvm -cpu host 

If you are on an X86 host and want to use qemu to run an aarch64 image, replace the last line with "-cpu cortex-a57". Now, since the example uses user networking with tcp port redirect, you can ssh into the VM:

ssh -p 2222 you@localhost
Welcome to Ubuntu 16.04 LTS (GNU/Linux 4.4.0-22-generic aarch64)
....

Using networkd for kvm tap networking

Setting up basic systemd-network was recently described by Joachim, and the post inspired me to try it as well. The twist is that in my case I need a bridge for my KVM with Lava server and arm/aarch64 qemu system emulators...

For background, qemu/kvm support a few ways to provide network to guests. The default is user networking, which requires no privileges, but is slow and based on ancient SLIRP code. The other common option is tap networking, which is fast, but complicated to set up. Turns out, with networkd and qemu bridge helper, tap is easy to set up.

$ for file in /etc/systemd/network/*; do echo $file; cat $file; done
/etc/systemd/network/eth.network
[Match]
Name=eth1
[Network]
Bridge=br0

/etc/systemd/network/kvm.netdev
[NetDev]
Name=br0
Kind=bridge

/etc/systemd/network/kvm.network
[Match]
Name=br0
[Network]
DHCP=yes

Diverging from Joachims simple example, we replaced "DHCP=yes" with "Bridge=br0". Then we proceed to define the bridge (in the kvm.netdev) and give it an ip via dhcp in kvm.network. From the kvm side, if you haven't used the bridge helper before, you need to give the helper permissions (setuid root or cap_net_admin) to create a tap device to attach on the bridge. The helper needs an configuration file to tell what bridge it may meddle with.

# cat > /etc/qemu/bridge.conf <<__END__
allow br0
__END__
# setcap cap_net_admin=ep /usr/lib/qemu/qemu-bridge-helper

Now we can start kvm with bridge networking as easily as with user networking:

$ kvm -m 2048 -drive file=jessie.img,if=virtio -net bridge -net nic,model=virtio -serial stdio

The manpages systemd.network(5) and systemd.netdev(5) do a great job explaining the files. Qemu/kvm networking docs are unfortunately not as detailed.