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virtiofs-root.md

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HOWTO VirtioFS rootfs

A quick guide for using virtiofs as a cloud-hypervisor guest's rootfs (i.e. with no root block device). This document is a quick getting started guide. There are many more steps to take to make this a production ready, secure setup.

Prerequisites

  1. virtiofsd from the qemu project
    • We are using the Qemu version for now
    • There is a Rust version being worked on that may be a better option in the future
    • Part of the qemu-system-common package on Ubuntu
    • Part of the qemu-common package on Fedora
  2. cloud-hypervisor - the newer the better, but I tested with 0.12
  3. a rootfs - This howto uses an alpine rootfs available here:

To create the VM rootfs

mkdir rootfs/
cd rootfs
# this needs sudo to be able to set root permissions on fs components
sudo tar -xf /path/to/alpine-minirootfs-3.13.1-x86_64.tar.gz
# this will get created when the VM actually boots by the dhcp client
# but we need it in the chroot to download packages
sudo cp /etc/resolv.conf etc/
# the alpine mini rootfs is meant for docker containers, we need a few extra
# things for a working rootfs
sudo chroot $PWD apk add openrc busybox-initscripts
# we are using the paravirt console in cloud-hypervisor, so enable it in init
# append it after the other console since it doesn't work just appending it
sudo sed -i '/vt100/a \n# paravirt console\nhvc0::respawn:/sbin/getty -L hvc0 115200 vt100' etc/inittab
# set no password for root user... you obviously don't want to do this for
# any sort of production setup
sudo sed -i 's/root:!::0:::::/root:::0:::::/' etc/shadow
# set up init scripts
for i in acpid crond
    sudo ln -sf /etc/init.d/$i etc/runlevels/default/$i
end
for i in bootmisc hostname hwclock loadkmap modules networking swap sysctl syslog urandom
    sudo ln -sf /etc/init.d/$i etc/runlevels/boot/$i
end

for i in killprocs mount-ro savecache
    sudo ln -sf /etc/init.d/$i etc/runlevels/shutdown/$i
end

for i in devfs dmesg hwdrivers mdev
    sudo ln -sf /etc/init.d/$i etc/runlevels/sysinit/$i
end
# setup network config
echo 'auto lo
iface lo inet loopback

auto eth0
iface eth0 inet dhcp
' | sudo tee etc/network/interfaces

To run the VM

# starting in the directory above rootfs
sudo virtiofsd --socket-path=$PWD/virtiofs-rootfs.sock -o source=$PWD/rootfs -o cache=none &
sudo cloud-hypervisor \
    --cpus boot=1,max=1 \
    --kernel vmlinux \
    --fs tag=/dev/root,socket=$PWD/virtiofs-rootfs.sock \
    --memory size=2G,shared=on \
    --cmdline "console=hvc0 rootfstype=virtiofs root=/dev/root ro debug" \
    --api-socket $PWD/ch.sock \
    --rng \
    --net ...

Note: an important part of the above is the tag=/dev/root and root=/dev/root parts. For whatever reason, it would only work with that as the tag.

Note: another important bit is that the memory is shared. This is required for virtiofs

Message from the author

If you find any issues or have suggestions, feel free to reach out to @iggy on the cloud-hypervisor slack. Also if this works for you, I'd like to know as well. It would also be nice to get steps for preparing other distribution root filesystems.