docs: Add virtiofs root howto

Add a quick HowTo for using a virtiofs root filesystem in a
cloud-hypervisor guest.

Signed-off-by: Iggy Jackson <iggy@theiggy.com>

docs/virtiofs-root.md

@@ -0,0 +1,92 @@
+# 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:
+   * https://dl-cdn.alpinelinux.org/alpine/v3.13/releases/x86_64/alpine-minirootfs-3.13.2-x86_64.tar.gz
+   * Others should work
+
+## To create the VM rootfs
+
+```bash
+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/$i
+end
+# setup network config
+echo 'auto lo
+iface lo inet loopback
+
+auto eth0
+iface eth0 inet dhcp
+' | sudo tee etc/network/interfaces
+
+```
+
+## To run the VM
+
+```bash
+# starting in the directory above rootfs
+sudo virtiofsd --socket-path=$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.