OpenWrt as QEMU/KVM host server

It's possible to use OpenWrt as a QEMU host and run guests on it. If you want to run OpenWrt as a QEMU guest itself, see OpenWrt in QEMU.

OpenWrt provides QEMU packages for ARM and x86 platforms. This article focuses on the x86 target, the networking is done via qemu-bridge-helper.

You need the following packages on your device: kmod-tun, qemu-bridge-helper. Depending on the guest architecture, install qemu-x86_64-softmmu or qemu-arm-softmmu. If your hardware supports it, also install kmod-kvm-amd or kmod-kvm-intel for better performance.

Example for an Intel system and a x86_64 guest:

opkg install kmod-tun qemu-bridge-helper qemu-x86_64-softmmu kmod-kvm-intel

After the first installation, reboot your device.

For the guest OS, use a distribution that comes with virtio drivers by default (Debian or Fedora for example).

The following QEMU command uses a physical disk, but you can use a disk image as well. Below are explanations of the networking options.

qemu-system-x86_64 -enable-kvm -cpu host -smp 2 -m 2G \
    -drive file=/dev/sda,cache=none,if=virtio,format=raw \
    -device virtio-net-pci,mac=E2:F2:6A:01:9D:C9,netdev=br0 \
    -netdev bridge,br=br-lan,id=br0

Line 3 creates a virtio net device with a fixed MAC address that is told to use the network backend with id=br0, which gets created in line 4. The network backend is of type bridge and br=br-lan tells QEMU to connect to the bridge network br-lan of the OpenWrt host. For that qemu-bridge-helper will automatically create the needed TUN/TAP interfaces. If your guest uses DHCP it should now receive an address by the server running in br-lan.

You can choose any other bridge interface available on your OpenWrt host as well. See Network basics on how to configure networking interfaces. If you want to change which networks QEMU is allowed to connect to, you can do so via /etc/qemu/bridge.conf.

If you don't have a prepared disk image, you can install a guest OS directly on your OpenWrt device. There are several guides available on how to install a Linux distribution on a QEMU image: Debian for example. A quick way is to install Debian using the kernel and initrd of a Debian netboot installer.

qemu-img create -f qcow2 debian.img 4G

More details on disk images: https://en.wikibooks.org/wiki/QEMU/Images

wget https://ftp.debian.org/debian/dists/stable/main/installer-amd64/current/images/netboot/debian-installer/amd64/linux
wget https://ftp.debian.org/debian/dists/stable/main/installer-amd64/current/images/netboot/debian-installer/amd64/initrd.gz

Both files can be savely removed after finishing the installation.

The following command will launch the Debian installer inside QEMU, debian.img will appear as /dev/vda

qemu-system-x86_64 -enable-kvm -cpu host -smp 2 -m 2G \
	-initrd initrd.gz \
	-kernel linux \
	-append "root=/dev/ram console=ttyS0" \
	-drive file=debian.img,if=virtio \
	-device virtio-net-pci,mac=E2:F2:6A:01:9D:C9,netdev=br0 \
	-netdev bridge,br=br-lan,id=br0 \
	-nographic

Follow the installation instructions and install GRUB on /dev/vda. It's also useful to install sshd (enabled by default).

After finishing the installation, you can launch the new installation with QEMU:

qemu-system-x86_64 -enable-kvm -cpu host -smp 2 -m 2G \
	-drive file=debian.img,if=virtio \
	-device virtio-net-pci,mac=E2:F2:6A:01:9D:C9,netdev=br0 \
	-netdev bridge,br=br-lan,id=br0 \
	-nographic

You should now be able to reach the VM via SSH from within br-lan. If you want to control the VM using the command line, you have to enable a serial console. To do this, edit the GRUB entry during boot and add console=ttyS0 to the kernel command line. After the VM finished booting, edit /etc/default/grub and add console=ttyS0 to GRUB_CMDLINE_LINUX_DEFAULT as well. After that run update-grub.

To automatically start the VM at boot and shut it down cleanly using QMP, create an init script like the one below.

/etc/init.d/qemu

#!/bin/sh /etc/rc.common
 
START=99
STOP=1
 
qemu_pidfile="/var/run/qemu.pid"
 
start() {
qemu-system-x86_64 -enable-kvm -cpu host -smp 2 -m 2G \
	-drive file=/dev/sda,cache=none,if=virtio,format=raw \
	-device virtio-net-pci,mac=E2:F2:6A:01:9D:C9,netdev=br0 \
	-netdev bridge,br=br-lan,id=br0 \
	-qmp tcp:127.0.0.1:4444,server,nowait \
	-daemonize &> /var/log/qemu.log
 
/usr/bin/pgrep qemu-system-x86_64 > $qemu_pidfile
echo "QEMU: Started VM with PID $(cat $qemu_pidfile)."
}
 
stop() {
echo "QEMU: Sending 'system_powerdown' to VM with PID $(cat $qemu_pidfile)."
nc localhost 4444 <<QMP 
{ "execute": "qmp_capabilities" } 
{ "execute": "system_powerdown" } 
QMP
 
if [ -e $qemu_pidfile ]; then
	if [ -e /proc/$(cat $qemu_pidfile) ]; then
		echo "QEMU: Waiting for VM shutdown."
		while [ -e /proc/$(cat $qemu_pidfile) ]; do sleep 1s; done
		echo "QEMU: VM Process $(cat $qemu_pidfile) finished."
	else
		echo "QEMU: Error: No VM with PID $(cat $qemu_pidfile) running."
	fi
 
	rm -f $qemu_pidfile
else
	echo "QEMU: Error: $qemu_pidfile doesn't exist."
fi
}

Test the the script by running /etc/init.d/qemu start and look for errors in /var/log/qemu.log. If the script works as desired, enable it for every boot: /etc/init.d/qemu enable

This script needs to be ported to procd. Problem: stop_service() is called after procd killed the service, but we must run it beforehand to let the VM shut down.