Xen Dom0 on USB or SD

This guide will show you how to perform a Xen Dom0 install on a USB device or SD card, so you can have your Dom0 running from tmpfs. This has some benefits, but you will need a place to store your guests (virtual machines) and a syslog server to redirect the logs to.

The first step is to set up a basic usb or SD install. To do that, boot from the Alpine install CD (minimal is fine) and follow one of the following guides:
Create a Bootable USB or Create a Bootable Compact Flash.

Once you finish installing Alpine Linux on your device, boot from it and configure your newly installed system:

Set up the network interfaces:

# setup-interfaces
# ifup eth0

Note: you may need to change eth0 to the name of your interface

Continue by setting up the apk repositories:

# setup-apkrepos
[...]
# apk update

Note: you may need to edit /etc/apk/repositories and change the first line so it points to /media/usb/apks.

Now that you have your system properly configured, it's time to install Xen:

# Should this not be apk add xen xen-hypervisor ?
# apk add xen

Now that we have Xen installed, we need to modify the usb bootloader in order to load the Xen kernel. The first step is to remount /media/usb with write permissions:

# mount -o remount,rw /media/usb

Then, copy the Xen kernel and mboot.c32 to the usb boot partition:

# cp /boot/xen.gz /media/usb/boot/
# cp /boot/mboot.c32 /media/usb/boot/

Now that we have the necessary files to boot Xen from the usb device, it's time to change the bootloader and add a Xen entry.
To do so, open /media/usb/syslinux.cfg with your favorite editor and add one of the following entries:

Normal boot:

LABEL xen
  KERNEL /boot/mboot.c32
  APPEND /boot/xen.gz --- /boot/vmlinuz-hardened alpine_dev=usbdisk:vfat modules=loop,squashfs,sd-mod,usb-storage modloop=/boot/modloop-hardened --- /boot/initramfs-hardened

Note: you might have to use alpine_dev=UUID=XXXX-XXXX:vfat. UUID can be read using blkid.

Note: alternatively, to use /media/UUID=XXX-XXXX instead of /media/usb, see /proc/mounts to detect where your usb/cf card is mounted.

Serial console boot:

LABEL xen
  KERNEL /boot/mboot.c32
  APPEND /boot/xen.gz com1=115200,8n1 console=com1 --- /boot/vmlinuz-hardened alpine_dev=usbdisk:vfat modules=loop,squashfs,sd-mod,usb-storage modloop=/boot/modloop-hardened --- /boot/initramfs-hardened

Change the UUID to the correct one, and add the necessary modules for your system.
It's also a good idea to check Xen Boot options and set, dom0_mem, dom0_vcpus_pin and dom0_max_vcpus.

The next step is to load the necessary kernel modules for Xen. We will add them to /etc/modules so they will be loaded automatically at boot time:

# echo "xen_netback" >> /etc/modules
# echo "xen_blkback" >> /etc/modules
# echo "tun" >> /etc/modules

The last step is to configure startup services. We will need udev and xencommons to start at boot time:

# rc-update add udev sysinit
# rc-update add udev-postmount
# rc-update add xenstored
# rc-update add xenconsoled

Now you have a fully functional Xen install. It's time to save your changes and boot into it.

# lbu commit
# reboot

Remember to configure at least one network bridge. Follow the Bridge guide.

See also