Xen Dom0 on USB or SD

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This guide will show you how to perform a Xen Dom0 install on a USB 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 this 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 finished installing Alpine Linux on your device, boot from it and configure your newly installed system:

Note: you might have to edit /etc/apk/repositories and change the first line so it points to /media/usb/apks.
# setup-alpine
[...]
# apk update

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

Note: you might have to use /media/UUID=XXX-XXXX instead of /media/usb, see /proc/mounts to detected where is your usb/cf card mounted.
# 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 we have all the necessary files to boot Xen from the usb, 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/grsec alpine_dev=UUID=XXXX-XXXX:vfat modules=loop,squashfs,sd-mod,usb-storage modloop=/boot/grsec.modloop.squashfs --- /boot/grsec.gz

Serial console boot:

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

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 at least.

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 on boot:

# 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 at least to be started on boot:

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

If you need the xm toolstack, also add xend to startup services:

# rc-update add xend

Ok, now you have a fully functional Xen install, it's time to save your changes and boot into it.

# lbu commit
# reboot

Also remember to configure at least one network bridge following the Bridge guide.

See also