Xen Hypervisor Dom0 and DomU Support

This is an old revision of the document!

Xen is a GPLv2-license hypervisor for the Intel architecture

FreeBSD 8.0 onward includes i386 and amd64 DomU and Amazon EC2 unprivileged domain (virtual machine) support.

FreeBSD 11.0 onward includes Xen Dom0 privileged domain (host) support.

FreeBSD Dom0 Control Domain Support

Hardware virtualized domains require Extended Page Table (EPT) and Input/Output Memory Management Unit (IOMMU) processor features to operate.

NB! FreeBSD Xen cannot be loaded with UEFI at this time

Dom0 Control Domain Installation

The 'xen-kernel' and 'xen-tools' packages provide the Xen Dom0 kernel and support utilities. The following example will assume VNC output for unprivileged domains which will be accessed from a another system using a tool such as net/tightvnc.

Xen Dom0 Package Installation

pkg install -y xen-kernel xen-tools

Follow the instructions provided at the end of the package which modifies:

/etc/sysctl.conf
/etc/ttys
/boot/loader.conf
/boot/menu.rc.local

In addition to these instructions, Xen also requires:

/boot/loader.conf

if_tap_load="YES"

sysrc xencommons_enable=YES

Reboot the host. Pressing “6” in the loader will give you the option to disable the Xen kernel.

You should see the Xen kernel output followed by either the FreeBSD kernel output or a blank screen until the login prompt. The FreeBSD control domain system will behave in the established manner and note the dom0_mem and dom0_max_vcpus options in /boot/loader.conf.

Unprivileged Domain Configuration

Unprivileged Domains consist of a configuration file and logical or physical optical and hard disks. For this example we will assume a ISO optical disk file freebsd.iso and a hard disk file created with truncate -s 20G freebsd.img.

An example FreeBSD DomU configuration file freebsd.cfg with 2048M RAM and two virtual CPUs without networking:

builder = "hvm"
memory = 2048
vcpus = 2
name = "FreeBSD"
disk = [ '/root/freebsd.img,raw,hda,w',
         '/root/freebsd.iso,raw,hdc:cdrom,r' ]
# boot = "c" # Boot to hard disk image
boot = "d" # Boot to ISO image
usbdevice = 'tablet'
vnc = 1
vnclisten = 0.0.0.0

To boot this DomU with -vvvv level of verbose logging:

# xl -vvvv create freebsd.cfg

# xl list
Name                        ID   Mem VCPUs      State   Time(s)
Domain-0                     0  2047     4     r-----     660.6
FreeBSD                      1  2048     2     r-----       3.5

To connect to this DomU using net/tightvnc:

# vncviewer <IP Address of the Xen Dom0>

The DomU should reboot and shutdown properly but to manually destroy it:

# xl destroy FreeBSD

An example Windows DomU configuration file windows.cfg with 4096M RAM and a two virtual CPUs without networking:

builder = "hvm"
memory = 4096
vcpus = 2
name = "Windows"
disk = [ '/root/freebsd.img,raw,hda,w',
         '/root/windows.iso,raw,hdc:cdrom,r' ]
# boot = "c" # Boot to hard disk image
boot = "d" # Boot to ISO image
vnc = 1
vnclisten = 0.0.0.0
usbdevice = 'tablet'
on_poweroff = 'destroy'
on_reboot = 'restart'
on_crash = 'restart'
acpi = 1

Network Configuration

Xen DomU networking is fundamentally similar to bhyve VirtIO networking and to add a vif virtual network interface attached to bridge0, add to the DomU configuration file:

vif = [ 'bridge=bridge0' ]

To prepare the bridge on the Dom0 with network interface em0:

# ifconfig bridge0 create
# ifconfig bridge0 addm em0 SYNCDHCP
# ifconfig bridge0 up

This will result in the xn0 network interface appearing in the DomU.

Note that the tap and bridge devices must be present or the DomU will not boot. Add these to /etc/rc.conf to suit your unique network.

Serial Configuration

The additional configuration option serial='pty' should allow for attachment to the DomU serial console with xl create -c <DomU>.cfg. Be sure that your DomU is configured for serial console output. For a FreeBSD DomU add console="comconsole" to /boot/loader.conf. CTRL-] will disconnect from the DomU console.

Live migration, save and restore

The version of Xen available in the ports tree supports live migration, save and restore of live guests. Keep in mind that in order to use live migration your virtual disk must be shared between the different Xen Dom0s (NFS, iSCSI…), and it must reside in the same path on the Dom0 filesystem. The usage is the following:

# xl migrate <vm name> <destination host>

The default transport used by xl is ssh, check the man page (xl(1)) for other options.

With save and restore you can take a snapshot of the current guest state and save it to disk as a regular file, including memory and device state. This allows you to restore a guest to a specific state without having to boot it again:

# xl save <vm name> <file>

Then in order to restore the guest:

# xl restore <file>

Of course, you can also take a snapshot of the disk in order to be able to do a complete checkpoint of a guest state.

Suspend and Resume may not work
The console mouse may not work
The FreeBSD kernel debugger may not work
MSI-X PV interrupts are not supported (FreeBSD)
PCI devices with virtual functions work inconsistently (FreeBSD)
PCI Passthrough is not supported (Xen/FreeBSD)
No Xen ARM support
Loading vmm.ko will panic the Dom0, as may VirtualBox
~~Inconsistent SC console support~~ (Fixed in r289033)
~~DomU Save, Restore and Migration is not supported~~ (Fixed in r398918)

Error creating domain 0 on boot indicates that VT-x extensions are unavailable or disabled in BIOS.

If your DomU “FreeBSD” does not boot, check the log in /var/log/xen/qemu-dm-FreeBSD.log

ZFS zvol-backed DomU virtual machines are supported with (must verify):

disk = ['/dev/zvol/zroot/freebsd,raw,hda,rw']
disk = ['phy:/dev/zvol/zroot/linux,xvda,w']

Some DomU virtual machines such as OpenBSD require one virtual CPU.

If your VNC display freezes or does not scale during the various DomU boot stages, try reconnecting.

DomU configuration files are not re-read on reboot.

bhyve virtual machines should boot as DomU unprivileged domains with the caveat that their network devices may change name.

The freebsd-virtualization mailing list and official Xen project resources may be helpful but at this stage you may be the first person to try your given configuration.

The established Xen documentation is generally applicable to FreeBSD.

More Dom0 development information can be found at: https://wiki.xen.org/wiki/FreeBSD_Dom0

More Amazon EC2 unprivileged domain information can be found at: http://www.daemonology.net/freebsd-on-ec2/