22.8. FreeBSD as a Xen™-Host

Xen is a GPLv2-licensed type 1 hypervisor for Intel® and ARM® architectures. FreeBSD has included i386™ and AMD® 64-Bit DomU and Amazon EC2 unprivileged domain (virtual machine) support since FreeBSD 8.0 and includes Dom0 control domain (host) support in FreeBSD 11.0. Support for para-virtualized (PV) domains has been removed from FreeBSD 11 in favor of hardware virtualized (HVM) domains, which provides better performance.

Xen™ is a bare-metal hypervisor, which means that it is the first program loaded after the BIOS. A special privileged guest called the Domain-0 (Dom0 for short) is then started. The Dom0 uses its special privileges to directly access the underlying physical hardware, making it a high-performance solution. It is able to access the disk controllers and network adapters directly. The Xen™ management tools to manage and control the Xen™ hypervisor are also used by the Dom0 to create, list, and destroy VMs. Dom0 provides virtual disks and networking for unprivileged domains, often called DomU. Xen™ Dom0 can be compared to the service console of other hypervisor solutions, while the DomU is where individual guest VMs are run.

Xen™ can migrate VMs between different Xen™ servers. When the two xen hosts share the same underlying storage, the migration can be done without having to shut the VM down first. Instead, the migration is performed live while the DomU is running and there is no need to restart it or plan a downtime. This is useful in maintenance scenarios or upgrade windows to ensure that the services provided by the DomU are still provided. Many more features of Xen™ are listed on the Xen Wiki Overview page. Note that not all features are supported on FreeBSD yet.

22.8.1. Hardware Requirements for Xen™ Dom0

To run the Xen™ hypervisor on a host, certain hardware functionality is required. Hardware virtualized domains require Extended Page Table (EPT) and Input/Output Memory Management Unit (IOMMU) support in the host processor.


In order to run a FreeBSD Xen™ Dom0 the box must be booted using legacy boot (BIOS).

22.8.2. Xen™ Dom0 Control Domain Setup

Users of FreeBSD 11 should install the emulators/xen-kernel47 and sysutils/xen-tools47 packages that are based on Xen version 4.7. Systems running on FreeBSD-12.0 or newer can use Xen 4.11 provided by emulators/xen-kernel411 and sysutils/xen-tools411, respectively.

Configuration files must be edited to prepare the host for the Dom0 integration after the Xen packages are installed. An entry to /etc/sysctl.conf disables the limit on how many pages of memory are allowed to be wired. Otherwise, DomU VMs with higher memory requirements will not run.

# echo 'vm.max_wired=-1' >> /etc/sysctl.conf

Another memory-related setting involves changing /etc/login.conf, setting the memorylocked option to unlimited. Otherwise, creating DomU domains may fail with Cannot allocate memory errors. After making the change to /etc/login.conf, run cap_mkdb to update the capability database. See Section 14.13, “Resource Limits” for details.

# sed -i '' -e 's/memorylocked=64K/memorylocked=unlimited/' /etc/login.conf
# cap_mkdb /etc/login.conf

Add an entry for the Xen™ console to /etc/ttys:

# echo 'xc0     "/usr/libexec/getty Pc"         xterm   onifconsole  secure' >> /etc/ttys

Selecting a Xen™ kernel in /boot/loader.conf activates the Dom0. Xen™ also requires resources like CPU and memory from the host machine for itself and other DomU domains. How much CPU and memory depends on the individual requirements and hardware capabilities. In this example, 8 GB of memory and 4 virtual CPUs are made available for the Dom0. The serial console is also activated and logging options are defined.

The following command is used for Xen 4.7 packages:

# sysrc -f /boot/loader.conf hw.pci.mcfg=0
# sysrc -f /boot/loader.conf if_tap_load="YES"
# sysrc -f /boot/loader.conf xen_kernel="/boot/xen"
# sysrc -f /boot/loader.conf xen_cmdline="dom0_mem=8192M dom0_max_vcpus=4 dom0pvh=1 console=com1,vga com1=115200,8n1 guest_loglvl=all loglvl=all"

For Xen versions 4.11 and higher, the following command should be used instead:

# sysrc -f /boot/loader.conf if_tap_load="YES"
# sysrc -f /boot/loader.conf xen_kernel="/boot/xen"
# sysrc -f /boot/loader.conf xen_cmdline="dom0_mem=8192M dom0_max_vcpus=4 dom0=pvh console=com1,vga com1=115200,8n1 guest_loglvl=all loglvl=all"


Log files that Xen™ creates for the DomU VMs are stored in /var/log/xen. Please be sure to check the contents of that directory if experiencing issues.

Activate the xencommons service during system startup:

# sysrc xencommons_enable=yes

These settings are enough to start a Dom0-enabled system. However, it lacks network functionality for the DomU machines. To fix that, define a bridged interface with the main NIC of the system which the DomU VMs can use to connect to the network. Replace em0 with the host network interface name.

# sysrc cloned_interfaces="bridge0"
# sysrc ifconfig_bridge0="addm em0 SYNCDHCP"
# sysrc ifconfig_em0="up"

Restart the host to load the Xen™ kernel and start the Dom0.

# reboot

After successfully booting the Xen™ kernel and logging into the system again, the Xen™ management tool xl is used to show information about the domains.

# xl list
Name                                        ID   Mem VCPUs      State   Time(s)
Domain-0                                     0  8192     4     r-----     962.0

The output confirms that the Dom0 (called Domain-0) has the ID 0 and is running. It also has the memory and virtual CPUs that were defined in /boot/loader.conf earlier. More information can be found in the Xen™ Documentation. DomU guest VMs can now be created.

22.8.3. Xen™ DomU Guest VM Configuration

Unprivileged domains consist of a configuration file and virtual or physical hard disks. Virtual disk storage for the DomU can be files created by truncate(1) or ZFS volumes as described in Section 20.4.2, “Creating and Destroying Volumes”. In this example, a 20 GB volume is used. A VM is created with the ZFS volume, a FreeBSD ISO image, 1 GB of RAM and two virtual CPUs. The ISO installation file is retrieved with fetch(1) and saved locally in a file called freebsd.iso.

# fetch ftp://ftp.freebsd.org/pub/FreeBSD/releases/ISO-IMAGES/12.0/FreeBSD-12.0-RELEASE-amd64-bootonly.iso -o freebsd.iso

A ZFS volume of 20 GB called xendisk0 is created to serve as the disk space for the VM.

# zfs create -V20G -o volmode=dev zroot/xendisk0

The new DomU guest VM is defined in a file. Some specific definitions like name, keymap, and VNC connection details are also defined. The following freebsd.cfg contains a minimum DomU configuration for this example:

# cat freebsd.cfg
builder = "hvm" 1
name = "freebsd" 2
memory = 1024 3
vcpus = 2 4
vif = [ 'mac=00:16:3E:74:34:32,bridge=bridge0' ] 5
disk = [
'/dev/zvol/tank/xendisk0,raw,hda,rw', 6
'/root/freebsd.iso,raw,hdc:cdrom,r' 7
vnc = 1 8
vnclisten = ""
serial = "pty"
usbdevice = "tablet"

These lines are explained in more detail:


This defines what kind of virtualization to use. hvm refers to hardware-assisted virtualization or hardware virtual machine. Guest operating systems can run unmodified on CPUs with virtualization extensions, providing nearly the same performance as running on physical hardware. generic is the default value and creates a PV domain.


Name of this virtual machine to distinguish it from others running on the same Dom0. Required.


Quantity of RAM in megabytes to make available to the VM. This amount is subtracted from the hypervisor's total available memory, not the memory of the Dom0.


Number of virtual CPUs available to the guest VM. For best performance, do not create guests with more virtual CPUs than the number of physical CPUs on the host.


Virtual network adapter. This is the bridge connected to the network interface of the host. The mac parameter is the MAC address set on the virtual network interface. This parameter is optional, if no MAC is provided Xen™ will generate a random one.


Full path to the disk, file, or ZFS volume of the disk storage for this VM. Options and multiple disk definitions are separated by commas.


Defines the Boot medium from which the initial operating system is installed. In this example, it is the ISO imaged downloaded earlier. Consult the Xen™ documentation for other kinds of devices and options to set.


Options controlling VNC connectivity to the serial console of the DomU. In order, these are: active VNC support, define IP address on which to listen, device node for the serial console, and the input method for precise positioning of the mouse and other input methods. keymap defines which keymap to use, and is english by default.

After the file has been created with all the necessary options, the DomU is created by passing it to xl create as a parameter.

# xl create freebsd.cfg


Each time the Dom0 is restarted, the configuration file must be passed to xl create again to re-create the DomU. By default, only the Dom0 is created after a reboot, not the individual VMs. The VMs can continue where they left off as they stored the operating system on the virtual disk. The virtual machine configuration can change over time (for example, when adding more memory). The virtual machine configuration files must be properly backed up and kept available to be able to re-create the guest VM when needed.

The output of xl list confirms that the DomU has been created.

# xl list
Name                                        ID   Mem VCPUs      State   Time(s)
Domain-0                                     0  8192     4     r-----  1653.4
freebsd                                      1  1024     1     -b----   663.9

To begin the installation of the base operating system, start the VNC client, directing it to the main network address of the host or to the IP address defined on the vnclisten line of freebsd.cfg. After the operating system has been installed, shut down the DomU and disconnect the VNC viewer. Edit freebsd.cfg, removing the line with the cdrom definition or commenting it out by inserting a # character at the beginning of the line. To load this new configuration, it is necessary to remove the old DomU with xl destroy, passing either the name or the id as the parameter. Afterwards, recreate it using the modified freebsd.cfg.

# xl destroy freebsd
# xl create freebsd.cfg

The machine can then be accessed again using the VNC viewer. This time, it will boot from the virtual disk where the operating system has been installed and can be used as a virtual machine.

22.8.4. Troubleshooting

This section contains basic information in order to help troubleshoot issues found when using FreeBSD as a Xen™ host or guest. Host Boot Troubleshooting

Please note that the following troubleshooting tips are intended for Xen™ 4.11 or newer. If you are still using Xen™ 4.7 and having issues consider migrating to a newer version of Xen™.

In order to troubleshoot host boot issues you will likely need a serial cable, or a debug USB cable. Verbose Xen™ boot output can be obtained by adding options to the xen_cmdline option found in loader.conf. A couple of relevant debug options are:

  • iommu=debug: can be used to print additional diagnostic information about the iommu.

  • dom0=verbose: can be used to print additional diagnostic information about the dom0 build process.

  • sync_console: flag to force synchronous console output. Useful for debugging to avoid losing messages due to rate limiting. Never use this option in production environments since it can allow malicious guests to perform DoS attacks against Xen™ using the console.

FreeBSD should also be booted in verbose mode in order to identify any issues. To activate verbose booting, run this command:

# sysrc -f /boot/loader.conf boot_verbose="YES"

If none of these options help solving the problem, please send the serial boot log to and for further analysis. Guest Creation Troubleshooting

Issues can also arise when creating guests, the following attempts to provide some help for those trying to diagnose guest creation issues.

The most common cause of guest creation failures is the xl command spitting some error and exiting with a return code different than 0. If the error provided is not enough to help identify the issue, more verbose output can also be obtained from xl by using the v option repeatedly.

# xl -vvv create freebsd.cfg
Parsing config from freebsd.cfg
libxl: debug: libxl_create.c:1693:do_domain_create: Domain 0:ao 0x800d750a0: create: how=0x0 callback=0x0 poller=0x800d6f0f0
libxl: debug: libxl_device.c:397:libxl__device_disk_set_backend: Disk vdev=xvda spec.backend=unknown
libxl: debug: libxl_device.c:432:libxl__device_disk_set_backend: Disk vdev=xvda, using backend phy
libxl: debug: libxl_create.c:1018:initiate_domain_create: Domain 1:running bootloader
libxl: debug: libxl_bootloader.c:328:libxl__bootloader_run: Domain 1:not a PV/PVH domain, skipping bootloader
libxl: debug: libxl_event.c:689:libxl__ev_xswatch_deregister: watch w=0x800d96b98: deregister unregistered
domainbuilder: detail: xc_dom_allocate: cmdline="", features=""
domainbuilder: detail: xc_dom_kernel_file: filename="/usr/local/lib/xen/boot/hvmloader"
domainbuilder: detail: xc_dom_malloc_filemap    : 326 kB
libxl: debug: libxl_dom.c:988:libxl__load_hvm_firmware_module: Loading BIOS: /usr/local/share/seabios/bios.bin

If the verbose output does not help diagnose the issue there are also QEMU and Xen™ toolstack logs in /var/log/xen. Note that the name of the domain is appended to the log name, so if the domain is named freebsd you should find a /var/log/xen/xl-freebsd.log and likely a /var/log/xen/qemu-dm-freebsd.log. Both log files can contain useful information for debugging. If none of this helps solve the issue, please send the description of the issue you are facing and as much information as possible to and in order to get help.

All FreeBSD documents are available for download at https://download.freebsd.org/ftp/doc/

Questions that are not answered by the documentation may be sent to <freebsd-questions@FreeBSD.org>.
Send questions about this document to <freebsd-doc@FreeBSD.org>.