Chapter 24. Virtualization

24.1. Synopsis

Virtualization software allows multiple operating systems to run simultaneously on the same computer. Such software systems for PCs often involve a host operating system which runs the virtualization software and supports any number of guest operating systems.

After reading this chapter, you will know:

  • The difference between a host operating system and a guest operating system.

  • How to install FreeBSD on the following virtualization platforms:

    • Parallels Desktop(Apple® macOS®)

    • VMware Fusion(Apple® macOS®)

    • VirtualBox™(Microsoft® Windows®, Intel®-based Apple® macOS®, Linux)

    • bhyve(FreeBSD)

  • How to tune a FreeBSD system for best performance under virtualization.

Before reading this chapter, you should:

24.2. FreeBSD as a Guest on Parallels Desktop for macOS®

Parallels Desktop for Mac® is a commercial software product available for Apple® Mac® computers running macOS® 10.14.6 or higher. FreeBSD is a fully supported guest operating system. Once Parallels has been installed on macOS®, the user must configure a virtual machine and then install the desired guest operating system.

24.2.1. Installing FreeBSD on Parallels Desktop on Mac®

The first step in installing FreeBSD on Parallels is to create a new virtual machine for installing FreeBSD.

Choose Install Windows or another OS from a DVD or image file and proceed.

Parallels setup wizard showing Install Windows or another OS from a DVD or image file chosen

Select the FreeBSD image file.

Parallels setup wizard showing FreeBSD image file selected

Choose Other as operating system.

Choosing FreeBSD will cause boot error on startup.

Parallels setup wizard showing Other selected as operating system

Name the virtual machine and check Customize settings before installation

Parallels setup wizard showing the checkbox checked for customizing settings before installation

When the configuration window pops up, go to Hardware tab, choose Boot order, and click Advanced. Then, choose EFI 64-bit as BIOS.

Parallels setup wizard showing EFI 64-bit chosen as BIOS

Click OK, close the configuration window, and click Continue.

Parallels setup wizard showing the summary of the new virtual machine

The virtual machine will automatically boot. Install FreeBSD following the general steps.

FreeBSD booted on Parallels

24.2.2. Configuring FreeBSD on Parallels

After FreeBSD has been successfully installed on macOS® X with Parallels, there are a number of configuration steps that can be taken to optimize the system for virtualized operation.

  1. Set Boot Loader Variables

    The most important step is to reduce the kern.hz tunable to reduce the CPU utilization of FreeBSD under the Parallels environment. This is accomplished by adding the following line to /boot/loader.conf:


    Without this setting, an idle FreeBSD Parallels guest will use roughly 15% of the CPU of a single processor iMac®. After this change the usage will be closer to 5%.

    If installing FreeBSD 14.0 or later, and CPU utilization is still high, add the following additional line to /boot/loader.conf:

  2. Create a New Kernel Configuration File

    All SCSI, FireWire, and USB device drivers can be removed from a custom kernel configuration file. Parallels provides a virtual network adapter used by the ed(4) driver, so all network devices except for ed(4) and miibus(4) can be removed from the kernel.

  3. Configure Networking

    The most basic networking setup uses DHCP to connect the virtual machine to the same local area network as the host Mac®. This can be accomplished by adding ifconfig_ed0="DHCP" to /etc/rc.conf. More advanced networking setups are described in Advanced Networking.

24.3. FreeBSD as a Guest on VMware Fusion for macOS®

VMware Fusion for Mac® is a commercial software product available for Apple® Mac® computers running macOS® 12 or higher. FreeBSD is a fully supported guest operating system. Once VMware Fusion has been installed on macOS®, the user can configure a virtual machine and then install the desired guest operating system.

24.3.1. Installing FreeBSD on VMware Fusion

The first step is to start VMware Fusion which will load the Virtual Machine Library. Click +→New to create the virtual machine:

vmware freebsd01

This will load the New Virtual Machine Assistant. Choose Create a custom virtual machine and click Continue to proceed:

vmware freebsd02

Select Other as the Operating System and either FreeBSD X or FreeBSD X 64-bit, as the Version when prompted:

vmware freebsd03

Choose the firmware(UEFI is recommended):

vmware freebsd04

Choose Create a new virtual disk and click Continue:

vmware freebsd05

Check the configuration and click Finish:

vmware freebsd06

Choose the name of the virtual machine and the directory where it should be saved:

vmware freebsd07

Press command+E to open virtual machine settings and click CD/DVD:

vmware freebsd08

Choose FreeBSD ISO image or from a CD/DVD:

vmware freebsd09

Start the virtual machine:

vmware freebsd10

Install FreeBSD as usual:

vmware freebsd11

Once the install is complete, the settings of the virtual machine can be modified, such as memory usage and the number of CPUs the virtual machine will have access to:

The System Hardware settings of the virtual machine cannot be modified while the virtual machine is running.

vmware freebsd12

The status of the CD-ROM device. Normally the CD/DVD/ISO is disconnected from the virtual machine when it is no longer needed.

vmware freebsd09

The last thing to change is how the virtual machine will connect to the network. To allow connections to the virtual machine from other machines besides the host, choose Connect directly to the physical network (Bridged). Otherwise, Share the host’s internet connection (NAT) is preferred so that the virtual machine can have access to the Internet, but the network cannot access the virtual machine.

vmware freebsd13

After modifying the settings, boot the newly installed FreeBSD virtual machine.

24.3.2. Configuring FreeBSD on VMware Fusion

After FreeBSD has been successfully installed on macOS® X with VMware Fusion, there are a number of configuration steps that can be taken to optimize the system for virtualized operation.

  1. Set Boot Loader Variables

    The most important step is to reduce the kern.hz tunable to reduce the CPU utilization of FreeBSD under the VMware Fusion environment. This is accomplished by adding the following line to /boot/loader.conf:


    Without this setting, an idle FreeBSD VMware Fusion guest will use roughly 15% of the CPU of a single processor iMac®. After this change, the usage will be closer to 5%.

  2. Create a New Kernel Configuration File

    All FireWire, and USB device drivers can be removed from a custom kernel configuration file. VMware Fusion provides a virtual network adapter used by the em(4) driver, so all network devices except for em(4) can be removed from the kernel.

  3. Configure Networking

    The most basic networking setup uses DHCP to connect the virtual machine to the same local area network as the host Mac®. This can be accomplished by adding ifconfig_em0="DHCP" to /etc/rc.conf. More advanced networking setups are described in Advanced Networking.

  4. Install drivers and open-vm-tools

    To run FreeBSD smoothly on VMWare, drivers should be installed:

    # pkg install xf86-video-vmware xf86-input-vmmouse open-vm-tools

24.4. FreeBSD as a Guest on VirtualBox™

FreeBSD works well as a guest in VirtualBox™. The virtualization software is available for most common operating systems, including FreeBSD itself.

The VirtualBox™ guest additions provide support for:

  • Clipboard sharing.

  • Mouse pointer integration.

  • Host time synchronization.

  • Window scaling.

  • Seamless mode.

These commands are run in the FreeBSD guest.

First, install the emulators/virtualbox-ose-additions package or port in the FreeBSD guest. This will install the port:

# cd /usr/ports/emulators/virtualbox-ose-additions && make install clean

Add these lines to /etc/rc.conf:


If ntpd(8) or ntpdate(8) is used, disable host time synchronization:


Xorg will automatically recognize the vboxvideo driver. It can also be manually entered in /etc/X11/xorg.conf:

Section "Device"
	Identifier "Card0"
	Driver "vboxvideo"
	VendorName "InnoTek Systemberatung GmbH"
	BoardName "VirtualBox Graphics Adapter"

To use the vboxmouse driver, adjust the mouse section in /etc/X11/xorg.conf:

Section "InputDevice"
	Identifier "Mouse0"
	Driver "vboxmouse"

Shared folders for file transfers between host and VM are accessible by mounting them using mount_vboxvfs. A shared folder can be created on the host using the VirtualBox GUI or via vboxmanage. For example, to create a shared folder called myshare under /mnt/bsdboxshare for the VM named BSDBox, run:

# vboxmanage sharedfolder add 'BSDBox' --name myshare --hostpath /mnt/bsdboxshare

Note that the shared folder name must not contain spaces. Mount the shared folder from within the guest system like this:

# mount_vboxvfs -w myshare /mnt

24.5. FreeBSD as a Host with VirtualBox™

VirtualBox™ is an actively developed, complete virtualization package, that is available for most operating systems including Windows®, macOS®, Linux® and FreeBSD. It is equally capable of running Windows® or UNIX®-like guests. It is released as open source software, but with closed-source components available in a separate extension pack. These components include support for USB 2.0 devices. More information may be found on the Downloads page of the VirtualBox™ wiki. Currently, these extensions are not available for FreeBSD.

24.5.1. Installing VirtualBox™

VirtualBox™ is available as a FreeBSD package or port in emulators/virtualbox-ose. The port can be installed using these commands:

# cd /usr/ports/emulators/virtualbox-ose
# make install clean

One useful option in the port’s configuration menu is the GuestAdditions suite of programs. These provide a number of useful features in guest operating systems, like mouse pointer integration (allowing the mouse to be shared between host and guest without the need to press a special keyboard shortcut to switch) and faster video rendering, especially in Windows® guests. The guest additions are available in the Devices menu, after the installation of the guest is finished.

A few configuration changes are needed before VirtualBox™ is started for the first time. The port installs a kernel module in /boot/modules which must be loaded into the running kernel:

# kldload vboxdrv

To ensure the module is always loaded after a reboot, add this line to /boot/loader.conf:


To use the kernel modules that allow bridged or host-only networking, add this line to /etc/rc.conf and reboot the computer:


The vboxusers group is created during installation of VirtualBox™. All users that need access to VirtualBox™ will have to be added as members of this group. pw can be used to add new members:

# pw groupmod vboxusers -m yourusername

The default permissions for /dev/vboxnetctl are restrictive and need to be changed for bridged networking:

# chown root:vboxusers /dev/vboxnetctl
# chmod 0660 /dev/vboxnetctl

To make this permissions change permanent, add these lines to /etc/devfs.conf:

own     vboxnetctl root:vboxusers
perm    vboxnetctl 0660

To launch VirtualBox™, type from an Xorg session:

% VirtualBox

For more information on configuring and using VirtualBox™, refer to the official website. For FreeBSD-specific information and troubleshooting instructions, refer to the relevant page in the FreeBSD wiki.

24.5.2. VirtualBox™ USB Support

VirtualBox™ can be configured to pass USB devices through to the guest operating system. The host controller of the OSE version is limited to emulating USB 1.1 devices until the extension pack supporting USB 2.0 and 3.0 devices becomes available on FreeBSD.

For VirtualBox™ to be aware of USB devices attached to the machine, the user needs to be a member of the operator group.

# pw groupmod operator -m yourusername

Then, add the following to /etc/devfs.rules, or create this file if it does not exist yet:

add path 'usb/*' mode 0660 group operator

To load these new rules, add the following to /etc/rc.conf:


Then, restart devfs:

# service devfs restart

Restart the login session and VirtualBox™ for these changes to take effect, and create USB filters as necessary.

24.5.3. VirtualBox™ Host DVD/CD Access

Access to the host DVD/CD drives from guests is achieved through the sharing of the physical drives. Within VirtualBox™, this is set up from the Storage window in the Settings of the virtual machine. If needed, create an empty IDECD/DVD device first. Then choose the Host Drive from the popup menu for the virtual CD/DVD drive selection. A checkbox labeled Passthrough will appear. This allows the virtual machine to use the hardware directly. For example, audio CDs or the burner will only function if this option is selected.

In order for users to be able to use VirtualBox™DVD/CD functions, they need access to /dev/xpt0, /dev/cdN, and /dev/passN. This is usually achieved by making the user a member of operator. Permissions to these devices have to be corrected by adding these lines to /etc/devfs.conf:

perm cd* 0660
perm xpt0 0660
perm pass* 0660
# service devfs restart

24.6. FreeBSD as a Host with bhyve

The bhyve BSD-licensed hypervisor became part of the base system with FreeBSD 10.0-RELEASE. This hypervisor supports several guests, including FreeBSD, OpenBSD, many Linux® distributions, and Microsoft Windows®. By default, bhyve provides access to a serial console and does not emulate a graphical console. Virtualization offload features of newer CPUs are used to avoid the legacy methods of translating instructions and manually managing memory mappings.

The bhyve design requires

  • an Intel® processor that supports Intel Extended Page Tables (EPT),

  • or an AMD® processor that supports AMD Rapid Virtualization Indexing (RVI), or Nested Page Tables (NPT),

  • or an ARM® aarch64 CPU.

Only pure ARMv8.0 virtualization is supported on ARM, the Virtualization Host Extensions are not currently used. Hosting Linux® guests or FreeBSD guests with more than one vCPU requires VMX unrestricted mode support (UG).

The easiest way to tell if an Intel or AMD processor supports bhyve is to run dmesg or look in /var/run/dmesg.boot for the POPCNT processor feature flag on the Features2 line for AMD® processors or EPT and UG on the VT-x line for Intel® processors.

24.6.1. Preparing the Host

The first step to creating a virtual machine in bhyve is configuring the host system. First, load the bhyve kernel module:

# kldload vmm

There are several ways to connect a virtual machine guest to a host’s network; one straightforward way to accomplish this is to create a tap interface for the network device in the virtual machine to attach to. For the network device to participate in the network, also create a bridge interface containing the tap interface and the physical interface as members. In this example, the physical interface is igb0:

# ifconfig tap0 create
# sysctl 0 -> 1
# ifconfig bridge0 create
# ifconfig bridge0 addm igb0 addm tap0
# ifconfig bridge0 up

24.6.2. Creating a FreeBSD Guest

Create a file to use as the virtual disk for the guest machine. Specify the size and name of the virtual disk:

# truncate -s 16G guest.img

Download an installation image of FreeBSD to install:

# fetch
FreeBSD-14.0-RELEASE-amd64-bootonly.iso                426 MB   16 MBps    22s

FreeBSD comes with an example script for running a virtual machine in bhyve. It will start the virtual machine and run it in a loop, so it will automatically restart if it crashes. takes several options to control the configuration of the machine, including:

  • -c controls the number of virtual CPUs,

  • -m limits the amount of memory available to the guest,

  • -t defines which tap device to use,

  • -d indicates which disk image to use,

  • -i tells bhyve to boot from the CD image instead of the disk, and

  • -I defines which CD image to use.

The last parameter is the name of the virtual machine and is used to track the running machines. The following command lists all available program argument options:

# sh /usr/share/examples/bhyve/ -h

This example starts the virtual machine in installation mode:

# sh /usr/share/examples/bhyve/ -c 1 -m 1024M -t tap0 -d guest.img \
     -i -I FreeBSD-14.0-RELEASE-amd64-bootonly.iso guestname

The virtual machine will boot and start the installer. After installing a system in the virtual machine, when the system asks about dropping into a shell at the end of the installation, choose Yes.

Reboot the virtual machine. While rebooting the virtual machine causes bhyve to exit, the script runs bhyve in a loop and will automatically restart it. When this happens, choose the reboot option from the boot loader menu to escape the loop. Now the guest can be started from the virtual disk:

# sh /usr/share/examples/bhyve/ -c 4 -m 1024M -t tap0 -d guest.img guestname

24.6.3. Creating a Linux® Guest

Linux guests can be booted either like any other regular UEFI-based guest virtual machine, or alternatively, you can make use of the sysutils/grub2-bhyve port.

To do this, first ensure that the port is installed, then create a file to use as the virtual disk for the guest machine:

# truncate -s 16G linux.img

Starting a Linux virtual machine with grub2-bhyve is a two-step process.

  1. First a kernel must be loaded, then the guest can be started.

  2. The Linux® kernel is loaded with sysutils/grub2-bhyve.

Create a that grub will use to map the virtual devices to the files on the host system:

(hd0) ./linux.img
(cd0) ./somelinux.iso

Use sysutils/grub2-bhyve to load the Linux® kernel from the ISO image:

# grub-bhyve -m -r cd0 -M 1024M linuxguest

This will start grub. If the installation CD contains a grub.cfg, a menu will be displayed. If not, the vmlinuz and initrd files must be located and loaded manually:

grub> ls
(hd0) (cd0) (cd0,msdos1) (host)
grub> ls (cd0)/isolinux boot.msg grub.conf initrd.img isolinux.bin isolinux.cfg memtest
splash.jpg TRANS.TBL vesamenu.c32 vmlinuz
grub> linux (cd0)/isolinux/vmlinuz
grub> initrd (cd0)/isolinux/initrd.img
grub> boot

Now that the Linux® kernel is loaded, the guest can be started:

# bhyve -A -H -P -s 0:0,hostbridge -s 1:0,lpc -s 2:0,virtio-net,tap0 \
    -s 3:0,virtio-blk,./linux.img -s 4:0,ahci-cd,./somelinux.iso \
    -l com1,stdio -c 4 -m 1024M linuxguest

The system will boot and start the installer. After installing a system in the virtual machine, reboot the virtual machine. This will cause bhyve to exit. The instance of the virtual machine needs to be destroyed before it can be started again:

# bhyvectl --destroy --vm=linuxguest

Now the guest can be started directly from the virtual disk. Load the kernel:

# grub-bhyve -m -r hd0,msdos1 -M 1024M linuxguest
grub> ls
(hd0) (hd0,msdos2) (hd0,msdos1) (cd0) (cd0,msdos1) (host)
(lvm/VolGroup-lv_swap) (lvm/VolGroup-lv_root)
grub> ls (hd0,msdos1)/
lost+found/ grub/ efi/ config-2.6.32-431.el6.x
86_64 symvers-2.6.32-431.el6.x86_64.gz vmlinuz-2.6.32-431.el6.x86_64
grub> linux (hd0,msdos1)/vmlinuz-2.6.32-431.el6.x86_64 root=/dev/mapper/VolGroup-lv_root
grub> initrd (hd0,msdos1)/initramfs-2.6.32-431.el6.x86_64.img
grub> boot

Boot the virtual machine:

# bhyve -A -H -P -s 0:0,hostbridge -s 1:0,lpc -s 2:0,virtio-net,tap0 \
    -s 3:0,virtio-blk,./linux.img -l com1,stdio -c 4 -m 1024M linuxguest

Linux® will now boot in the virtual machine and eventually present you with the login prompt. Login and use the virtual machine. When you are finished, reboot the virtual machine to exit bhyve. Destroy the virtual machine instance:

# bhyvectl --destroy --vm=linuxguest

24.6.4. Booting bhyve Virtual Machines with UEFI Firmware

In addition to bhyveload and grub-bhyve, the bhyve hypervisor can also boot virtual machines using the UEFI firmware. This option may support guest operating systems that are not supported by the other loaders.

To make use of the UEFI support in bhyve, first obtain the UEFI firmware images. This can be done by installing sysutils/bhyve-firmware port or package.

With the firmware in place, add the flags -l bootrom,/path/to/firmware to your bhyve command line. The actual bhyve command may look like this:

# bhyve -AHP -s 0:0,hostbridge -s 1:0,lpc \
  	-s 2:0,virtio-net,tap1 -s 3:0,virtio-blk,./disk.img \
	-s 4:0,ahci-cd,./install.iso -c 4 -m 1024M \
	-l bootrom,/usr/local/share/uefi-firmware/BHYVE_UEFI.fd \

To allow a guest to store UEFI variables, you can use a variables file appended to the -l flag. Note that bhyve will write guest modifications to the given variables file. Therefore, be sure to first create a per-guest-copy of the variables template file:

# cp /usr/local/share/uefi-firmware/BHYVE_UEFI_VARS.fd /path/to/vm-image/BHYVE_UEFI_VARS.fd

Then, add that variables file into your bhyve arguments:

# bhyve -AHP -s 0:0,hostbridge -s 1:0,lpc \
  	-s 2:0,virtio-net,tap1 -s 3:0,virtio-blk,./disk.img \
	-s 4:0,ahci-cd,./install.iso -c 4 -m 1024M \
	-l bootrom,/usr/local/share/uefi-firmware/BHYVE_UEFI.fd,/path/to/vm-image/BHYVE_UEFI_VARS.fd \

Some Linux distributions require the use of UEFI variables to store the path for their UEFI boot file (using linux64.efi or grubx64.efi instead of bootx64.efi, for example). It is therefore recommended to use a variables file for Linux virtual machines to avoid having to manually alter the boot partition files.

To view or modify the variables file contents, use efivar(8) from the host.

sysutils/bhyve-firmware also contains a CSM-enabled firmware, to boot guests with no UEFI support in legacy BIOS mode:

# bhyve -AHP -s 0:0,hostbridge -s 1:0,lpc \
  	-s 2:0,virtio-net,tap1 -s 3:0,virtio-blk,./disk.img \
	-s 4:0,ahci-cd,./install.iso -c 4 -m 1024M \
	-l bootrom,/usr/local/share/uefi-firmware/BHYVE_UEFI_CSM.fd \

24.6.5. Graphical UEFI Framebuffer for bhyve Guests

The UEFI firmware support is particularly useful with predominantly graphical guest operating systems such as Microsoft Windows®.

Support for the UEFI-GOP framebuffer may also be enabled with the -s 29,fbuf,tcp= flags. The framebuffer resolution may be configured with w=800 and h=600, and bhyve can be instructed to wait for a VNC connection before booting the guest by adding wait. The framebuffer may be accessed from the host or over the network via the VNC protocol. Additionally, -s 30,xhci,tablet can be added to achieve precise mouse cursor synchronization with the host.

The resulting bhyve command would look like this:

# bhyve -AHP -s 0:0,hostbridge -s 31:0,lpc \
  	-s 2:0,virtio-net,tap1 -s 3:0,virtio-blk,./disk.img \
	-s 4:0,ahci-cd,./install.iso -c 4 -m 1024M \
	-s 29,fbuf,tcp=,w=800,h=600,wait \
	-s 30,xhci,tablet \
	-l bootrom,/usr/local/share/uefi-firmware/BHYVE_UEFI.fd \

Note, in BIOS emulation mode, the framebuffer will cease receiving updates once control is passed from firmware to guest operating system.

24.6.6. Creating a Microsoft Windows® Guest

Setting up a guest for Windows versions 10 or earlier can be done directly from the original installation media and is a relatively straightforward process. Aside from minimum resource requirements, running Windows as guest requires

  • wiring virtual machine memory (flag -w) and

  • booting with an UEFI bootrom.

An example for booting a virtual machine guest with a Windows installation ISO:

bhyve \
      -c 2 \
      -s 0,hostbridge \
      -s 3,nvme,windows2016.img \
      -s 4,ahci-cd,install.iso \
      -s 10,virtio-net,tap0 \
      -s 31,lpc \
      -s 30,xhci,tablet \
      -l bootrom,/usr/local/share/uefi-firmware/BHYVE_UEFI.fd \
      -m 8G -H -w \

Only one or two VCPUs should be used during installation but this number can be increased once Windows is installed.

VirtIO drivers must be installed to use the defined virtio-net network interface. An alternative is to switch to E1000 (Intel E82545) emulation by changing virtio-net to e1000 in the above command line. However, performance will be impacted. Creating a Windows 11 Guest

Beginning with Windows 11, Microsoft introduced a hardware requirement for a TPM 2 module. bhyve supports passing a hardware TPM through to a guest. The installation media can be modified to disable the relevant hardware checks. A detailed description for this process can be found on the FreeBSD Wiki.

Modifying Windows installation media and running Windows guests without a TPM module are unsupported by the manufacturer. Consider your application and use case before implementing such approaches.

24.6.7. Using ZFS with bhyve Guests

If ZFS is available on the host machine, using ZFS volumes instead of disk image files can provide significant performance benefits for the guest VMs. A ZFS volume can be created by:

# zfs create -V16G -o volmode=dev zroot/linuxdisk0

When starting the VM, specify the ZFS volume as the disk drive:

# bhyve -A -H -P -s 0:0,hostbridge -s 1:0,lpc -s 2:0,virtio-net,tap0 \
  	-s3:0,virtio-blk,/dev/zvol/zroot/linuxdisk0 \
	-l com1,stdio -c 4 -m 1024M linuxguest

If you are using ZFS for the host as well as inside a guest, keep in mind the competing memory pressure of both systems caching the virtual machine’s contents. To alleviate this, consider setting the host’s ZFS filesystems to use metadata-only cache. To do this, apply the following settings to ZFS filesystems on the host, replacing <name> with the name of the specific zvol dataset name of the virtual machine.

# zfs set primarycache=metadata <name>

24.6.8. Creating a Virtual Machine Snapshot

Modern hypervisors allow their users to create "snapshots" of their state; such a snapshot includes a guest’s disk, CPU, and memory contents. A snapshot can usually be taken independent of whether the guest is running or shut down. One can then reset and return the virtual machine to the precise state when the snapshot was taken. ZFS Snapshots

Using ZFS volumes as the backing storage for a virtual machine enables the snapshotting of the guest’s disk. For example:

zfs snapshot zroot/path/to/zvol@snapshot_name

Though it is possible to snapshot a ZFS volume this way while the guest is running, keep in mind that the contents of the virtual disk may be in an inconsistent state while the guest is active. It is therefore recommended to first shutdown or pause the guest before executing this command. Pausing a guest is not supported by default and needs to be enabled first (see Memory and CPU Snapshots)

Rolling back a ZFS zvol to a snapshot while a virtual machine is using it may corrupt the file system contents and crash the guest. All unsaved data in the guest will be lost and modifications since the last snapshot may get destroyed.

A second rollback may be required once the virtual machine is shut down to restore the file system to a useable state. This in turn will ultimately destroy any changes made after the snapshot. Memory and CPU Snapshots (Experimental Feature)

As of FreeBSD 13, bhyve has an experimental "snapshot" feature for dumping a guest’s memory and CPU state to a file and then halting the virtual machine. The guest can be resumed from the snapshot file contents later.

However, this feature is not enabled by default and requires the system to be rebuilt from source. See Building from Source for an in-depth description on the process of compiling the kernel with custom options.

The functionality is not ready for production use and limited to work for specific virtual machine configurations. There are multiple limitations:

  • nvme and virtio-blk storage backends do not work yet

  • snapshots are only supported when the guest uses a single kind of each device, i.e. if there is more than one ahci-hd disk attached, snapshot creation will fail

  • additionally, the feature may be reasonably stable on Intel, but it probably won’t work on AMD CPUs.

Make sure the /usr/src directory is up-to date before taking the following steps. See Updating the Source for the detailed procedure how to do this.

First, add the following to /etc/src.conf:


If the system was partially or wholly rebuilt, it is recommended to run

# cd /usr/src
# make cleanworld

before proceeding.

Then follow the steps outlined in the Quick Start section of the Updating FreeBSD from Source chapter to build and install world and kernel.

To verify successful activation of the snapshot feature, enter

# bhyvectl --usage

and check if the output lists a --suspend flag. If the flag is missing, the feature did not activate correctly.

Then, you can snapshot and suspend a running virtual machine of your choice:

# bhyvectl --vm=vmname --suspend=/path/to/snapshot/filename

Provide an absolute path and filename to --suspend. Otherwise, bhyve will write snapshot data to whichever directory bhyve was started from.

Make sure to write the snapshot data to a secure directory. The generated output contains a full memory dump of the guest and may thus contain sensitive data (i.e. passwords)!

This creates three files:

  • memory snapshot - named like the input to --suspend

  • kernel file - name like the input to --suspend with the suffix .kern

  • metadata - contains meta data about the system state, named with the suffix .meta

To restore a guest from a snapshot, use the -r flag with bhyve:

# bhyve -r /path/to/snapshot/filename

Restoring a guest snapshot on a different CPU architecture will not work. Generally, attempting to restore on a system not identical to the snapshot creator will likely fail.

24.6.9. Jailing bhyve

For improved security and separation of virtual machines from the host operating system, it is possible to run bhyve in a jail. See Jails for an in-depth description of jails and their security benefits. Creating a Jail for bhyve

First, create a jail environment. If using a UFS file system, simply run:

# mkdir -p /jails/bhyve

If using a ZFS filesystem, use the following commands:

# zfs create zroot/jails
# zfs create zroot/jails/bhyve

Then create a ZFS zvol for the virtual machine bhyvevm0:

# zfs create zroot/vms
# zfs create -V 20G zroot/vms/bhyvevm0

If not using ZFS, use the following commands to create a disk image file directly in the jail directory structure:

# mkdir /jails/bhyve/vms
# truncate -s 20G /jails/bhyve/vms/bhyvevm0

Download a FreeBSD image, preferably a version equal to or older than the host and extract it into the jail directory:

# cd /jails
# fetch -o base.txz
# tar -C /jails/bhyve -xvf base.txz

Running a higher FreeBSD version in a jail than the host is unsupported (i.e. running 14.0-RELEASE in a jail, embedded in a 13.2-RELEASE host).

Next, add a devfs ruleset to /etc/devfs.rules:

add include $devfsrules_hide_all
add include $devfsrules_unhide_login
add path 'urandom' unhide
add path 'random' unhide
add path 'crypto' unhide
add path 'shm' unhide
add path 'zero' unhide
add path 'null' unhide
add path 'mem' unhide
add path 'vmm' unhide
add path 'vmm/*' unhide
add path '' unhide
add path '*' unhide
add path 'nmdmbhyve*' unhide
add path 'zvol' unhide
add path 'zvol/zroot' unhide
add path 'zvol/zroot/vms' unhide
add path 'zvol/zroot/vms/bhyvevm0' unhide
add path 'zvol/zroot/vms/bhyvevm1' unhide
add path 'tap10*' unhide

If there’s another devfs rule with the numeric ID 100 in your /etc/devfs.rules file, replace the one in the listing with another yet unused ID number.

If not using a ZFS filesystem, skip the related zvol rules in /etc/devfs.rules:

add path 'zvol' unhide
add path 'zvol/zroot' unhide
add path 'zvol/zroot/vms' unhide
add path 'zvol/zroot/vms/bhyvevm0' unhide
add path 'zvol/zroot/vms/bhyvevm1' unhide

These rules will cause bhyve to

  • create a virtual machine with disk volumes called bhyvevm0 and bhyvevm1,

  • use tap network interfaces with the name prefix tap10. That means, valid interface names will be tap10, tap100, tap101, …​ tap109, tap1000 and so on.

    Limiting the access to a subset of possible tap interface names will prevent the jail (and thus bhyve) from seeing tap interfaces of the host and other jails.

  • use nmdm devices prefixed with "bhyve", i.e. /dev/nmdmbhyve0.

Those rules can be expanded and varied with different guest and interface names as desired.

If you intend to use bhyve on the host as well as in a one or more jails, remember that tap and nmdm interface names will operate in a shared environment. For example, you can use /dev/nmdmbhyve0 only either for bhyve on the host or in a jail.

Restart devfs for the changes to be loaded:

# service devfs restart

Then add a definition for your new jail into /etc/jail.conf or /etc/jail.conf.d. Replace the interface number $if and IP address with your personal variations.

Example 1. Using NAT or routed traffic with a firewall
bhyve {
        $if = 0;
        exec.prestart = "/sbin/ifconfig epair${if} create up";
        exec.prestart += "/sbin/ifconfig epair${if}a up";
        exec.prestart += "/sbin/ifconfig epair${if}a name ${name}0";
        exec.prestart += "/sbin/ifconfig epair${if}b name jail${if}";
        exec.prestart += "/sbin/ifconfig ${name}0 inet";
        exec.prestart += "/sbin/sysctl net.inet.ip.forwarding=1";


        host.hostname = "your-hostname-here";
        vnet.interface = "em${if}";
        path = "/jails/${name}";
        securelevel = 3;
        devfs_ruleset = 100;


        exec.start += "/bin/sh /etc/rc";
        exec.stop = "/bin/sh /etc/rc.shutdown";

        exec.poststop += "/sbin/ifconfig ${name}0 destroy";

This example assumes use of a firewall like pf or ipfw to NAT your jail traffic. See the Firewalls chapter for more details on the available options to implement this.

Example 2. Using a bridged network connection
bhyve {
        $if = 0;
        exec.prestart = "/sbin/ifconfig epair${if} create up";
        exec.prestart += "/sbin/ifconfig epair${if}a up";
        exec.prestart += "/sbin/ifconfig epair${if}a name ${name}0";
        exec.prestart += "/sbin/ifconfig epair${if}b name jail${if}";
        exec.prestart += "/sbin/ifconfig bridge0 addm ${name}0";
        exec.prestart += "/sbin/sysctl net.inet.ip.forwarding=1";


        host.hostname = "your-hostname-here";
        vnet.interface = "em${if}";
        path = "/jails/${name}";
        securelevel = 3;
        devfs_ruleset = 100;


        exec.start += "/bin/sh /etc/rc";
        exec.stop = "/bin/sh /etc/rc.shutdown";

        exec.poststop += "/sbin/ifconfig ${name}0 destroy";

If you previously replaced the devfs ruleset ID 100 in /etc/devfs.rules with your own unique number, remember to replace the numeric ID also in your jails.conf too. Configuring the Jail

To start the jail for the first time and do some additional configuration work, enter:

# cp /etc/resolv.conf /jails/bhyve/etc
# service jail onestart bhyve
# jexec bhyve
# sysrc ifconfig_jail0="inet"
# sysrc defaultrouter=""
# sysrc sendmail_enable=NONE
# sysrc cloned_interfaces="tap100"
# exit

Restart and enable the jail:

# sysrc jail_enable=YES
# service jail restart bhyve

Afterwards, you can create a virtual machine within the jail. For a FreeBSD guest, download an installation ISO first:

# jexec bhyve
# cd /vms
# fetch -o freebsd.iso Creating a Virtual Machine Inside the Jail

To create a virtual machine, use bhyvectl to initialize it first:

# jexec bhyve
# bhyvectl --create --vm=bhyvevm0

Creating the guest with bhyvectl may be required when initiating the virtual machine from a jail. Skipping this step may cause the following error message when starting bhyve:

vm_open: vm-name could not be opened. No such file or directory

Finally, use your preferred way of starting the guest.

Example 3. Starting with and ZFS

Using on a ZFS filesystems:

# jexec bhyve
# sh /usr/share/examples/bhyve/ -c 1 -m 1024M \
     -t tap100 -d /dev/zvols/zroot/vms/bhyvevm0 -i -I /vms/FreeBSD-14.0-RELEASE-amd64-bootonly.iso bhyvevm0
Example 4. Starting with and UFS

Using on a UFS filesystem:

# jexec bhyve
# sh /usr/share/examples/bhyve/ -c 1 -m 1024M \
     -t tap100 -d /vms/bhyvevm0 -i -I /vms/FreeBSD-14.0-RELEASE-amd64-bootonly.iso bhyvevm0
Example 5. Starting bhyve for an UEFI guest with ZFS

If instead you want to use an UEFI guest, remember to first install the required firmware package sysutils/bhyve-firmware in the jail:

# pkg -j bhyve install bhyve-firmware

Then use bhyve directly:

# bhyve -A -c 4 -D -H -m 2G \
        -s 0,hostbridge \
        -s 1,lpc \
        -s 2,virtio-net,tap100 \
        -s 3,virtio-blk,/dev/zvol/zroot/vms/bhyvevm0 \
	-s 4,ahci-cd,/vms/FreeBSD-14.0-RELEASE-amd64-bootonly.iso \
        -s 31,fbuf,tcp=,w=1024,h=800,tablet \
        -l bootrom,/usr/local/share/uefi-firmware/BHYVE_UEFI.fd \
        -l com1,/dev/nmdbbhyve0A \

This will allow you to connect to your virtual machine bhyvevm0 through VNC as well as a serial console at /dev/nmdbbhyve0B.

24.6.10. Virtual Machine Consoles

It is advantageous to wrap the bhyve console in a session management tool such as sysutils/tmux or sysutils/screen in order to detach and reattach to the console. It is also possible to have the console of bhyve be a null modem device that can be accessed with cu. To do this, load the nmdm kernel module and replace -l com1,stdio with -l com1,/dev/nmdm0A. The /dev/nmdm devices are created automatically as needed, where each is a pair, corresponding to the two ends of the null modem cable (/dev/nmdm0A and /dev/nmdm0B). See nmdm(4) for more information.

# kldload nmdm
# bhyve -A -H -P -s 0:0,hostbridge -s 1:0,lpc -s 2:0,virtio-net,tap0 -s 3:0,virtio-blk,./linux.img \
    -l com1,/dev/nmdm0A -c 4 -m 1024M linuxguest
# cu -l /dev/nmdm0B

Ubuntu 13.10 handbook ttyS0

handbook login:

To disconnect from a console, enter a newline (i.e. press RETURN) followed by tilde (~), and finally dot (.). Keep in mind that only the connection is dropped while the login session remains active. Another user connecting to the same console could therefore make use of any active sessions without having to first authenticate. For security reasons, it’s therefore recommended to logout before disconnecting.

The number in the nmdm device path must be unique for each virtual machine and must not be used by any other processes before bhyve starts. The number can be chosen arbitrarily and does not need to be taken from a consecutive sequence of numbers. The device node pair (i.e. /dev/nmdm0a and /dev/nmdm0b) are created dynamically when bhyve connects its console and destroyed when it shuts down. Keep this in mind when creating scripts to start your virtual machines: you need to make sure that all virtual machines are assigned unique nmdm devices.

24.6.11. Managing Virtual Machines

A device node is created in /dev/vmm for each virtual machine. This allows the administrator to easily see a list of the running virtual machines:

# ls -al /dev/vmm
total 1
dr-xr-xr-x   2 root  wheel    512 Mar 17 12:19 ./
dr-xr-xr-x  14 root  wheel    512 Mar 17 06:38 ../
crw-------   1 root  wheel  0x1a2 Mar 17 12:20 guestname
crw-------   1 root  wheel  0x19f Mar 17 12:19 linuxguest
crw-------   1 root  wheel  0x1a1 Mar 17 12:19 otherguest

A specified virtual machine can be destroyed using bhyvectl:

# bhyvectl --destroy --vm=guestname

Destroying a virtual machine this way means killing it immediately. Any unsaved data will be lost, open files and filesystems may get corrupted. To gracefully shut down a virtual machine, send a TERM signal to its bhyve process instead. This triggers an ACPI shutdown event for the guest:

# ps ax | grep bhyve
17424  -  SC      56:48.27 bhyve: guestvm (bhyve)
# kill 17424

24.6.12. Tools and Utilities

There are numerous utilities and applications available in ports to help simplify setting up and managing bhyve virtual machines:

Table 1. bhyve Managers













Bhyve RC Script












24.6.13. Persistent Configuration

In order to configure the system to start bhyve guests at boot time, some configuration file changes are required.

  1. /etc/sysctl.conf

    When using tap interfaces as network backend, you either need to manually set each used tap interface to UP or simply set the following sysctl:
  2. /etc/rc.conf

    To connect your virtual machine’s tap device to the network via a bridge, you need to persist the device settings in /etc/rc.conf. Additionally, you can load the necessary kernel modules vmm for bhyve and nmdm for nmdm devices through the kld_list configuration variable. When configuring ifconfig_bridge0, make sure to replace <ipaddr>/<netmask> with the actual IP address of your physical interface (igb0 in this example) and remove IP settings from your physical device.

    # sysrc cloned_interfaces+="bridge0 tap0"
    # sysrc ifconfig_bridge0="inet <ipaddr>/<netmask> addm igb0 addm tap0"
    # sysrc kld_list+="nmdm vmm"
    # sysrc ifconfig_igb0="up"
Example 6. Setting the IP for a bridge device

For a host with an igb0 interface connected to the network with IP and netmask, you would use the following commands:

# sysrc ifconfig_igb0="up"
# sysrc ifconfig_bridge0="inet addm igb0 addm tap0"
# sysrc kld_list+="nmdm vmm"
# sysrc cloned_interfaces+="bridge0 tap0"

Modifying the IP address configuration of a system may lock you out if you are executing these commands while you are connected remotely (i.e. via SSH)! Take precautions to maintain system access or make those modifications while logged in on a local terminal session.

24.7. 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.

24.7.1. 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).

24.7.2. Xen™ Dom0 Control Domain Setup

Users should install the emulators/xen-kernel and sysutils/xen-tools packages, based on Xen™ 4.18.

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 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:

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

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

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

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.

24.7.3. 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 “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 -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:

1This 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.
2Name of this virtual machine to distinguish it from others running on the same Dom0. Required.
3Quantity 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.
4Number 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.
5Virtual 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.
6Full path to the disk, file, or ZFS volume of the disk storage for this VM. Options and multiple disk definitions are separated by commas.
7Defines the Boot medium from which the initial operating system is installed. In this example, it is the ISO image downloaded earlier. Consult the Xen™ documentation for other kinds of devices and options to set.
8Options 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.

24.7.4. 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:

# echo 'boot_verbose="YES"' >> /boot/loader.conf

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.

Last modified on: May 29, 2024 by Chris Moerz