Xen is a Type 1 hypervisor which supports running multiple guest operating systems on a single physical machine. One uses the Xen kernel to control the CPU, memory and console, a dom0 operating system which mediates access to other hardware (e.g., disks, network, USB), and one or more domU operating systems which operate in an unprivileged virtualized environment. IO requests from the domU systems are forwarded by the Xen hypervisor to the dom0 to be fulfilled.

Xen supports different styles of guest:

Style of guest Supported by NetBSD
PV Yes (dom0, domU)
HVM Yes (domU)

In Para-Virtualized (PV) mode, the guest OS does not attempt to access hardware directly, but instead makes hypercalls to the hypervisor; PV guests must be specifically coded for Xen. In HVM mode, no guest modification is required; however, hardware support is required, such as VT-x on Intel CPUs and SVM on AMD CPUs.

At boot, the dom0 kernel is loaded as a module with Xen as the kernel. The dom0 can start one or more domUs. (Booting is explained in detail in the dom0 section.)

This HOWTO presumes a basic familiarity with the Xen system architecture, with installing NetBSD on i386/amd64 hardware, and with installing software from pkgsrc. See also the Xen website.

  1. Versions and Support
  2. Creating a dom0
  3. Unprivileged domains (domU)
  4. Creating a domU
  5. NetBSD as a domU in a VPS

Versions and Support

In NetBSD, Xen is provided in pkgsrc, via matching pairs of packages xenkernel and xentools. We will refer only to the kernel versions, but note that both packages must be installed together and must have matching versions.

Versions available in pkgsrc:

Xen Version Package Name Xen CPU Support xm? EOL'ed By Upstream
4.2 xenkernel42 i386 x86_64 yes Yes
4.5 xenkernel45 x86_64 Yes
4.6 xenkernel46 x86_64 Yes
4.8 xenkernel48 x86_64 Yes
4.11 xenkernel411 x86_64 No

See also the Xen Security Advisory page.

Multiprocessor (SMP) support in NetBSD differs depending on the domain:

Domain Supports SMP
dom0 No
domU Yes

Note: NetBSD support is called XEN3. However, it does support Xen 4, because the hypercall interface has remained identical.

Older Xen had a python-based management tool called xm, now replaced by xl. xm is obsolete, but 4.2 remains in pkgsrc because migrating from xm to xl is not always trivial, and because 4.2 is the last version to run on an i386 dom0.


Xen 4.5 and later runs on x86_64 hardware (the NetBSD amd64 port). Xen 4.2 can in theory use i386 hardware, but we do not have recent reports of success.

The dom0 system, plus each domU, can be either i386PAE or amd64. i386 without PAE is not supported.

The standard approach is to use NetBSD/amd64 for the dom0.

To use an i386PAE dom0 (other than on 4.2), one must build or obtain a 64bit Xen kernel and install it on the system.

For domUs, i386PAE is considered as faster than amd64.

Creating a dom0

In order to install a NetBSD as a dom0, one must first install a normal NetBSD system, and then pivot the install to a dom0 install by changing the kernel and boot configuration.

In 2018-05, trouble booting a dom0 was reported with 256M of RAM: with 512M it worked reliably. This does not make sense, but if you see "not ELF" after Xen boots, try increasing dom0 RAM.

Installation of NetBSD

Install NetBSD/amd64 just as you would if you were not using Xen.

Installation of Xen

We will consider that you chose to use Xen 4.8, with NetBSD/amd64 as dom0. In the dom0, install xenkernel48 and xentools48 from pkgsrc.

Once this is done, install the Xen kernel itself:

# cp /usr/pkg/xen48-kernel/xen.gz /

Then, place a NetBSD XEN3_DOM0 kernel in the / directory. Such kernel can either be compiled manually, or downloaded from the NetBSD FTP, for example at:


Add a line to /boot.cfg to boot Xen:


menu=Xen:load /netbsd-XEN3_DOM0.gz console=pc;multiboot /xen.gz dom0_mem=512M

This specifies that the dom0 should have 512MB of ram, leaving the rest to be allocated for domUs. To use a serial console, use:


menu=Xen:load /netbsd-XEN3_DOM0.gz;multiboot /xen.gz dom0_mem=512M console=com1 com1=9600,8n1

which will use the first serial port for Xen (which counts starting from 1, unlike NetBSD which counts starting from 0), forcing speed/parity. Because the NetBSD command line lacks a "console=pc" argument, it will use the default "xencons" console device, which directs the console I/O through Xen to the same console device Xen itself uses (in this case, the serial port).

In an attempt to add performance, one can also add dom0_max_vcpus=1 dom0_vcpus_pin, to force only one vcpu to be provided (since NetBSD dom0 can't use more) and to pin that vcpu to a physical CPU. Xen has many boot options, and other than dom0 memory and max_vcpus, they are generally not necessary.

Copy the boot scripts into /etc/rc.d:

# cp /usr/pkg/share/examples/rc.d/xen* /etc/rc.d/

Enable xencommons:



Now, reboot so that you are running a DOM0 kernel under Xen, rather than GENERIC without Xen.

TODO: Recommend for/against xen-watchdog.

Once the reboot is done, use xl to inspect Xen's boot messages, available resources, and running domains. For example:

# xl dmesg
... xen's boot info ...
# xl info
... available memory, etc ...
# xl list
Name              Id  Mem(MB)  CPU  State  Time(s)  Console
Domain-0           0       64    0  r----     58.1

Xen logs will be in /var/log/xen.

Issues with xencommons

xencommons starts xenstored, which stores data on behalf of dom0 and domUs. It does not currently work to stop and start xenstored. Certainly all domUs should be shutdown first, following the sort order of the rc.d scripts. However, the dom0 sets up state with xenstored, and is not notified when xenstored exits, leading to not recreating the state when the new xenstored starts. Until there's a mechanism to make this work, one should not expect to be able to restart xenstored (and thus xencommons). There is currently no reason to expect that this will get fixed any time soon.

anita (for testing NetBSD)

With the setup so far (assuming 4.8/xl), one should be able to run anita (see pkgsrc/misc/py-anita) to test NetBSD releases, by doing (as root, because anita must create a domU):

anita --vmm=xl test file:///usr/obj/i386/

Xen-specific NetBSD issues

There are (at least) two additional things different about NetBSD as a dom0 kernel compared to hardware.

One is that the module ABI is different because some of the #defines change, so one must build modules for Xen. As of netbsd-7, the build system does this automatically.

The other difference is that XEN3_DOM0 does not have exactly the same options as GENERIC. While it is debatable whether or not this is a bug, users should be aware of this and can simply add missing config items if desired.

Updating NetBSD in a dom0

This is just like updating NetBSD on bare hardware, assuming the new version supports the version of Xen you are running. Generally, one replaces the kernel and reboots, and then overlays userland binaries and adjusts /etc.

Note that one must update both the non-Xen kernel typically used for rescue purposes and the DOM0 kernel used with Xen.

Converting from grub to /boot

These instructions were used to convert a system from grub to /boot. The system was originally installed in February of 2006 with a RAID1 setup and grub to boot Xen 2, and has been updated over time. Before these commands, it was running NetBSD 6 i386, Xen 4.1 and grub, much like the message linked earlier in the grub section.

# Install MBR bootblocks on both disks.
fdisk -i /dev/rwd0d
fdisk -i /dev/rwd1d
# Install NetBSD primary boot loader (/ is FFSv1) into RAID1 components.
installboot -v /dev/rwd0d /usr/mdec/bootxx_ffsv1
installboot -v /dev/rwd1d /usr/mdec/bootxx_ffsv1
# Install secondary boot loader
cp -p /usr/mdec/boot /
# Create boot.cfg following earlier guidance:
menu=Xen:load /netbsd-XEN3PAE_DOM0.gz console=pc;multiboot /xen.gz dom0_mem=512M
menu=Xen.ok:load /netbsd-XEN3PAE_DOM0.ok.gz console=pc;multiboot /xen.ok.gz dom0_mem=512M
menu=GENERIC single-user:boot -s
menu=GENERIC.ok:boot netbsd.ok
menu=GENERIC.ok single-user:boot netbsd.ok -s
menu=Drop to boot prompt:prompt

Upgrading Xen versions

Minor version upgrades are trivial. Just rebuild/replace the xenkernel version and copy the new xen.gz to / (where /boot.cfg references it), and reboot.

Unprivileged domains (domU)

This section describes general concepts about domUs. It does not address specific domU operating systems or how to install them. The config files for domUs are typically in /usr/pkg/etc/xen, and are typically named so that the file name, domU name and the domU's host name match.

The domU is provided with CPU and memory by Xen, configured by the dom0. The domU is provided with disk and network by the dom0, mediated by Xen, and configured in the dom0.

Entropy in domUs can be an issue; physical disks and network are on the dom0. NetBSD's /dev/random system works, but is often challenged.

Config files

See /usr/pkg/share/examples/xen/xlexample* for a small number of well-commented examples, mostly for running GNU/Linux.

The following is an example minimal domain configuration file. The domU serves as a network file server.


name = "domU-id"
kernel = "/netbsd-XEN3PAE_DOMU-i386-foo.gz"
memory = 1024
vif = [ 'mac=aa:00:00:d1:00:09,bridge=bridge0' ]
disk = [ 'file:/n0/xen/foo-wd0,0x0,w',
         'file:/n0/xen/foo-wd1,0x1,w' ]

The domain will have name given in the name setting. The kernel has the host/domU name in it, so that on the dom0 one can update the various domUs independently. The vif line causes an interface to be provided, with a specific mac address (do not reuse MAC addresses!), in bridge mode. Two disks are provided, and they are both writable; the bits are stored in files and Xen attaches them to a vnd(4) device in the dom0 on domain creation. The system treats xbd0 as the boot device without needing explicit configuration.

By convention, domain config files are kept in /usr/pkg/etc/xen. Note that "xl create" takes the name of a config file, while other commands take the name of a domain.

Examples of commands:

xl create /usr/pkg/etc/xen/foo
xl console domU-id
xl create -c /usr/pkg/etc/xen/foo
xl shutdown domU-id
xl list

Typing ^] will exit the console session. Shutting down a domain is equivalent to pushing the power button; a NetBSD domU will receive a power-press event and do a clean shutdown. Shutting down the dom0 will trigger controlled shutdowns of all configured domUs.

CPU and memory

A domain is provided with some number of vcpus, up to the number of CPUs seen by the hypervisor. For a domU, it is controlled from the config file by the "vcpus = N" directive.

A domain is provided with memory; this is controlled in the config file by "memory = N" (in megabytes). In the straightforward case, the sum of the the memory allocated to the dom0 and all domUs must be less than the available memory.

Xen also provides a "balloon" driver, which can be used to let domains use more memory temporarily.

Virtual disks

In domU config files, the disks are defined as a sequence of 3-tuples:



disk = [ 'file:/n0/xen/foo-wd0,0x0,w' ]

Note that NetBSD by default creates only vnd[0123]. If you need more than 4 total virtual disks at a time, run e.g. "./MAKEDEV vnd4" in the dom0.

Note that NetBSD by default creates only xbd[0123]. If you need more virtual disks in a domU, run e.g. "./MAKEDEV xbd4" in the domU.

Virtual Networking

Xen provides virtual Ethernets, each of which connects the dom0 and a domU. For each virtual network, there is an interface "xvifN.M" in the dom0, and a matching interface xennetM (NetBSD name) in domU index N. The interfaces behave as if there is an Ethernet with two adapters connected. From this primitive, one can construct various configurations. We focus on two common and useful cases for which there are existing scripts: bridging and NAT.

With bridging (in the example above), the domU perceives itself to be on the same network as the dom0. For server virtualization, this is usually best. Bridging is accomplished by creating a bridge(4) device and adding the dom0's physical interface and the various xvifN.0 interfaces to the bridge. One specifies "bridge=bridge0" in the domU config file. The bridge must be set up already in the dom0; an example /etc/ifconfig.bridge0 is:


!brconfig bridge0 add wm0

With NAT, the domU perceives itself to be behind a NAT running on the dom0. This is often appropriate when running Xen on a workstation. TODO: NAT appears to be configured by "vif = [ '' ]".

The MAC address specified is the one used for the interface in the new domain. The interface in dom0 will use this address XOR'd with 00:00:00:01:00:00. Random MAC addresses are assigned if not given.

Starting domains automatically

To start domains domU-netbsd and domU-linux at boot and shut them down cleanly on dom0 shutdown, add the following in rc.conf:


xendomains="domU-netbsd domU-linux"

Creating a domU

Creating domUs is almost entirely independent of operating system. We have already presented the basics of config files. Note that you must have already completed the dom0 setup so that "xl list" works.

Creating a NetBSD domU

See the earlier config file, and adjust memory. Decide on how much storage you will provide, and prepare it (file or LVM).

While the kernel will be obtained from the dom0 file system, the same file should be present in the domU as /netbsd so that tools like savecore(8) can work. (This is helpful but not necessary.)

The kernel must be specifically for Xen and for use as a domU. The i386 and amd64 provide the following kernels:

    i386 XEN3PAE_DOMU
    amd64 XEN3_DOMU

This will boot NetBSD, but this is not that useful if the disk is empty. One approach is to unpack sets onto the disk outside of xen (by mounting it, just as you would prepare a physical disk for a system you can't run the installer on).

A second approach is to run an INSTALL kernel, which has a miniroot and can load sets from the network. To do this, copy the INSTALL kernel to / and change the kernel line in the config file to:

    kernel = "/home/bouyer/netbsd-INSTALL_XEN3_DOMU"

Then, start the domain as "xl create -c configfile".

Alternatively, if you want to install NetBSD/Xen with a CDROM image, the following line should be used in the config file.

disk = [ 'phy:/dev/wd0e,0x1,w', 'phy:/dev/cd0a,0x2,r' ]

After booting the domain, the option to install via CDROM may be selected. The CDROM device should be changed to xbd1d.

Once done installing, "halt -p" the new domain (don't reboot or halt, it would reload the INSTALL_XEN3_DOMU kernel even if you changed the config file), switch the config file back to the XEN3_DOMU kernel, and start the new domain again. Now it should be able to use "root on xbd0a" and you should have a, functional NetBSD domU.

TODO: check if this is still accurate. When the new domain is booting you'll see some warnings about wscons and the pseudo-terminals. These can be fixed by editing the files /etc/ttys and /etc/wscons.conf. You must disable all terminals in /etc/ttys, except console, like this:

console "/usr/libexec/getty Pc"         vt100   on secure
ttyE0   "/usr/libexec/getty Pc"         vt220   off secure
ttyE1   "/usr/libexec/getty Pc"         vt220   off secure
ttyE2   "/usr/libexec/getty Pc"         vt220   off secure
ttyE3   "/usr/libexec/getty Pc"         vt220   off secure

Finally, all screens must be commented out from /etc/wscons.conf.

It is also desirable to add


in rc.conf. This way, the domain will be properly shut down if xm shutdown -R or xm shutdown -H is used on the dom0.

It is not strictly necessary to have a kernel (as /netbsd) in the domU file system. However, various programs (e.g. netstat) will use that kernel to look up symbols to read from kernel virtual memory. If /netbsd is not the running kernel, those lookups will fail. (This is not really a Xen-specific issue, but because the domU kernel is obtained from the dom0, it is far more likely to be out of sync or missing with Xen.)

Creating a Linux domU

Creating unprivileged Linux domains isn't much different from unprivileged NetBSD domains, but there are some details to know.

First, the second parameter passed to the disk declaration (the '0x1' in the example below)

disk = [ 'phy:/dev/wd0e,0x1,w' ]

does matter to Linux. It wants a Linux device number here (e.g. 0x300 for hda). Linux builds device numbers as: (major \<< 8 + minor). So, hda1 which has major 3 and minor 1 on a Linux system will have device number 0x301. Alternatively, devices names can be used (hda, hdb, ...) as xentools has a table to map these names to devices numbers. To export a partition to a Linux guest we can use:

    disk = [ 'phy:/dev/wd0e,0x300,w' ]
    root = "/dev/hda1 ro"

and it will appear as /dev/hda on the Linux system, and be used as root partition.

To install the Linux system on the partition to be exported to the guest domain, the following method can be used: install sysutils/e2fsprogs from pkgsrc. Use mke2fs to format the partition that will be the root partition of your Linux domain, and mount it. Then copy the files from a working Linux system, make adjustments in /etc (fstab, network config). It should also be possible to extract binary packages such as .rpm or .deb directly to the mounted partition using the appropriate tool, possibly running under NetBSD's Linux emulation. Once the file system has been populated, umount it. If desirable, the file system can be converted to ext3 using tune2fs -j. It should now be possible to boot the Linux guest domain, using one of the vmlinuz-*-xenU kernels available in the Xen binary distribution.

To get the Linux console right, you need to add:

extra = "xencons=tty1"

to your configuration since not all Linux distributions auto-attach a tty to the xen console.

Creating a Solaris domU

See possibly outdated Solaris domU instructions.

PCI passthrough: Using PCI devices in guest domains

The dom0 can give other domains access to selected PCI devices. This can allow, for example, a non-privileged domain to have access to a physical network interface or disk controller. However, keep in mind that giving a domain access to a PCI device most likely will give the domain read/write access to the whole physical memory, as PCs don't have an IOMMU to restrict memory access to DMA-capable device. Also, it's not possible to export ISA devices to non-dom0 domains, which means that the primary VGA adapter can't be exported. A guest domain trying to access the VGA registers will panic.

If the dom0 is NetBSD, it has to be running Xen 3.1, as support has not been ported to later versions at this time.

For a PCI device to be exported to a domU, is has to be attached to the "pciback" driver in dom0. Devices passed to the dom0 via the pciback.hide boot parameter will attach to "pciback" instead of the usual driver. The list of devices is specified as "(bus:dev.func)", where bus and dev are 2-digit hexadecimal numbers, and func a single-digit number:


pciback devices should show up in the dom0's boot messages, and the devices should be listed in the /kern/xen/pci directory.

PCI devices to be exported to a domU are listed in the "pci" array of the domU's config file, with the format "0000:bus:dev.func".

    pci = [ '0000:00:06.0', '0000:00:0a.0' ]

In the domU an "xpci" device will show up, to which one or more pci buses will attach. Then the PCI drivers will attach to PCI buses as usual. Note that the default NetBSD DOMU kernels do not have "xpci" or any PCI drivers built in by default; you have to build your own kernel to use PCI devices in a domU. Here's a kernel config example; note that only the "xpci" lines are unusual.

    include         "arch/i386/conf/XEN3_DOMU"

    # Add support for PCI buses to the XEN3_DOMU kernel
    xpci* at xenbus ?
    pci* at xpci ?

    # PCI USB controllers
    uhci*   at pci? dev ? function ?        # Universal Host Controller (Intel)

    # USB bus support
    usb*    at uhci?

    # USB Hubs
    uhub*   at usb?
    uhub*   at uhub? port ? configuration ? interface ?

    # USB Mass Storage
    umass*  at uhub? port ? configuration ? interface ?
    wd*     at umass?
    # SCSI controllers
    ahc*    at pci? dev ? function ?        # Adaptec [23]94x, aic78x0 SCSI

    # SCSI bus support (for both ahc and umass)
    scsibus* at scsi?

    # SCSI devices
    sd*     at scsibus? target ? lun ?      # SCSI disk drives
    cd*     at scsibus? target ? lun ?      # SCSI CD-ROM drives

NetBSD as a domU in a VPS

The bulk of the HOWTO is about using NetBSD as a dom0 on your own hardware. This section explains how to deal with Xen in a domU as a virtual private server where you do not control or have access to the dom0. This is not intended to be an exhaustive list of VPS providers; only a few are mentioned that specifically support NetBSD.

VPS operators provide varying degrees of access and mechanisms for configuration. The big issue is usually how one controls which kernel is booted, because the kernel is nominally in the dom0 file system (to which VPS users do not normally have access). A second issue is how to install NetBSD. A VPS user may want to compile a kernel for security updates, to run npf, run IPsec, or any other reason why someone would want to change their kernel.

One approach is to have an administrative interface to upload a kernel, or to select from a prepopulated list. Other approaches are pygrub (deprecated) and pvgrub, which are ways to have a bootloader obtain a kernel from the domU file system. This is closer to a regular physical computer, where someone who controls a machine can replace the kernel.

A second issue is multiple CPUs. With NetBSD 6, domUs support multiple vcpus, and it is typical for VPS providers to enable multiple CPUs for NetBSD domUs.


pvgrub is a version of grub that uses PV operations instead of BIOS calls. It is booted from the dom0 as the domU kernel, and then reads /grub/menu.lst and loads a kernel from the domU file system.

Panix lets users use pvgrub. Panix reports that pvgrub works with FFsv2 with 16K/2K and 32K/4K block/frag sizes (and hence with defaults from "newfs -O 2"). See Panix's pvgrub page, which describes only Linux but should be updated to cover NetBSD :-).

prgmr.com also lets users with pvgrub to boot their own kernel. See then prgmr.com NetBSD HOWTO (which is in need of updating).

It appears that grub's FFS code does not support all aspects of modern FFS, but there are also reports that FFSv2 works fine. At prgmr, typically one has an ext2 or FAT partition for the kernel with the intent that grub can understand it, which leads to /netbsd not being the actual kernel. One must remember to update the special boot partition.


pygrub runs in the dom0 and looks into the domU file system. This implies that the domU must have a kernel in a file system in a format known to pygrub.

pygrub doesn't seem to work to load Linux images under NetBSD dom0, and is inherently less secure than pvgrub due to running inside dom0. For both these reasons, pygrub should not be used, and is only still present so that historical DomU images using it still work.

As of 2014, pygrub seems to be of mostly historical interest. New DomUs should use pvgrub.


See the Amazon EC2 page.