[[!meta title="Xen HowTo"]]
Xen is a hypervisor (or virtual machine monitor) for x86 hardware
(i686-class or higher), which supports running multiple guest
operating systems on a single physical machine. Xen is a Type 1 or
bare-metal hypervisor; 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
hypervisor (Xen) to the dom0 to be fulfilled.
Xen supports two styles of guests. The original is Para-Virtualized
(PV) which means that the guest OS does not attempt to access hardware
directly, but instead makes hypercalls to the hypervisor. This is
analogous to a user-space program making system calls. (The dom0
operating system uses PV calls for some functions, such as updating
memory mapping page tables, but has direct hardware access for disk
and network.) PV guests must be specifically coded for Xen.
The more recent style is HVM, which means that the guest does not have
code for Xen and need not be aware that it is running under Xen.
Attempts to access hardware registers are trapped and emulated. This
style is less efficient but can run unmodified guests.
Generally any machine that runs NetBSD/amd64 will work with Xen and PV
guests. In theory i386 computers (without x86_64/amd64 support) can
be used for Xen <= 4.2, but we have no recent reports of this working
(this is a hint). For HVM guests, hardware support is needed, but it
is common on recent machines. For Intel CPUs, one needs the VT-x
extension, shown in "cpuctl identify 0" as VMX. For AMD CPUs, one
needs the AMD-V extensions, shown in "cpuctl identify 0" as SVM.
There are further features for IOMMU virtualization, Intel's VT-d and
AMD's AMD-Vi. TODO: Explain whether Xen on NetBSD makes use of these
features. TODO: Review by someone who really understands this.
Note that a FreeBSD dom0 requires VT-x and VT-d (or equivalent); this
is because the FreeBSD dom0 does not run in PV mode.
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.)
NetBSD supports Xen in that it can serve as dom0, be used as a domU,
and that Xen kernels and tools are available in pkgsrc. This HOWTO
attempts to address both the case of running a NetBSD dom0 on hardware
and running domUs under it (NetBSD and other), and also running NetBSD
as a domU in a VPS.
Xen 3.1 in pkgsrc used to support "PCI passthrough", which means that
specific PCI devices can be made available to a specific domU instead
of the dom0. This can be useful to let a domU run X11, or access some
network interface or other peripheral.
NetBSD 6 and earlier supported Xen 2; support was removed from NetBSD
7. Xen 2 has been removed from pkgsrc.
Installing NetBSD/Xen is not extremely difficult, but it is more
complex than a normal installation of NetBSD.
In general, this HOWTO is occasionally overly restrictive about how
things must be done, guiding the reader to stay on the established
path when there are no known good reasons to stray.
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
Versions of Xen and NetBSD
Most of the installation concepts and instructions are independent
of Xen version and NetBSD version. This section gives advice on
which version to choose. Versions not in pkgsrc and older unsupported
versions of NetBSD are intentionally ignored.
The term "amd64" is used to refer to both the NetBSD port and to the
hardware architecture on which it runs. (Such hardware is made by
both Intel and AMD, and in 2016 a normal PC has this CPU
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
Versions available in pkgsrc:
Xen Version |Package Name |Xen CPU Support |EOL'ed By Upstream
4.2 |xenkernel42 |32bit, 64bit |Yes
4.5 |xenkernel45 |64bit |Yes
4.6 |xenkernel46 |64bit |Partially
4.8 |xenkernel48 |64bit |No
4.11 |xenkernel411 |64bit |No
See also the [Xen Security Advisory page](http://xenbits.xen.org/xsa/).
Note: Xen 4.2 was the last version to support 32bit CPUs.
Note that NetBSD support is called XEN3. It works with Xen 3 and Xen
4 because the hypercall interface has been stable.
Xen command program
Early Xen used a program called xm to manipulate the system from the
dom0. Starting in 4.1, a replacement program with similar behavior
called xl is provided, but it does not work well in 4.1. In 4.2, both
xm and xl work fine. 4.4 is the last version that has xm.
You must make a global choice to use xm or xl, because it affects not
only which command you use, but the command used by rc.d scripts
(specifically xendomains) and which daemons should be run. The
xentools packages provide xl for 4.2 and up.
In 4.2, you can choose to use xm by simply changing the ctl_command
variable and setting xend=YES in rc.conf.
With xl, virtual devices are configured in parallel, which can cause
problems if they are written assuming serial operation (e.g., updating
firewall rules without explicit locking). There is now locking for
the provided scripts, which works for normal casses (e.g, file-backed
xbd, where a vnd must be allocated). But, as of 201612, it has not
been adequately tested for a complex custom setup with a large number
The netbsd-7, netbsd-8, and -current branches are all reasonable
choices, with more or less the same considerations for non-Xen use.
Therefore, netbsd-8 is recommended as the stable version of the most
recent release for production use.
For developing Xen, netbsd-current may be appropriate.
As of NetBSD 6, a NetBSD domU will support multiple vcpus. There is
no SMP support for NetBSD as dom0. (The dom0 itself doesn't really
need SMP for dom0 functions; the lack of support is really a problem
when using a dom0 as a normal computer.)
Xen itself can run on i386 (Xen < 4.2) or amd64 hardware (all Xen
versions). (Practically, almost any computer where one would want to
run Xen today supports amd64.)
Xen, the dom0 system, and each domU system can be either i386 or
amd64. When building a xenkernel package, one obtains an i386 Xen
kernel on an i386 host, and an amd64 Xen kernel on an amd64 host. If
the Xen kernel is i386, then the dom0 kernel and all domU kernels must
be i386. With an amd64 Xen kernel, an amd64 dom0 kernel is known to
work, and an i386 dom0 kernel should in theory work. An amd64
Xen/dom0 is known to support both i386 and amd64 domUs.
i386 dom0 and domU kernels must be PAE (except for an i386 Xen 3.1
kernel, where one can use non-PAE for dom0 and all domUs); PAE kernels
are included in the NetBSD default build. (Note that emacs (at least)
fails if run on i386 with PAE when built without, and vice versa,
presumably due to bugs in the undump code.)
Because of the above, the standard approach is to use an amd64 Xen
kernel and NetBSD/amd64 for the dom0. For domUs, NetBSD/i386 (with
the PAE kernel) and NetBSD/amd64 are in widespread use, and there is
little to no Xen-specific reason to prefer one over the other.
Note that to use an i386 dom0 with Xen 4.5 or higher, one must build
(or obtain from pre-built packages) an amd64 Xen kernel and install
that on the system. (One must also use a PAE i386 kernel, but this is
also required with an i386 Xen kernel.). Almost no one in the
NetBSD/Xen community does this, and the standard, well-tested,
approach is to use an amd64 dom0.
A [posting on
explained that PV system call overhead was higher on amd64, and thus
there is some notion that i386 guests are faster. It goes on to
caution that the total situation is complex and not entirely
understood. On top of that caution, the post is about Linux, not
NetBSD. TODO: Include link to benchmarks, if someone posts them.
Mostly, NetBSD as a dom0 or domU is quite stable. However, just like every
other architecture, there are some open PRs indicating problems.
Note also that there are issues with sparse vnd(4) instances, but
these are not about Xen -- they just are noticed with sparse vnd(4)
instances in support of virtual disks in a dom0.
Therefore, this HOWTO recommends running xenkernel46, xl, the NetBSD 7
stable branch, and therefore to use an amd64 kernel as the dom0.
Either the i386PAE or amd64 version of NetBSD may be used as domUs.
A tentative replacement recommendation is xenkernel48, xl, and NetBSD
Because bugs are fixed quite often, and because of Xen security
advisories, it is good to stay up to date with NetBSD (tracking a
stable branch), with the Xen kernel (tracking a Xen version via
pkgsrc), and with the Xen tools. Specifically, NetBSD (-7 and
-current) got an important fix affecting dom0/domU timesharing in
November, 2015, and xentools46 got a fix to enable Ubuntu guests to
boot in December, 2016.
NetBSD as a dom0
NetBSD can be used as a dom0 and works very well. The following
sections address installation, updating NetBSD, and updating Xen.
Note that it doesn't make sense to talk about installing a dom0 OS
without also installing Xen itself. We first address installing
NetBSD, which is not yet a dom0, and then adding Xen, pivoting the
NetBSD install to a dom0 install by just changing the kernel and boot
For experimenting with Xen, a machine with as little as 1G of RAM and
100G of disk can work. For running many domUs in productions, far
more will be needed; e.g. 4-8G and 1T of disk is reasonable for a
half-dozen domUs of 512M and 32G each. Basically, the RAM and disk
have to be bigger than the sum of the RAM/disk needs of the dom0 and
all the domUs.
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.
Styles of dom0 operation
There are two basic ways to use Xen. The traditional method is for
the dom0 to do absolutely nothing other than providing support to some
number of domUs. Such a system was probably installed for the sole
purpose of hosting domUs, and sits in a server room on a UPS.
The other way is to put Xen under a normal-usage computer, so that the
dom0 is what the computer would have been without Xen, perhaps a
desktop or laptop. Then, one can run domUs at will. Purists will
deride this as less secure than the previous approach, and for a
computer whose purpose is to run domUs, they are right. But Xen and a
dom0 (without domUs) is not meaningfully less secure than the same
things running without Xen. One can boot Xen or boot regular NetBSD
alternately with little problems, simply refraining from starting the
Xen daemons when not running Xen.
Note that NetBSD as dom0 does not support multiple CPUs. This will
limit the performance of the Xen/dom0 workstation approach. In theory
the only issue is that the "backend drivers" are not yet MPSAFE:
Installation of NetBSD
just as you would if you were not using Xen.
However, the partitioning approach is very important.
If you want to use RAIDframe for the dom0, there are no special issues
for Xen. Typically one provides RAID storage for the dom0, and the
domU systems are unaware of RAID. The 2nd-stage loader bootxx_* skips
over a RAID1 header to find /boot from a file system within a RAID
partition; this is no different when booting Xen.
There are 4 styles of providing backing storage for the virtual disks
used by domUs: raw partitions, LVM, file-backed vnd(4), and SAN.
With raw partitions, one has a disklabel (or gpt) partition sized for
each virtual disk to be used by the domU. (If you are able to predict
how domU usage will evolve, please add an explanation to the HOWTO.
Seriously, needs tend to change over time.)
One can use [lvm(8)](/guide/lvm/) to create logical devices to use
for domU disks. This is almost as efficient as raw disk partitions
and more flexible. Hence raw disk partitions should typically not
One can use files in the dom0 file system, typically created by dd'ing
/dev/zero to create a specific size. This is somewhat less efficient,
but very convenient, as one can cp the files for backup, or move them
between dom0 hosts.
Finally, in theory one can place the files backing the domU disks in a
SAN. (This is an invitation for someone who has done this to add a
Installation of Xen
In the dom0, install sysutils/xenkernel42 and sysutils/xentools42 from
pkgsrc (or another matching pair). See [the pkgsrc
documentation](https://www.NetBSD.org/docs/pkgsrc/) for help with
pkgsrc. Ensure that your packages are recent; the HOWTO does not
contemplate old builds.
For Xen 3.1, support for HVM guests is in sysutils/xentool3-hvm. More
recent versions have HVM support integrated in the main xentools
package. It is entirely reasonable to run only PV guests.
Next you need to install the selected Xen kernel itself, which is
installed by pkgsrc as "/usr/pkg/xen*-kernel/xen.gz". Copy it to /.
For debugging, one may copy xen-debug.gz; this is conceptually similar
to DIAGNOSTIC and DEBUG in NetBSD. xen-debug.gz is basically only
useful with a serial console. Then, place a NetBSD XEN3_DOM0 kernel
in /, copied from releasedir/amd64/binary/kernel/netbsd-XEN3_DOM0.gz
of a NetBSD build. If using i386, use
releasedir/i386/binary/kernel/netbsd-XEN3PAE_DOM0.gz. (If using Xen
3.1 and i386, you may use XEN3_DOM0 with the non-PAE Xen. But you
should not use Xen 3.1.) Both xen and the NetBSD kernel may be (and
typically are) left compressed.
In a dom0, kernfs is mandatory for xend to communicate with the
kernel, so ensure that /kern is in fstab. (A standard NetBSD install
should already mount /kern.)
Because you already installed NetBSD, you have a working boot setup
with an MBR bootblock, either bootxx_ffsv1 or bootxx_ffsv2 at the
beginning of your root file system, have /boot, and likely also
/boot.cfg. (If not, fix before continuing!)
Add a line to to /boot.cfg to boot Xen. See boot.cfg(5) for an
example. The basic line is
menu=Xen:load /netbsd-XEN3_DOM0.gz console=pc;multiboot /xen.gz dom0_mem=512M
which specifies that the dom0 should have 512M, 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
to force only one vcpu to be provided (since NetBSD dom0 can't use
more) and to pin that vcpu to a physical CPU. TODO: benchmark this.
Xen has [many boot
and other than dom0 memory and max_vcpus, they are generally not
As with non-Xen systems, you should have a line to boot /netbsd (a
kernel that works without Xen). Consider a line to boot /netbsd.ok (a
fallback version of the non-Xen kernel, updated manually when you are
sure /netbsd is ok). Consider also a line to boot fallback versions
of Xen and the dom0 kernel, but note that non-Xen NetBSD can be used
to resolve Xen booting issues.
Probably you want a default=N line to choose Xen in the absence of
Now, reboot so that you are running a DOM0 kernel under Xen, rather
than GENERIC without Xen.
Using grub (historic)
Before NetBSD's native bootloader could support Xen, the use of
grub was recommended. If necessary, see the
[old grub information](/ports/xen/howto-grub).
The [HowTo on Installing into
explains how to set up booting a dom0 with Xen using grub with
NetBSD's RAIDframe. (This is obsolete with the use of NetBSD's native
boot. Now, just create a system with RAID-1, and alter /boot.cfg as
Xen logs will be in /var/log/xen.
Now, you have a system that will boot Xen and the dom0 kernel, but not
do anything else special. Make sure that you have rebooted into Xen.
There will be no domUs, and none can be started because you still have
to configure the dom0 daemons.
The daemons which should be run vary with Xen version and with whether
one is using xm or xl. The Xen 3.1, 3.3 and 4.1 packages use xm. Xen
4.2 and up packages use xl. To use xm with 4.2, edit xendomains to
use xm instead.
For 3.1 and 3.3, you should enable xend and xenbackendd:
For 4.1 and up, you should enable xencommons. Not enabling xencommons
will result in a hang; it is necessary to hit ^C on the console to let
the machine finish booting. If you are using xm (default in 4.1, or
if you changed xendomains in 4.2), you should also enable xend:
xend=YES # only if using xm, and only installed <= 4.2
TODO: Recommend for/against xen-watchdog.
After you have configured the daemons and either started them (in the
order given) or rebooted, use xm or xl to inspect Xen's boot messages,
available resources, and running domains. An example with xl follows:
# 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
### 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.
### No-longer needed advice about devices
The installation of NetBSD should already have created devices for xen
(xencons, xenevt, xsd_kva), but if they are not present, create them:
cd /dev && sh MAKEDEV xen
anita (for testing NetBSD)
With the setup so far (assuming 4.2/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/
Alternatively, one can use --vmm=xm to use xm-based domU creation
instead (and must, on Xen <= 4.1). TODO: confirm that anita xl really works.
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. TODO: check this. (Before building
Xen modules was added, it was awkward to use modules to the point
where it was considered that it did not work.)
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 [TODO: will be] 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
# 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 single-user:boot netbsd.ok -s
menu=Drop to boot prompt:prompt
TODO: actually do this and fix it if necessary.
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.
Major version upgrades are conceptually not difficult, but can run
into all the issues found when installing Xen. Assuming migration
from 4.1 to 4.2, remove the xenkernel41 and xentools41 packages and
install the xenkernel42 and xentools42 packages. Copy the 4.2 xen.gz
Ensure that the contents of /etc/rc.d/xen* are correct. Specifically,
they must match the package you just installed and not be left over
from some previous installation.
Enable the correct set of daemons; see the configuring section above.
(Upgrading from 3.x to 4.x without doing this will result in a hang.)
Ensure that the domU config files are valid for the new version.
Specifically, for 4.x remove autorestart=True, and ensure that disks
are specified with numbers as the second argument, as the examples
above show, and not NetBSD device names.
Hardware known to work
Arguably, this section is misplaced, and there should be a page of
hardware that runs NetBSD/amd64 well, with the mostly-well-founded
assumption that NetBSD/xen runs fine on any modern hardware that
NetBSD/amd64 runs well on. Until then, we give motherboard/CPU (and
sometimes RAM) pairs/triples to aid those choosing a motherboard.
Note that Xen systems usually do not run X, so a listing here does not
imply that X works at all.
Supermicro X9SRL-F, Xeon E5-1650 v2, 96 GiB ECC
Supermicro ??, Atom C2758 (8 core), 32 GiB ECC
ASUS M5A78L-M/USB3 AM3+ microATX, AMD Piledriver X8 4000MHz, 16 GiB ECC
Intel D915GEV, Pentium4 CPU 3.40GHz, 4GB 533MHz Synchronous DDR2
INTEL DG33FB, "Intel(R) Core(TM)2 Duo CPU E6850 @ 3.00GHz"
INTEL DG33FB, "Intel(R) Core(TM)2 Duo CPU E8400 @ 3.00GHz"
Running Xen under qemu
The astute reader will note that this section is somewhat twisted.
However, it can be useful to run Xen under qemu either because the
version of NetBSD as a dom0 does not run on the hardware in use, or to
generate automated test cases involving Xen.
In 2015-01, the following combination was reported to mostly work:
host OS: NetBSD/amd64 6.1.4
qemu: 2.2.0 from pkgsrc
Xen kernel: xenkernel42-4.2.5nb1 from pkgsrc
dom0 kernel: NetBSD/amd64 6.1.5
Xen tools: xentools42-4.2.5 from pkgsrc
See [PR 47720](https://gnats.netbsd.org/47720) for a problem with dom0
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
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.
There is no good order to present config files and the concepts
surrounding what is being configured. We first show an example config
file, and then in the various sections give details.
See (at least in xentools41) /usr/pkg/share/examples/xen/xmexample*,
for a large number of well-commented examples, mostly for running
The following is an example minimal domain configuration file
"/usr/pkg/etc/xen/foo". It is (with only a name change) an actual
known working config file on Xen 4.1 (NetBSD 5 amd64 dom0 and NetBSD 5
i386 domU). The domU serves as a network file server.
# -*- mode: python; -*-
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',
The domain will have the same name as the file. 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 default xm looks for domain config files in /usr/pkg/etc/xen. Note
that "xm create" takes the name of a config file, while other commands
take the name of a domain. To create the domain, connect to the
console, create the domain while attaching the console, shutdown the
domain, and see if it has finished stopping, do (or xl with Xen >=
xm create foo
xm console foo
xm create -c foo
xm shutdown foo
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.
On a physical computer, the BIOS reads sector 0, and a chain of boot
loaders finds and loads a kernel. Normally this comes from the root
file system. With Xen domUs, the process is totally different. The
normal path is for the domU kernel to be a file in the dom0's
file system. At the request of the dom0, Xen loads that kernel into a
new domU instance and starts execution. While domU kernels can be
anyplace, reasonable places to store domU kernels on the dom0 are in /
(so they are near the dom0 kernel), in /usr/pkg/etc/xen (near the
config files), or in /u0/xen (where the vdisks are).
Note that loading the domU kernel from the dom0 implies that boot
blocks, /boot, /boot.cfg, and so on are all ignored in the domU.
See the VPS section near the end for discussion of alternate ways to
obtain domU kernels.
CPU and memory
A domain is provided with some number of vcpus, less than the number
of CPUs seen by the hypervisor. (For a dom0, this is controlled by
the boot argument "dom0_max_vcpus=1".) 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. TODO: Explain better, and explain how
well it works with NetBSD.
With the file/vnd style, typically one creates a directory,
e.g. /u0/xen, on a disk large enough to hold virtual disks for all
domUs. Then, for each domU disk, one writes zeros to a file that then
serves to hold the virtual disk's bits; a suggested name is foo-xbd0
for the first virtual disk for the domU called foo. Writing zeros to
the file serves two purposes. One is that preallocating the contents
improves performance. The other is that vnd on sparse files has
failed to work. TODO: give working/notworking NetBSD versions for
sparse vnd and gnats reference. Note that the use of file/vnd for Xen
is not really different than creating a file-backed virtual disk for
some other purpose, except that xentools handles the vnconfig
commands. To create an empty 4G virtual disk, simply do
dd if=/dev/zero of=foo-xbd0 bs=1m count=4096
Do not use qemu-img-xen, because this will create sparse file. There
have been recent (2015) reports of sparse vnd(4) devices causing
lockups, but there is apparently no PR.
With the lvm style, one creates logical devices. They are then used
similarly to vnds. TODO: Add an example with lvm.
In domU config files, the disks are defined as a sequence of 3-tuples.
The first element is "method:/path/to/disk". Common methods are
"file:" for file-backed vnd. and "phy:" for something that is already
a (TODO: character or block) device.
The second element is an artifact of how virtual disks are passed to
Linux, and a source of confusion with NetBSD Xen usage. Linux domUs
are given a device name to associate with the disk, and values like
"hda1" or "sda1" are common. In a NetBSD domU, the first disk appears
as xbd0, the second as xbd1, and so on. However, xm/xl demand a
second argument. The name given is converted to a major/minor by
calling stat(2) on the name in /dev and this is passed to the domU.
In the general case, the dom0 and domU can be different operating
systems, and it is an unwarranted assumption that they have consistent
numbering in /dev, or even that the dom0 OS has a /dev. With NetBSD
as both dom0 and domU, using values of 0x0 for the first disk and 0x1
for the second works fine and avoids this issue. For a GNU/Linux
guest, one can create /dev/hda1 in /dev, or to pass 0x301 for
The third element is "w" for writable disks, and "r" for read-only
Note that NetBSD by default creates only vnd. If you need more
than 4 total virtual disks at a time, run e.g. "./MAKEDEV vnd4" in the
Note that NetBSD by default creates only xbd. If you need more
virtual disks in a domU, run e.g. "./MAKEDEV xbd4" in the domU.
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 in domU index N, a matching interface xennetM (NetBSD
name). 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.
Modern x86 hardware has vast amounts of resources. However, many
virtual servers can function just fine on far less. A system with
512M of RAM and a 4G disk can be a reasonable choice. Note that it is
far easier to adjust virtual resources than physical ones. For
memory, it's just a config file edit and a reboot. For disk, one can
create a new file and vnconfig it (or lvm), and then dump/restore,
just like updating physical disks, but without having to be there and
without those pesky connectors.
Starting domains automatically
To start domains foo at bar at boot and shut them down cleanly on dom0
shutdown, in rc.conf add:
Note that earlier versions of the xentools41 xendomains rc.d script
used xl, when one should use xm with 4.1.
Creating specific unprivileged domains (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" (or "xm list")
Creating an unprivileged NetBSD domain (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:
Unless using Xen 3.1 (and you shouldn't) with i386-mode Xen, you must
use the PAE version of the i386 kernel.
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 configname".
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 an unprivileged Linux domain (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
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 an unprivileged Solaris domain (domU)
See possibly outdated
[Solaris domU instructions](/ports/xen/howto-solaris/).
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
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.
# 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 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
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.
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. As of 2014, pygrub seems to be of mostly historical
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](http://www.panix.com/) 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](http://www.panix.com/v-colo/grub.html), which describes only
Linux but should be updated to cover NetBSD :-).
[prgmr.com](http://prgmr.com/) also lets users with pvgrub to boot
their own kernel. See then [prgmr.com NetBSD
(which is in need of updating).
It appears that [grub's FFS
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.
See the [Amazon EC2 page](/amazon_ec2/).
TODO items for improving NetBSD/xen
* Make the NetBSD dom0 kernel work with SMP.
* Test the Xen 4.5 packages adequately to be able to recommend them as
the standard approach.
* Get PCI passthrough working on Xen 4.5
* Get pvgrub into pkgsrc, either via xentools or separately.
* Check/add support to pkgsrc grub2 for UFS2 and arbitrary
fragsize/blocksize (UFS2 support may be present; the point is to
make it so that with any UFS1/UFS2 file system setup that works
with NetBSD grub will also work).
* Push patches upstream.
* Get UFS2 patches into pvgrub.
* Add support for PV ops to a version of /boot, and make it usable as
a kernel in Xen, similar to pvgrub.
This section contains links from elsewhere not yet integrated into the
HOWTO, and other guides.
CVSweb for NetBSD wikisrc <wikimaster@NetBSD.org> software: FreeBSD-CVSweb