File:  [NetBSD Developer Wiki] / wikisrc / ports / xen / howto.mdwn
Revision 1.160: download - view: text, annotated - select for diffs
Wed Sep 5 09:37:37 2018 UTC (4 years, 8 months ago) by maxv
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CVS tags: HEAD
more noise reduction, and also add a summary at the beginning

[[!meta title="Xen HowTo"]]

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

Xen supports different styles of guest:

[[!table data="""
Style of guest	|Supported by NetBSD
PV		|Yes (dom0, domU)
HVM		|Yes (domU)
PVH		|No

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


#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:

[[!table data="""
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](

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

[[!table data="""
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.


Xen itself runs on x86_64 hardware.

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, one must build or obtain a 64bit Xen kernel and
install it on the system.

For domUs, i386PAE is considered as
than amd64.

#NetBSD as 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](/guide/inst/)
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:

[[!template id=programlisting text="""
# 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:

[[!template id=programlisting text="""

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

[[!template id=filecontent name="/boot.cfg" text="""
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:

[[!template id=filecontent name="/boot.cfg" text="""
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

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

[[!template id=programlisting text="""
# cp /usr/pkg/share/examples/rc.d/xen* /etc/rc.d/

Enable `xencommons`:

[[!template id=filecontent name="/etc/rc.conf" text="""

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:

[[!template id=programlisting text="""
# 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):

[[!template id=programlisting text="""
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

[[!template id=programlisting text="""
# 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 large number of well-commented examples, mostly for running

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

[[!template id=filecontent name="/usr/pkg/etc/xen/foo" text="""
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 default, `xl` looks for domain config files 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:

[[!template id=programlisting text="""
xl create foo
xl console foo
xl create -c foo
xl shutdown foo
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, less than 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:

 * The first element is "method:/path/to/disk". Common methods are
   "file:" for file-backed vnd, and "phy:" for something that is already
   a 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, xl demands 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

[[!template id=filecontent name="/usr/pkg/etc/xen/foo" text="""
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

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:

[[!template id=filecontent name="/etc/ifconfig.bridge0" text="""
!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:

[[!template id=filecontent name="/etc/rc.conf" text="""
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 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 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

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](/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
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.


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]( 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 :-).

[]( also lets users with pvgrub to boot
their own kernel.  See then [ 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/).

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