File:  [NetBSD Developer Wiki] / wikisrc / ports / xen / howto.mdwn
Revision 1.127: download - view: text, annotated - select for diffs
Tue Dec 20 19:25:35 2016 UTC (3 years, 1 month ago) by gdt
Branches: MAIN
CVS tags: HEAD
Update xm/xl info

Also, explain /dev file creattion better.



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
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.

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 supports "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
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.


but note that both packages must be installed together and must have
matching versions.

xenkernel3 provides Xen 3.1.  It is no longer maintained by Xen, and the last applied security patch was in
2011. Thus, it should not be used.  It supports PCI passthrough,
which is why people use it anyway. Xen 3.1 supports i386, both PAE and

xenkernel33 provides Xen 3.3.  It is no longer maintained by Xen, and
the last applied security patch was in 2012.  Thus, it should not be
used.  Xen 3.3 supports i386, but only in PAE mode.  There are no good
reasons to run this version.

xenkernel41 provides Xen 4.1.  It is no longer maintained by Xen, but
as of 2016-12 received backported security patches.  Xen 4.1 supports
i386, but only in PAE mode.  There are no good reasons to run this

xenkernel42 provides Xen 4.2.  It is no longer maintained by Xen, but
as of 2016-12 received backported security patches.  Xen 4.2 supports
i386, but only in PAE mode.  The only reason to run this is if you
need to use xm instead of xl, or if you need to run an i386 dom0
(because your hardware is i386 only).

xenkernel45 provides Xen 4.5.  As of 2016-12, security patches were
released by Xen and applied to pkgsrc.  Xen 4.5 requires using a dom0
running NetBSD's amd64 port (Intel or AMD hardware is fine), but domUs
can be amd64 or i386 PAE.  TODO: It is either a conservative choice or
somewhat old.

xenkernel46 provides Xen 4.6.  It is new to pkgsrc as of 2016-05.  As
of 2016-12, security patches were released by Xen and applied to
pkgsrc.  Xen 4.6 similarly requires a NetBSD/amd64 dom0, but domUs can
be amd64 or i386 PAE.  TODO: It is either a somewhat aggressive choice
or the standard choice.

Xen 4.7 (released 2016-06) and 4.8 (released 2016-12) are not yet in


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 xm for 3.1, 3.3 and 4.1 and xl for 4.2 and up.

In 4.2, you can choose to use xm by simply changing the ctl_command

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 xl startup, but as of 201612 it has not been
adequately tested.


The netbsd-6, netbsd-7, and -current branches are all reasonable
choices, with more or less the same considerations for non-Xen use.
Therefore, netbsd-7 is recommended as the stable version of the most
recent release for production use.  In addition, netbsd-7 and -current
have a important scheduler fix (in November of 2015) affecting
contention between dom0 and domUs; see
https://releng.netbsd.org/cgi-bin/req-7.cgi?show=1040 for a
description.  For those wanting to learn Xen or without production
stability concerns, netbsd-7 is still likely most appropriate, but
-current is also a reasonable choice.  (Xen runs ok on netbsd-5, but
the xentools packages are likely difficult to build, and netbsd-5 is
not supported.)

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 < 3.1) or amd64 machines (all Xen
versions).  (Practically, almost any computer where one would want to
run Xen today supports amd64.)

Xen, the dom0 kernel, and each domU kernel can be either i386 or
amd64.  When building a xenkernel package, one obtains i386 on an i386
host, and amd64 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 i386PAE dom0
kernel should in theory work.  An amd64 Xen/dom0 is known to support
both i386PAE and amd64 domUs.

i386 dom0 and domU kernels must be PAE (except for Xen 3.1); these are
built by default.  (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 amd64 for the

Xen 4.2 is the last version to support i386 as a host.  TODO: Clarify
if this is about the CPU, the Xen kernel, or the dom0 kernel having to
be amd64.


Mostly, NetBSD as a dom0 or domU is quite stable.
However, there are some open PRs indicating problems.

 - [PR 48125](http://gnats.netbsd.org/48125)
 - [PR 47720](http://gnats.netbsd.org/47720)

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 xenkernel45 or xenkernel46,
xl, the NetBSD 7 stable branch, and to use an amd64 kernel as the
dom0.  Either the i386PAE or amd64 version of NetBSD may be used as


Ideally, all versions of Xen in pkgsrc would build on all supported
versions of NetBSD/amd64, to the point where this section would be
silly.  However, that has not always been the case.  Besides aging
code and aging compilers, qemu (included in xentools for HVM support)
is difficult to build.  Note that there is intentionally no data for
4.5+ up for i386, and often omits xentools info if the corresponding
kernel fails.

The following table gives status, with the date last checked
(generally on the most recent quarterly branch).  The first code is
"builds" if it builds ok, and "FAIL" for a failure to build.  The
second code/date only appears for xenkernel* and is "works" if it runs
ok as a dom0, and "FAIL" if it won't boot or run a domU.

	xenkernel3 netbsd-6 i386 FAIL 201612
	xenkernel33 netbsd-6 i386 FAIL 201612
	xenkernel41 netbsd-6 i386 builds 201612
	xenkernel42 netbsd-6 i386 builds 201612
	xentools3 netbsd-6 i386 FAIL 201612
	xentools3-hvm netbsd-6 i386 FAIL 201412
	xentools33 netbsd-6 i386 builds 201412
	xentools41 netbsd-6 i386 builds 201612
	xentools42 netbsd-6 i386 FAIL 201612

	xenkernel3 netbsd-7 i386 FAIL 201412
	xenkernel33 netbsd-7 i386 FAIL 201412
	xenkernel41 netbsd-7 i386 builds 201412
	xenkernel42 netbsd-7 i386 builds 201412
	xentools41 netbsd-7 i386 builds 201412
	xentools42 netbsd-7 i386 ??FAIL 201412

	xenkernel3 netbsd-6 amd64 FAIL 201612
	xenkernel33 netbsd-6 amd64 FAIL 201612
	xenkernel41 netbsd-6 amd64 builds 201612 works 201612
	xenkernel42 netbsd-6 amd64 builds 201612 works 201612
	xenkernel45 netbsd-6 amd64 builds 201612
	xenkernel46 netbsd-6 amd64 builds 201612
	xentools41 netbsd-6 amd64 builds 201612
	xentools42 netbsd-6 amd64 builds 201612
	xentools45 netbsd-6 amd64 builds 201612
	xentools46 netbsd-6 amd64 FAIL 201612

	xenkernel3 netbsd-7 amd64 builds 201612
	xenkernel33 netbsd-7 amd64 builds 201612
	xenkernel41 netbsd-7 amd64 builds 201612
	xenkernel42 netbsd-7 amd64 builds 201612
	xenkernel45 netbsd-7 amd64 builds 201612
	xenkernel46 netbsd-7 amd64 builds 201612
	xentools3 netbsd-7 amd64 builds 201612
	xentools3-hvm netbsd-7 amd64 builds 201612
	xentools33 netbsd-7 amd64 FAIL 201612
	xentools41 netbsd-7 amd64 builds 201612
	xentools42 netbsd-7 amd64 builds 201612
	xentools45 netbsd-7 amd64 builds 201612
	xentools46 netbsd-7 amd64 builds 201612

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.

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

[install NetBSD/amd64](/guide/inst/)
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
be used.

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
HOWTO page.)

Installation of Xen

In the dom0, install sysutils/xenkernel42 and sysutils/xentools42 from
pkgsrc (or another matching pair).  See [the pkgsrc
documentation](http://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 kernel, kernfs is mandatory for xend to communicate with the
kernel, so ensure that /kern is in fstab.  TODO: Say this is default,
or file a PR and give a reference.

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, /boot present, and likely
/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


which specifies that the dom0 should have 256M, leaving the rest to be
allocated for domUs.  To use a serial console, use

        menu=Xen:load /netbsd-XEN3_DOM0.gz console=com0;multiboot /xen.gz dom0_mem=256M console=com1 com1=9600,8n1

from 1), forcing speed/parity, and also for NetBSD (which counts
starting at 0).  In an attempt to add performance, one can also add

        dom0_max_vcpus=1 dom0_vcpus_pin

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

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

Configuring Xen

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 "xm", you should enable xend and xenbackendd:


For "xl", you should enable xend and xencommons (xenstored).  Trying
to use xl without xencommons=YES will result in a hang; it is
necessary to hit ^C on the console to let the machine finish booting.
TODO: Check if xend really should be used, and if not why it is


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
        Domain-0           0       64    0  r----     58.1

### Issues with xencommons

xencommons starts xenstored, which stores data on behalf of dom0 and
Certainly all domUs should be shutdown first, following the sort order
and is not notified when xenstored exits, leading to not recreating
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
root, because anita must create a domU):


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

dom0 kernel compared to hardware.

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
items if desired.

Updating NetBSD in a dom0

replaces the kernel and reboots, and then overlays userland binaries
and adjusts /etc.

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

        # Install MBR bootblocks on both disks. 
        fdisk -i /dev/rwd0d
        fdisk -i /dev/rwd1d
        # Install secondary boot loader
        cp -p /usr/mdec/boot /
        # Create boot.cfg following earlier guidance:
        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

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
to /.

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/RAM
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

        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

Older hardware:

        Intel D915GEV, Pentium4 CPU 3.40GHz, 4GB 533MHz Synchronous DDR2

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](http://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
name match.

mediated by Xen, and configured in the dom0.


Config files

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*,

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

The domain will have the same name as the file.  The kernel has the
domUs independently.  The vif line causes an interface to be provided,
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
        xm list

Typing ^] will exit the console session.  Shutting down a domain is
will trigger controlled shutdowns of all configured domUs.

domU kernels

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

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

than the available memory.

use more memory temporarily.  TODO: Explain better, and explain how
well it works with NetBSD.

Virtual disks

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.
"file:" for file-backed vnd. and "phy:" for something that is already
a (TODO: character or block) device.

as xbd0, the second as xbd1, and so on.  However, xm/xl demand a
systems, and it is an unwarranted assumption that they have consistent




Virtual Networking

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.

example /etc/ifconfig.bridge0 is:

        !brconfig bridge0 add wm0


domain.  The interface in dom0 will use this address XOR'd with

Sizing domains

Modern x86 hardware has vast amounts of resources.  However, many
virtual servers can function just fine on far less.  A system with
256M 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:

        xendomains="foo bar"

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 completed the dom0 setup so that "xl list" (or "xm list")

Creating an unprivileged NetBSD domain (domU)



i386 and amd64 provide the following kernels:

        i386 XEN3_DOMU
        i386 XEN3PAE_DOMU
        amd64 XEN3_DOMU

Unless using Xen 3.1 (and you shouldn't) with i386-mode Xen, you must
use the PAE version of the i386 kernel.

system you can't run the installer on).


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


line should be used in the config file.



it would reload the INSTALL_XEN3_DOMU kernel even if you changed the
xbd0a" and you should have a, functional NetBSD domU.

TODO: check if this is still accurate.



It is also desirable to add



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.

the example below)



        root = "/dev/hda1 ro"


guest domain, the following method can be used: install
sysutils/e2fsprogs from pkgsrc.  Use mke2fs to format the partition
using the appropriate tool, possibly running under NetBSD's Linux

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

PCI passthrough: Using PCI devices in guest domains

The dom0 can give other domains access to selected PCI
access to a physical network interface or disk controller.  However,
A guest domain trying to access the VGA registers will panic.

not been ported to later versions at this time.

pciback.hide boot parameter will attach to "pciback" instead of the
single-digit number:





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

        # 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

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

        # SCSI devices

NetBSD as a domU in a VPS

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
to install NetBSD.
their kernel.

or to select from a prepopulated list.  Other approaches are pygrub

CPUs for NetBSD domUs.





Linux but should be updated to cover NetBSD :-).

their own kernel.  See then [prgmr.com NetBSD
(which is in need of updating).

It appears that [grub's FFS
to update the special boot partition.


See the [Amazon EC2 page](../amazon_ec2/).

Using npf

In standard kernels, npf is a module, and thus cannot be loaded in a
DOMU kernel.

TODO: Explain how to compile npf into a custom kernel, answering (but
note that the problem was caused by not booting the right kernel)
[this email to

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.
* grub
  * 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).
    See [pkg/40258](http://gnats.netbsd.org/40258).
  * 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.
* Solve somehow the issue with modules for GENERIC not being loadable
  in a Xen dom0 or domU kernel.

Random pointers

This section contains links from elsewhere not yet integrated into the
HOWTO, and other guides.

* http://www.lumbercartel.ca/library/xen/
* http://pbraun.nethence.com/doc/sysutils/xen_netbsd_dom0.html
* https://gmplib.org/~tege/xen.html
