File:  [NetBSD Developer Wiki] / wikisrc / ports / xen / howto.mdwn
Revision 1.207: download - view: text, annotated - select for diffs
Thu Apr 15 15:23:22 2021 UTC (2 months ago) by gdt
Branches: MAIN
CVS tags: HEAD
xen howto: retract hopeful statement about EFI

on rereading the thread, it isn't supported

[[!meta title="Xen Status and 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
fulfilled.

This document provides status on what Xen things work on NetBSD
(upstream documentation might say something works if it works on some
particular Linux system).

This document is also a HOWTO that presumes a basic familiarity with
the Xen system architecture, with installing NetBSD on amd64 hardware,
and with installing software from pkgsrc.  See also the [Xen
website](http://www.xenproject.org/).

If this document says that something works, and you find that it does
not, it is best to ask on port-xen and if you are correct to file a
PR.

[[!toc]]

# Overview

The basic concept of Xen is that the hypervisor (xenkernel) runs on
the hardware, and runs a privileged domain ("dom0") that can access
disks/networking/etc.  One then runs additional unprivileged domains
(each a "domU"), presumably to do something useful.

This HOWTO addresses how to run a NetBSD dom0 (and hence also build
xen itself).  It also addresses how to run domUs in that environment,
and how to deal with having a domU in a Xen environment run by someone
else and/or not running NetBSD.

There are many choices one can make; the HOWTO recommends the standard
approach and limits discussion of alternatives in many cases.

## Guest Styles

Xen supports different styles of guests.  See
https://wiki.xenproject.org/wiki/Virtualization_Spectrum for a
discussion.

This table shows the styles, and if a NetBSD dom0 can run in that
style, if a NetBSD dom0 can sypport that style of guest in a domU, and
if NetBSD as a domU can support that style.

[[!table data="""
Style of guest	|dom0 can be?	|dom0 can support?	|domU can be?
PV		|yes		|yes			|yes
HVM		|N/A		|yes			|yes
PVHVM		|N/A		|yes			|current only
PVH		|not yet	|current only		|current only
"""]]

In PV (paravirtualized) 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.  See
[PV](https://wiki.xen.org/wiki/Paravirtualization_(PV\)).

In HVM (Hardware Virtual Machine) mode, no guest modification is
required.  However, hardware support is required, such as VT-x on
Intel CPUs and SVM on AMD CPUs to assist with the processor emulation.
The dom0 runs qemu to emulate hardware other than the processor.  It
is therefore non-sensical to have an HVM dom0, because there is no
underlying system to provide emulation.

In PVHVM mode, the guest runs as HVM, but additionally uses PV
drivers for efficiency.  Therefore it is non-sensical for to have a
PVHVM dom0.  See [PV on HVM](https://wiki.xen.org/wiki/PV_on_HVM).

There have been two PVH modes: original PVH and PVHv2.  Original PVH
was based on PV mode and is no longer relevant at all.  Therefore
PVHv2 is written as PVH, here and elsewhere.  PVH is basically
lightweight HVM with PV drivers.  A critical feature of it is that
qemu is not needed; the hypervisor can do the emulation that is
required.  Thus, a dom0 can be PVH.  The source code uses PVH and
config files use pvh, but NB that this refers to PVHv2.  See
[PVH(v2)](https://wiki.xenproject.org/wiki/PVH_(v2\)_Domu).

At system 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.)

## CPU Architecture

Xen runs on x86_64 hardware (the NetBSD amd64 port).

There is a concept of Xen running on ARM, but there are no reports of this working with NetBSD.

The dom0 system should be amd64.  (Instructions for i386PAE dom0 have been removed from the HOWTO.)

The domU can be i386 PAE or amd64.
i386 PAE at one point was considered as [faster](https://lists.xen.org/archives/html/xen-devel/2012-07/msg00085.html) than amd64.
However, as of 2021 it is normal to use amd64 as the domU architecture, and use of i386 is dwindling.

## Xen Versions

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
4.11		|xenkernel411	|x86_64			|No
4.13		|xenkernel413	|x86_64			|No
"""]]

See also the [Xen Security Advisory page](http://xenbits.xen.org/xsa/).

Older Xen had a python-based management tool called xm; this has been
replaced by xl.

## NetBSD versions

Xen has been supported in NetBSD for a long time, at least since 2005.
Initially Xen was PV only.

NetBSD Xen has always supported PV, in both dom0 and domU; for a long
time this was the only way.  NetBSD >=8 as a dom0 supports HVM mode in
domUs.

Support for PVHVM and PVH is available only in NetBSD-current; this is
currently somewhat experimental, although PVHVM appears reasonably
solid.

NetBSD up to and including NetBSD 9 as a dom0 cannot safely run SMP.
Even if one added "options MULTIPROCESSOR" and configured multiple
vcpus, the kernel is likely to crash because of drivers without
adequate locking.

NetBSD-current supports SMP in dom0, and XEN3_DOM0 includes "options
MULTIPROCESSOR".

NetBSD (since NetBSD 6), when run as a domU, can run SMP, using
multiple CPUs if provided.  The XEN3_DOMU kernel is built
with "options MULITPROCESSOR".

Note that while Xen 4.13 is current, the kernel support is still
called XEN3, because the hypercall interface has not changed
significantly.

# Creating a NetBSD dom0

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

NB: As of 2021-04, you must arrange to have the system use BIOS boot,
not EFI boot.

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.  Therefore, use the most recent release, or a build from
the most recent stable branch.  Alternatively, use -current, being
mindful of all the usual caveats of lower stability of current, and
likely a bit more so.  Think about how you will provide storage for
disk images.

## Installation of Xen

### Building Xen

Use the most recent version of Xen in pkgsrc, unless the DESCR says
that it is not suitable.  Therefore, choose 4.13.  In the dom0,
install xenkernel413 and xentools413 from pkgsrc.

Once this is done, copy the Xen kernel from where pkgsrc puts it to
where the boot process will be able to find it:

[[!template id=programlisting text="""
# cp -p /usr/pkg/xen413-kernel/xen.gz /
"""]]

Then, place a NetBSD XEN3_DOM0 kernel in the `/` directory. Such
kernel can either be taken from a local release build.sh run, compiled
manually, or downloaded from the NetBSD FTP, for example at:

[[!template id=programlisting text="""
ftp.netbsd.org/pub/NetBSD/NetBSD-9.1/amd64/binary/kernel/netbsd-XEN3_DOM0.gz
"""]]

### Configuring booting

Read boot.cfg(8) carefully.  Add lines to /boot.cfg to boot Xen,
adjusting for your root filesystem:

[[!template id=filecontent name="/boot.cfg" text="""
menu=Xen:load /netbsd-XEN3_DOM0.gz root=wd0a console=pc;multiboot /xen.gz dom0_mem=512M
menu=Xen single user:load /netbsd-XEN3_DOM0.gz root=wd0a console=pc -s;multiboot /xen.gz dom0_mem=512M
"""]]

This specifies that the dom0 should have 512MB of ram, leaving the rest
to be allocated for domUs.

NB: This says add, not replace, so that you will be able to more
easily boot a NetBSD kernel without Xen.  Once Xen boots ok, you may
want to set it as default.  It is highly likely that you will have
trouble at some point, and keeping an up-to-date GENERIC for use in
fixing problems is the standard prudent approach.

\todo Explain why rndseed is not set with Xen as part of the dom0
subconfiguration.

Note that you are likely to have to set root= because the boot device
from /boot is not passed via Xen to the dom0 kernel.  With one disk,
it will work, but e.g. plugging in USB disk to a machine with root on
wd0a causes boot to fail.

Beware that userconf statements must be attached to the dom0 load, and
may not be at top-level, because then they would try to configure the
hypervisor, if there is a way to pass them via multiboot.  It appears
that adding `userconf=pckbc` to `/boot.cfg` causes Xen to crash very
early with a heap overflow.

### Console selection

See boot_console(8).  Understand that you should start from a place of
having console setup correct for booting GENERIC before trying to
configure Xen.

Generally for GENERIC, one sets the console in bootxx_ffsv1 or
equivalent, and this is passed on to /boot (where one typically does
not set the console).  This configuration of bootxx_ffsv1 should also
be in place for Xen systems, to allow seeing messages from /boot and
use of a keyboard to select a line from the menu.  And, one should
have a working boot path to GENERIC for rescue situations.

With GENERIC, the boot options are passed on to /netbsd, but there is
currently no mechanism to pass these via multiboot to the hypervisor.
Thus, in addition to configuring the console in the boot blocks, one
must also configure it for Xen.

By default, the hypervisor (Xen itself) will use some sort of vga
device as the console, much like GENERIC uses by default.  The vga
console is relinquished at the conclusion of hypervisor boot, before
the dom0 is started.  Xen when using a vga console does not process
console input.

The hypervisor can be configured to use a serial port console, e.g.
[[!template id=filecontent name="/boot.cfg" text="""
menu=Xen:load /netbsd-XEN3_DOM0.gz console=com0;multiboot /xen.gz dom0_mem=512M console=com1 com1=9600,8n1
"""]]
This example uses the first serial port (Xen counts from 1; this is
what NetBSD would call com0), and sets speed and parity.  (The dom0 is
then configured to use the same serial port in this example.)

With the hypervisor configured for a serial console, it can get input,
and there is a notion of passing this input to the dom0.  \todo
Explain why, if Xen has a serial console, the dom0 console is
typically also configured to open that same serial port, instead of
getting the passthrough input via the xen console.

One also configures the console for the dom0.  While one might expect
console=pc to be default, following behavior of GENERIC, a hasty read
of the code suggests there is no default and booting without a
selected console might lead to a panic.  Also, there is merit in
explicit configuration.  Therefore the standard approach is to place
console=pc as part of the load statement for the dom0 kernel, or
alternatively console=com0.

The NetBSD dom0 kernel will attach xencons(4) (the man page does not
exist), but this is not used as a console.  It is used to obtain the
messages from the hypervisor's console; run `xl dmesg` to see them.

### Tuning

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](http://xenbits.xenproject.org/docs/4.13-testing/misc/xen-command-line.html),
and other than dom0 memory and max_vcpus, they are generally not
necessary.

\todo Revisit this advice with current.
\todo Explain if anyone has ever actually measured that this helps.

### rc.conf

Ensure that the boot scripts installed in
`/usr/pkg/share/examples/rc.d` are in `/etc/rc.d`, either because you
have `PKG_RCD_SCRIPTS=yes`, or manually.  (This is not special to Xen,
but a normal part of pkgsrc usage.)

Set `xencommons=YES` in rc.conf:

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

\todo Recommend for/against xen-watchdog.

### Testing

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

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.
\todo Confirm if this is still true in 2020.

## Xen-specific NetBSD issues

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

One is that through NetBSD 9 the module ABI is different because some
of the #defines change, so there are separate sets of modules in
/stand.  (Further, zfs in Xen is troubled because of differing
MAXPHYS; see the zfs howto for more.)  In NetBSD-current, there is
only one set of modules.

The other difference is that XEN3_DOM0 does not have exactly the same
options as GENERIC.  While this is roughly agreed to be in large part
a bug, users should be aware of this and can simply add missing config
items if desired.

Finally, there have been occasional reports of trouble with X11
servers in NetBSD as a dom0.  Some hardware support is intentionally
disabled in XEN3_DOM0.

## Updating Xen in a dom0

Note the previous advice to maintain a working and tested boot config
into GENERIC without Xen.

Updating Xen in a dom0 consists of updating the xnekernel and xentools
packages, along with copying the xen.gz into place, and of course
rebooting.

If updating along a Xen minor version, e.g. from 4.13.1 to 4.13.2, or
from 4.13.2nb1 to 4.13.2nb3, it is very likely that this can be done
on a running system.  The point is that the xentools programs will be
replaced, and you will be using "xl" from the new installation to talk
to the older programs which are still running.  Problems from this
update path should be reported.

For added safety, shutdown all domUs before updating, to remove the
need for new xl to talk to old xenstored.  Note that Xen does not
guarantee stability of internal ABIs.

If updating across Xen minor versions, e.g. from 4.11 to 4.13, the
likelihood of trouble is increased.  Therefore, 'make replace' of
xentools on a dom0 with running domUs is not recommended.  A shutdown
on all domUs before replacing xentools is likely sufficient.  A safer
appraoch is to boot into GENERIC to replace the packages, as then no
Xen code will be running.  Single user is another option.

## 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 should update both the non-Xen kernel typically used for
rescue purposes, as well as the DOM0 kernel used with Xen.

## anita (for testing NetBSD)

With a NetBSD dom0, even without any domUs, one can 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/
"""]]

# 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 very small number of
examples for running GNU/Linux.

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.

There is not a type line; that implicitly defines a pv domU.
Otherwise, one sets type to the lower-case version of the domU type in
the table above; see later sections.

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

Examples of commands:

[[!template id=programlisting text="""
xl create /usr/pkg/etc/xen/foo
xl console domU-id
xl create -c /usr/pkg/etc/xen/foo
xl shutdown domU-id
xl list
"""]]

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

## CPU and memory

A domain is provided with some number of vcpus; any domain can have up
to the number of CPUs seen by the hypervisor. For a domU, it is
controlled from the config file by the "vcpus = N" directive.  It is
normal to overcommit vcpus; a 4-core machine machine might well provide 4
vcpus to each domU.  One might also configure fewer vcpus for a domU.

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.

## Balloon driver

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

\todo Explain how to set up a aystem to use the balloon scheme in a
useful manner.

## 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 a file-backed vnd, and "phy:" for something that is already
   a device, such as an LVM logical volume.

 * 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
   /dev/hda1.

 * The third element is "w" for writable disks, and "r" for read-only
   disks.

Example:
[[!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
dom0.

## 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="""
create
up
!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"
"""]]

# domU setup for specific systems

Creating domUs is almost entirely independent of operating system.  We
have already presented the basics of config files in the previous system.

Of course, this section presumes that you have a working dom0.

## Creating a NetBSD PV 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 built for Xen, to use PV interfacesas
a domU.  NetBSD release builds provide the following kernels:

        i386 XEN3PAE_DOMU
        amd64 XEN3_DOMU

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

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

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

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

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

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

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

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

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

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

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

One should also run `powerd` in a domU, but this should not need
configuring.  With powerd, the domain will run a controlled shutdown
if `xl shutdown -R` or `xl shutdown -H` is used on the dom0, via
receiving a synthetic `power button pressed` signal.  In 9 and
current, `powerd` is run by default under Xen kernels (or if ACPI is
present), and it can be added to rc.conf if not.

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.)

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.

## Creating a Linux PV domU

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

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

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

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

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

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

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

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

    extra = "xencons=tty1"

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

## Creating a NetBSD HVM domU

Use type='hvm', probably.  Use a GENERIC kernel within the disk image.

## Creating a NetBSD PVH domU

This only works with a current kernel in the domU.

Use type='pvh'.  Probably, use a GENERIC kernel within the disk image,
which in current has PV support.

\todo Verify.

\todo Verify if one can have current PVH domU on a 9 dom0.

## Creating a Solaris domU

See possibly outdated
[Solaris domU instructions](/ports/xen/howto-solaris/).

## PCI passthrough: Using PCI devices in guest domains

NB: PCI passthrough only works on some Xen versions and as of 2020 it
is not clear that it works on any version in pkgsrc.  \todo Reports
confirming or denying this notion should be sent to port-xen@.

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.hide=(00:0a.0)(00:06.0)

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


# Miscellaneous Information

## Nesting under Linux KVM

It is possible to run Xen and a NetBSD dom0 under Linux KVM.  One
can enable virtio in the dom0 for greater speed.

## Nesting under qemu

It is possible to run Xen and a NetBSD dom0 under qemu on NetBSD, and
also with nvmm.
\todo Check this.

## Other nesting

In theory, any full emulation should be able to run Xen and a NetBSD
dom0.  The HOWTO does not currently have information about Xen XVM
mode, Virtualbox, etc.

## NetBSD 5 as domU

[NetBSD 5 is known to panic.](http://mail-index.netbsd.org/port-xen/2018/04/17/msg009181.html)
(However, NetBSD 5 systems should be updated to a supported version.)

# 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.

## Complexities due to Xen changes

Xen has many security advisories and people running Xen systems make
different choices.

### stub domains

Some (Linux) dom0 systems use something called "stub domains" to
isolate qemu from the dom0 system, as a security and reliabilty
mechanism when running HVM domUs.  Somehow, NetBSD's GENERIC kernel
ends up using PIO for disks rather than DMA.  Of course, all of this
is emulated, but emulated PIO is unusably slow.  This problem is not
currently understood.

### Grant tables

There are multiple versions of using grant tables, and some security
advisories have suggested disabling some versions.  NetBSD through 9
uses version 1 and NetBSD-current uses version 2.  This can lead to
"NetBSD current doesn't run on hosting provider X" situations.

\todo Explain better.

## Boot methods

### pvgrub

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.

It appears that [grub's FFS
code](http://xenbits.xensource.com/hg/xen-unstable.hg/file/bca284f67702/tools/libfsimage/ufs/fsys_ufs.c)
does not support all aspects of modern FFS, but there are also reports
that FFSv2 works fine.

### pygrub

As of 2014, pygrub seems to be of mostly historical interest.  As of
2021, the section should perhaps be outright deleted.

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

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

## Specific Providers

The intent is to list providers only if they document support for
running NetBSD, and to point to their resources briefly.

### panix.com

[Panix](http://www.panix.com/) provides NetBSD as an OS option.  See
https://www.panix.com/v-colo/nupgrade.html for some information.
Users can 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 how to
boot NetBSD.

### prgmr.com

[prgmr.com](http://prgmr.com/) provides released versions of
NetBSD/amd64 as installation options.  Users can use pvgrub to boot
their own kernel, and a small FAT32 /boot is encouraged.  See the
[prgmr.com NetBSD
HOWTO](http://wiki.prgmr.com/mediawiki/index.php/NetBSD_as_a_DomU)
(which is in need of updating).

### Amazon

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

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