7: Xen is a hypervisor (or virtual machine monitor) for x86 hardware
8: (i686-class or higher), which supports running multiple guest
9: operating systems on a single physical machine. Xen is a Type 1 or
10: bare-metal hypervisor; one uses the Xen kernel to control the CPU,
11: memory and console, a dom0 operating system which mediates access to
12: other hardware (e.g., disks, network, USB), and one or more domU
13: operating systems which operate in an unprivileged virtualized
14: environment. IO requests from the domU systems are forwarded by the
15: hypervisor (Xen) to the dom0 to be fulfilled.
17: Xen supports two styles of guests. The original is Para-Virtualized
18: (PV) which means that the guest OS does not attempt to access hardware
19: directly, but instead makes hypercalls to the hypervisor. This is
20: analogous to a user-space program making system calls. (The dom0
21: operating system uses PV calls for some functions, such as updating
22: memory mapping page tables, but has direct hardware access for disk
23: and network.) PV guests must be specifically coded for Xen.
25: The more recent style is HVM, which means that the guest does not have
26: code for Xen and need not be aware that it is running under Xen.
27: Attempts to access hardware registers are trapped and emulated. This
28: style is less efficient but can run unmodified guests.
30: Generally any machine that runs NetBSD/amd64 will work with Xen and PV
31: guests. In theory i386 computers (without x86_64/amd64 support) can
32: be used for Xen <= 4.2, but we have no recent reports of this working
33: (this is a hint). For HVM guests, hardware support is needed, but it
34: is common on recent machines. For Intel CPUs, one needs the VT-x
35: extension, shown in "cpuctl identify 0" as VMX. For AMD CPUs, one
36: needs the AMD-V extensions, shown in "cpuctl identify 0" as SVM.
37: There are further features for IOMMU virtualization, Intel's VT-d and
38: AMD's AMD-Vi. TODO: Explain whether Xen on NetBSD makes use of these
39: features. TODO: Review by someone who really understands this.
41: Note that a FreeBSD dom0 requires VT-x and VT-d (or equivalent); this
42: is because the FreeBSD dom0 does not run in PV mode.
44: At boot, the dom0 kernel is loaded as a module with Xen as the kernel.
45: The dom0 can start one or more domUs. (Booting is explained in detail
46: in the dom0 section.)
48: NetBSD supports Xen in that it can serve as dom0, be used as a domU,
49: and that Xen kernels and tools are available in pkgsrc. This HOWTO
50: attempts to address both the case of running a NetBSD dom0 on hardware
51: and running domUs under it (NetBSD and other), and also running NetBSD
52: as a domU in a VPS.
54: Xen 3.1 in pkgsrc supports "PCI passthrough", which means that
55: specific PCI devices can be made available to a specific domU instead
56: of the dom0. This can be useful to let a domU run X11, or access some
57: network interface or other peripheral.
59: NetBSD 6 and earlier supported Xen 2; support was removed from NetBSD
60: 7. Xen 2 has been removed from pkgsrc.
65: Installing NetBSD/Xen is not extremely difficult, but it is more
66: complex than a normal installation of NetBSD.
67: In general, this HOWTO is occasionally overly restrictive about how
68: things must be done, guiding the reader to stay on the established
69: path when there are no known good reasons to stray.
71: This HOWTO presumes a basic familiarity with the Xen system
72: architecture, with installing NetBSD on i386/amd64 hardware, and with
73: installing software from pkgsrc. See also the [Xen
76: Versions of Xen and NetBSD
79: Most of the installation concepts and instructions are independent
80: of Xen version and NetBSD version. This section gives advice on
81: which version to choose. Versions not in pkgsrc and older unsupported
82: versions of NetBSD are intentionally ignored.
84: The term "amd64" is used to refer to both the NetBSD port and to the
85: hardware architecture on which it runs. (Such hardware is made by
86: both Intel and AMD, and in 2016 a normal PC has this CPU
92: In NetBSD, Xen is provided in pkgsrc, via matching pairs of packages
93: xenkernel and xentools. We will refer only to the kernel versions,
94: but note that both packages must be installed together and must have
95: matching versions.
97: xenkernel3 provides Xen 3.1. It is no longer maintained by Xen, and
98: the last applied security patch was in 2011. Thus, it should not be
99: used. It supports PCI passthrough, which is why people use it anyway.
100: Xen 3.1 runs on i386 (both non-PAE and PAE) and amd64 hardware.
102: xenkernel33 provides Xen 3.3. It is no longer maintained by Xen, and
103: the last applied security patch was in 2012. Thus, it should not be
104: used. Xen 3.3 runs on i386 PAE and amd64 hardware. There are no good
105: reasons to run this version.
107: xenkernel41 provides Xen 4.1. It is no longer maintained by Xen, but
108: as of 2016-12 received backported security patches. Xen 4.1 runs on
109: i386 PAE and amd64 hardware. There are no good reasons to run this
112: Note that 3.1, 3.3 and 4.1 have been removed from pkgsrc-current, but
113: are in 2016Q4. They will be removed from this HOWTO sometime after
116: xenkernel42 provides Xen 4.2. It is no longer maintained by Xen, but
117: as of 2016-12 received backported security patches. Xen 4.2 runs on
118: i386 PAE and amd64 hardware. The only reason to run this is if you
119: need to use xm instead of xl, or if you need to run on hardware that
120: supports i386 but not amd64. (This might also be useful if you need
121: an i386 dom0, if it turns out that an amd64 Xen kernel and an i386
122: dom0 is problematic.)
124: xenkernel45 provides Xen 4.5. As of 2016-12, security patches were
125: released by Xen and applied to pkgsrc. Xen 4.5 runs on amd64 hardware
126: only. While slightly old, 4.5 has been tested and run by others, so
127: it is the conservative choice.
129: xenkernel46 provides Xen 4.6. It is new to pkgsrc as of 2016-05. As
130: of 2016-12, security patches were released by Xen and applied to
131: pkgsrc. Xen 4.6 runs on amd64 hardware only For new installations,
132: 4.6 is probably the appropriate choice and it will likely soon be the
133: standard approach. (If using Ubuntu guests, be sure to have the
134: xentools46 from December, 2016).
136: Xen 4.7 (released 2016-06) and 4.8 (released 2016-12) are not yet in
139: See also the [Xen Security Advisory page](http://xenbits.xen.org/xsa/).
141: Note that NetBSD support is called XEN3. It works with Xen 3 and Xen
142: 4 because the hypercall interface has been stable.
144: Xen command program
147: Early Xen used a program called xm to manipulate the system from the
148: dom0. Starting in 4.1, a replacement program with similar behavior
149: called xl is provided, but it does not work well in 4.1. In 4.2, both
150: xm and xl work fine. 4.4 is the last version that has xm.
152: You must make a global choice to use xm or xl, because it affects not
153: only which command you use, but the command used by rc.d scripts
154: (specifically xendomains) and which daemons should be run. The
155: xentools packages provide xm for 3.1, 3.3 and 4.1 and xl for 4.2 and up.
157: In 4.2, you can choose to use xm by simply changing the ctl_command
158: variable and setting xend=YES in rc.conf.
160: With xl, virtual devices are configured in parallel, which can cause
161: problems if they are written assuming serial operation (e.g., updating
162: firewall rules without explicit locking). There is now locking for
163: the provided scripts, which works for normal casses (e.g, file-backed
164: xbd, where a vnd must be allocated). But, as of 201612, it has not
165: been adequately tested for a complex custom setup with a large number
166: of interfaces.
171: The netbsd-6, netbsd-7, and -current branches are all reasonable
172: choices, with more or less the same considerations for non-Xen use.
173: Therefore, netbsd-7 is recommended as the stable version of the most
174: recent release for production use. In addition, netbsd-7 and -current
175: have a important scheduler fix (in November of 2015) affecting
176: contention between dom0 and domUs; see
177: https://releng.netbsd.org/cgi-bin/req-7.cgi?show=1040 for a
178: description. For those wanting to learn Xen or without production
179: stability concerns, netbsd-7 is still likely most appropriate, but
180: -current is also a reasonable choice. (Xen runs ok on netbsd-5, but
181: the xentools packages are likely difficult to build, and netbsd-5 is
182: not supported.)
184: As of NetBSD 6, a NetBSD domU will support multiple vcpus. There is
185: no SMP support for NetBSD as dom0. (The dom0 itself doesn't really
186: need SMP for dom0 functions; the lack of support is really a problem
187: when using a dom0 as a normal computer.)
192: Xen itself can run on i386 (Xen < 4.2) or amd64 hardware (all Xen
193: versions). (Practically, almost any computer where one would want to
194: run Xen today supports amd64.)
196: Xen, the dom0 system, and each domU system can be either i386 or
197: amd64. When building a xenkernel package, one obtains an i386 Xen
198: kernel on an i386 host, and an amd64 Xen kernel on an amd64 host. If
199: the Xen kernel is i386, then the dom0 kernel and all domU kernels must
200: be i386. With an amd64 Xen kernel, an amd64 dom0 kernel is known to
201: work, and an i386 dom0 kernel should in theory work. An amd64
202: Xen/dom0 is known to support both i386 and amd64 domUs.
204: i386 dom0 and domU kernels must be PAE (except for an i386 Xen 3.1
205: kernel, where one can use non-PAE for dom0 and all domUs); PAE kernels
206: are included in the NetBSD default build. (Note that emacs (at least)
207: fails if run on i386 with PAE when built without, and vice versa,
208: presumably due to bugs in the undump code.)
210: Because of the above, the standard approach is to use an amd64 Xen
211: kernel and NetBSD/amd64 for the dom0. For domUs, NetBSD/i386 (with
212: the PAE kernel) and NetBSD/amd64 are in widespread use, and there is
213: little to no Xen-specific reason to prefer one over the other.
215: Note that to use an i386 dom0 with Xen 4.5 or higher, one must build
216: (or obtain from pre-built packages) an amd64 Xen kernel and install
217: that on the system. (One must also use a PAE i386 kernel, but this is
218: also required with an i386 Xen kernel.). Almost no one in the
219: NetBSD/Xen community does this, and the standard, well-tested,
220: approach is to use an amd64 dom0.
222: A [posting on
224: explained that PV system call overhead was higher on amd64, and thus
225: there is some notion that i386 guests are faster. It goes on to
226: caution that the total situation is complex and not entirely
227: understood. On top of that caution, the post is about Linux, not
228: NetBSD. TODO: Include link to benchmarks, if someone posts them.
233: Mostly, NetBSD as a dom0 or domU is quite stable.
234: However, there are some open PRs indicating problems.
236: - [PR 48125](http://gnats.netbsd.org/48125)
237: - [PR 47720](http://gnats.netbsd.org/47720)
239: Note also that there are issues with sparse vnd(4) instances, but
240: these are not about Xen -- they just are noticed with sparse vnd(4)
241: instances in support of virtual disks in a dom0.
246: Therefore, this HOWTO recommends running xenkernel45 or xenkernel46,
247: xl, the NetBSD 7 stable branch, and to use an amd64 kernel as the
248: dom0. Either the i386PAE or amd64 version of NetBSD may be used as
251: Because bugs are fixed quite often, and because of Xen security
252: advisories, it is good to stay up to date with NetBSD (tracking a
253: stable branch), with the Xen kernel (tracking a Xen version via
254: pkgsrc), and with the Xen tools. Specifically, NetBSD (-7 and
255: -current) got an important fix affecting dom0/domU timesharing in
256: November, 2015, and xentools46 got a fix to enable Ubuntu guests to
257: boot in December, 2016.
262: Ideally, all versions of Xen in pkgsrc would build on all supported
263: versions of NetBSD/amd64, to the point where this section would be
264: silly. However, that has not always been the case. Besides aging
265: code and aging compilers, qemu (included in xentools for HVM support)
266: is difficult to build. Note that there is intentionally no data for
267: 4.5+ up for i386, and often omits xentools info if the corresponding
268: kernel fails.
270: The following table gives status, with the date last checked
271: (generally on the most recent quarterly branch). The first code is
272: "builds" if it builds ok, and "FAIL" for a failure to build. The
273: second code/date only appears for xenkernel* and is "works" if it runs
274: ok as a dom0 and can support a domU, and "FAIL" if it won't boot or
275: run a domU.
277: xenkernel3 netbsd-6 i386 FAIL 201612
278: xenkernel33 netbsd-6 i386 FAIL 201612
279: xenkernel41 netbsd-6 i386 builds 201612
280: xenkernel42 netbsd-6 i386 builds 201612
281: xentools3 netbsd-6 i386 FAIL 201612
282: xentools33 netbsd-6 i386 FAIL 201612
283: xentools41 netbsd-6 i386 builds 201612
284: xentools42 netbsd-6 i386 FAIL 201612
286: xenkernel3 netbsd-7 i386 FAIL 201412
287: xenkernel33 netbsd-7 i386 FAIL 201412
288: xenkernel41 netbsd-7 i386 builds 201412
289: xenkernel42 netbsd-7 i386 builds 201412
290: xentools41 netbsd-7 i386 builds 201412
291: xentools42 netbsd-7 i386 ??FAIL 201412
293: xenkernel3 netbsd-6 amd64 FAIL 201612
294: xenkernel33 netbsd-6 amd64 FAIL 201612
295: xenkernel41 netbsd-6 amd64 builds 201612 works 201612
296: xenkernel42 netbsd-6 amd64 builds 201612 works 201612
297: xenkernel45 netbsd-6 amd64 builds 201612
298: xenkernel46 netbsd-6 amd64 builds 201612
299: xentools41 netbsd-6 amd64 builds 201612
300: xentools42 netbsd-6 amd64 builds 201612
301: xentools45 netbsd-6 amd64 builds 201612
302: xentools46 netbsd-6 amd64 FAIL 201612
304: xenkernel3 netbsd-7 amd64 builds 201612
305: xenkernel33 netbsd-7 amd64 builds 201612
306: xenkernel41 netbsd-7 amd64 builds 201612
307: xenkernel42 netbsd-7 amd64 builds 201612
308: xenkernel45 netbsd-7 amd64 builds 201612
309: xenkernel46 netbsd-7 amd64 builds 201612
310: xentools3 netbsd-7 amd64 builds 201612
311: xentools3-hvm netbsd-7 amd64 builds 201612
312: xentools33 netbsd-7 amd64 FAIL 201612
313: xentools41 netbsd-7 amd64 builds 201612
314: xentools42 netbsd-7 amd64 builds 201612
315: xentools45 netbsd-7 amd64 builds 201612
316: xentools46 netbsd-7 amd64 builds 201612
318: NetBSD as a dom0
321: NetBSD can be used as a dom0 and works very well. The following
322: sections address installation, updating NetBSD, and updating Xen.
323: Note that it doesn't make sense to talk about installing a dom0 OS
324: without also installing Xen itself. We first address installing
325: NetBSD, which is not yet a dom0, and then adding Xen, pivoting the
326: NetBSD install to a dom0 install by just changing the kernel and boot
329: For experimenting with Xen, a machine with as little as 1G of RAM and
330: 100G of disk can work. For running many domUs in productions, far
331: more will be needed; e.g. 4-8G and 1T of disk is reasonable for a
332: half-dozen domUs of 512M and 32G each. Basically, the RAM and disk
333: have to be bigger than the sum of the RAM/disk needs of the dom0 and
334: all the domUs.
336: Styles of dom0 operation
339: There are two basic ways to use Xen. The traditional method is for
340: the dom0 to do absolutely nothing other than providing support to some
341: number of domUs. Such a system was probably installed for the sole
342: purpose of hosting domUs, and sits in a server room on a UPS.
344: The other way is to put Xen under a normal-usage computer, so that the
345: dom0 is what the computer would have been without Xen, perhaps a
346: desktop or laptop. Then, one can run domUs at will. Purists will
347: deride this as less secure than the previous approach, and for a
348: computer whose purpose is to run domUs, they are right. But Xen and a
349: dom0 (without domUs) is not meaningfully less secure than the same
350: things running without Xen. One can boot Xen or boot regular NetBSD
351: alternately with little problems, simply refraining from starting the
352: Xen daemons when not running Xen.
354: Note that NetBSD as dom0 does not support multiple CPUs. This will
355: limit the performance of the Xen/dom0 workstation approach. In theory
356: the only issue is that the "backend drivers" are not yet MPSAFE:
359: Installation of NetBSD
363: [install NetBSD/amd64](/guide/inst/)
364: just as you would if you were not using Xen.
365: However, the partitioning approach is very important.
367: If you want to use RAIDframe for the dom0, there are no special issues
368: for Xen. Typically one provides RAID storage for the dom0, and the
369: domU systems are unaware of RAID. The 2nd-stage loader bootxx_* skips
370: over a RAID1 header to find /boot from a file system within a RAID
371: partition; this is no different when booting Xen.
373: There are 4 styles of providing backing storage for the virtual disks
374: used by domUs: raw partitions, LVM, file-backed vnd(4), and SAN.
376: With raw partitions, one has a disklabel (or gpt) partition sized for
377: each virtual disk to be used by the domU. (If you are able to predict
378: how domU usage will evolve, please add an explanation to the HOWTO.
379: Seriously, needs tend to change over time.)
381: One can use [lvm(8)](/guide/lvm/) to create logical devices to use
382: for domU disks. This is almost as efficient as raw disk partitions
383: and more flexible. Hence raw disk partitions should typically not
384: be used.
386: One can use files in the dom0 file system, typically created by dd'ing
387: /dev/zero to create a specific size. This is somewhat less efficient,
388: but very convenient, as one can cp the files for backup, or move them
389: between dom0 hosts.
391: Finally, in theory one can place the files backing the domU disks in a
392: SAN. (This is an invitation for someone who has done this to add a
393: HOWTO page.)
395: Installation of Xen
398: In the dom0, install sysutils/xenkernel42 and sysutils/xentools42 from
399: pkgsrc (or another matching pair). See [the pkgsrc
400: documentation](http://www.NetBSD.org/docs/pkgsrc/) for help with
401: pkgsrc. Ensure that your packages are recent; the HOWTO does not
402: contemplate old builds.
405: For Xen 3.1, support for HVM guests is in sysutils/xentool3-hvm. More
406: recent versions have HVM support integrated in the main xentools
407: package. It is entirely reasonable to run only PV guests.
409: Next you need to install the selected Xen kernel itself, which is
410: installed by pkgsrc as "/usr/pkg/xen*-kernel/xen.gz". Copy it to /.
411: For debugging, one may copy xen-debug.gz; this is conceptually similar
412: to DIAGNOSTIC and DEBUG in NetBSD. xen-debug.gz is basically only
413: useful with a serial console. Then, place a NetBSD XEN3_DOM0 kernel
414: in /, copied from releasedir/amd64/binary/kernel/netbsd-XEN3_DOM0.gz
415: of a NetBSD build. If using i386, use
416: releasedir/i386/binary/kernel/netbsd-XEN3PAE_DOM0.gz. (If using Xen
417: 3.1 and i386, you may use XEN3_DOM0 with the non-PAE Xen. But you
418: should not use Xen 3.1.) Both xen and the NetBSD kernel may be (and
419: typically are) left compressed.
421: In a dom0, kernfs is mandatory for xend to communicate with the
422: kernel, so ensure that /kern is in fstab. (A standard NetBSD install
423: should already mount /kern.)
425: Because you already installed NetBSD, you have a working boot setup
426: with an MBR bootblock, either bootxx_ffsv1 or bootxx_ffsv2 at the
427: beginning of your root file system, have /boot, and likely also
428: /boot.cfg. (If not, fix before continuing!)
430: Add a line to to /boot.cfg to boot Xen. See boot.cfg(5) for an
431: example. The basic line is
433: menu=Xen:load /netbsd-XEN3_DOM0.gz console=pc;multiboot /xen.gz dom0_mem=256M
435: which specifies that the dom0 should have 256M, leaving the rest to be
436: allocated for domUs. To use a serial console, use
438: menu=Xen:load /netbsd-XEN3_DOM0.gz console=com0;multiboot /xen.gz dom0_mem=256M console=com1 com1=9600,8n1
440: which will use the first serial port for Xen (which counts starting
441: from 1), forcing speed/parity, and also for NetBSD (which counts
442: starting at 0). In an attempt to add performance, one can also add
444: dom0_max_vcpus=1 dom0_vcpus_pin
446: to force only one vcpu to be provided (since NetBSD dom0 can't use
447: more) and to pin that vcpu to a physical CPU. TODO: benchmark this.
449: Xen has [many boot
451: and other than dom0 memory and max_vcpus, they are generally not
454: As with non-Xen systems, you should have a line to boot /netbsd (a
455: kernel that works without Xen). Consider a line to boot /netbsd.ok (a
456: fallback version of the non-Xen kernel, updated manually when you are
457: sure /netbsd is ok). Consider also a line to boot fallback versions
458: of Xen and the dom0 kernel, but note that non-Xen NetBSD can be used
459: to resolve Xen booting issues.
461: Probably you want a default=N line to choose Xen in the absence of
464: Now, reboot so that you are running a DOM0 kernel under Xen, rather
465: than GENERIC without Xen.
467: Using grub (historic)
470: Before NetBSD's native bootloader could support Xen, the use of
471: grub was recommended. If necessary, see the
472: [old grub information](/ports/xen/howto-grub).
474: The [HowTo on Installing into
476: explains how to set up booting a dom0 with Xen using grub with
477: NetBSD's RAIDframe. (This is obsolete with the use of NetBSD's native
478: boot. Now, just create a system with RAID-1, and alter /boot.cfg as
479: described above.)
481: Configuring Xen
484: Xen logs will be in /var/log/xen.
486: Now, you have a system that will boot Xen and the dom0 kernel, but not
487: do anything else special. Make sure that you have rebooted into Xen.
488: There will be no domUs, and none can be started because you still have
489: to configure the dom0 daemons.
491: The daemons which should be run vary with Xen version and with whether
492: one is using xm or xl. The Xen 3.1, 3.3 and 4.1 packages use xm. Xen
493: 4.2 and up packages use xl. To use xm with 4.2, edit xendomains to
494: use xm instead.
496: For 3.1 and 3.3, you should enable xend and xenbackendd:
501: For 4.1 and up, you should enable xencommons. Not enabling xencommons
502: will result in a hang; it is necessary to hit ^C on the console to let
503: the machine finish booting. If you are using xm (default in 4.1, or
504: if you changed xendomains in 4.2), you should also enable xend:
506: xend=YES # only if using xm, and only installed <= 4.2
509: TODO: Recommend for/against xen-watchdog.
511: After you have configured the daemons and either started them (in the
512: order given) or rebooted, use xm or xl to inspect Xen's boot messages,
513: available resources, and running domains. An example with xl follows:
515: # xl dmesg
516: [xen's boot info]
517: # xl info
518: [available memory, etc.]
519: # xl list
520: Name Id Mem(MB) CPU State Time(s) Console
521: Domain-0 0 64 0 r---- 58.1
523: ### Issues with xencommons
525: xencommons starts xenstored, which stores data on behalf of dom0 and
526: domUs. It does not currently work to stop and start xenstored.
527: Certainly all domUs should be shutdown first, following the sort order
528: of the rc.d scripts. However, the dom0 sets up state with xenstored,
529: and is not notified when xenstored exits, leading to not recreating
530: the state when the new xenstored starts. Until there's a mechanism to
531: make this work, one should not expect to be able to restart xenstored
532: (and thus xencommons). There is currently no reason to expect that
533: this will get fixed any time soon.
535: ### No-longer needed advice about devices
537: The installation of NetBSD should already have created devices for xen
538: (xencons, xenevt, xsd_kva), but if they are not present, create them:
540: cd /dev && sh MAKEDEV xen
542: anita (for testing NetBSD)
545: With the setup so far (assuming 4.2/xl), one should be able to run
546: anita (see pkgsrc/misc/py-anita) to test NetBSD releases, by doing (as
547: root, because anita must create a domU):
549: anita --vmm=xl test file:///usr/obj/i386/
551: Alternatively, one can use --vmm=xm to use xm-based domU creation
552: instead (and must, on Xen <= 4.1). TODO: confirm that anita xl really works.
554: Xen-specific NetBSD issues
557: There are (at least) two additional things different about NetBSD as a
558: dom0 kernel compared to hardware.
560: One is that the module ABI is different because some of the #defines
561: change, so one must build modules for Xen. As of netbsd-7, the build
562: system does this automatically. TODO: check this. (Before building
563: Xen modules was added, it was awkward to use modules to the point
564: where it was considered that it did not work.)
566: The other difference is that XEN3_DOM0 does not have exactly the same
567: options as GENERIC. While it is debatable whether or not this is a
568: bug, users should be aware of this and can simply add missing config
569: items if desired.
571: Updating NetBSD in a dom0
574: This is just like updating NetBSD on bare hardware, assuming the new
575: version supports the version of Xen you are running. Generally, one
576: replaces the kernel and reboots, and then overlays userland binaries
577: and adjusts /etc.
579: Note that one must update both the non-Xen kernel typically used for
580: rescue purposes and the DOM0 kernel used with Xen.
582: Converting from grub to /boot
585: These instructions were [TODO: will be] used to convert a system from
586: grub to /boot. The system was originally installed in February of
587: 2006 with a RAID1 setup and grub to boot Xen 2, and has been updated
588: over time. Before these commands, it was running NetBSD 6 i386, Xen
589: 4.1 and grub, much like the message linked earlier in the grub
592: # Install MBR bootblocks on both disks.
593: fdisk -i /dev/rwd0d
594: fdisk -i /dev/rwd1d
595: # Install NetBSD primary boot loader (/ is FFSv1) into RAID1 components.
596: installboot -v /dev/rwd0d /usr/mdec/bootxx_ffsv1
597: installboot -v /dev/rwd1d /usr/mdec/bootxx_ffsv1
598: # Install secondary boot loader
599: cp -p /usr/mdec/boot /
600: # Create boot.cfg following earlier guidance:
601: menu=Xen:load /netbsd-XEN3PAE_DOM0.gz console=pc;multiboot /xen.gz dom0_mem=256M
602: menu=Xen.ok:load /netbsd-XEN3PAE_DOM0.ok.gz console=pc;multiboot /xen.ok.gz dom0_mem=256M
604: menu=GENERIC single-user:boot -s
605: menu=GENERIC.ok:boot netbsd.ok
606: menu=GENERIC.ok single-user:boot netbsd.ok -s
607: menu=Drop to boot prompt:prompt
611: TODO: actually do this and fix it if necessary.
613: Upgrading Xen versions
616: Minor version upgrades are trivial. Just rebuild/replace the
617: xenkernel version and copy the new xen.gz to / (where /boot.cfg
618: references it), and reboot.
620: Major version upgrades are conceptually not difficult, but can run
621: into all the issues found when installing Xen. Assuming migration
622: from 4.1 to 4.2, remove the xenkernel41 and xentools41 packages and
623: install the xenkernel42 and xentools42 packages. Copy the 4.2 xen.gz
624: to /.
626: Ensure that the contents of /etc/rc.d/xen* are correct. Specifically,
627: they must match the package you just installed and not be left over
628: from some previous installation.
630: Enable the correct set of daemons; see the configuring section above.
631: (Upgrading from 3.x to 4.x without doing this will result in a hang.)
633: Ensure that the domU config files are valid for the new version.
634: Specifically, for 4.x remove autorestart=True, and ensure that disks
635: are specified with numbers as the second argument, as the examples
636: above show, and not NetBSD device names.
638: Hardware known to work
641: Arguably, this section is misplaced, and there should be a page of
642: hardware that runs NetBSD/amd64 well, with the mostly-well-founded
643: assumption that NetBSD/xen runs fine on any modern hardware that
644: NetBSD/amd64 runs well on. Until then, we give motherboard/CPU (and
645: sometimes RAM) pairs/triples to aid those choosing a motherboard.
646: Note that Xen systems usually do not run X, so a listing here does not
647: imply that X works at all.
649: Supermicro X9SRL-F, Xeon E5-1650 v2, 96 GiB ECC
650: Supermicro ??, Atom C2758 (8 core), 32 GiB ECC
651: ASUS M5A78L-M/USB3 AM3+ microATX, AMD Piledriver X8 4000MHz, 16 GiB ECC
653: Older hardware:
655: Intel D915GEV, Pentium4 CPU 3.40GHz, 4GB 533MHz Synchronous DDR2
656: INTEL DG33FB, "Intel(R) Core(TM)2 Duo CPU E6850 @ 3.00GHz"
657: INTEL DG33FB, "Intel(R) Core(TM)2 Duo CPU E8400 @ 3.00GHz"
659: Running Xen under qemu
662: The astute reader will note that this section is somewhat twisted.
663: However, it can be useful to run Xen under qemu either because the
664: version of NetBSD as a dom0 does not run on the hardware in use, or to
665: generate automated test cases involving Xen.
667: In 2015-01, the following combination was reported to mostly work:
669: host OS: NetBSD/amd64 6.1.4
670: qemu: 2.2.0 from pkgsrc
671: Xen kernel: xenkernel42-4.2.5nb1 from pkgsrc
672: dom0 kernel: NetBSD/amd64 6.1.5
673: Xen tools: xentools42-4.2.5 from pkgsrc
675: See [PR 47720](http://gnats.netbsd.org/47720) for a problem with dom0
678: Unprivileged domains (domU)
681: This section describes general concepts about domUs. It does not
682: address specific domU operating systems or how to install them. The
683: config files for domUs are typically in /usr/pkg/etc/xen, and are
684: typically named so that the file name, domU name and the domU's host
685: name match.
687: The domU is provided with CPU and memory by Xen, configured by the
688: dom0. The domU is provided with disk and network by the dom0,
689: mediated by Xen, and configured in the dom0.
691: Entropy in domUs can be an issue; physical disks and network are on
692: the dom0. NetBSD's /dev/random system works, but is often challenged.
694: Config files
697: There is no good order to present config files and the concepts
698: surrounding what is being configured. We first show an example config
699: file, and then in the various sections give details.
701: See (at least in xentools41) /usr/pkg/share/examples/xen/xmexample*,
702: for a large number of well-commented examples, mostly for running
705: The following is an example minimal domain configuration file
706: "/usr/pkg/etc/xen/foo". It is (with only a name change) an actual
707: known working config file on Xen 4.1 (NetBSD 5 amd64 dom0 and NetBSD 5
708: i386 domU). The domU serves as a network file server.
710: # -*- mode: python; -*-
712: kernel = "/netbsd-XEN3PAE_DOMU-i386-foo.gz"
713: memory = 1024
714: vif = [ 'mac=aa:00:00:d1:00:09,bridge=bridge0' ]
715: disk = [ 'file:/n0/xen/foo-wd0,0x0,w',
716: 'file:/n0/xen/foo-wd1,0x1,w' ]
718: The domain will have the same name as the file. The kernel has the
719: host/domU name in it, so that on the dom0 one can update the various
720: domUs independently. The vif line causes an interface to be provided,
721: with a specific mac address (do not reuse MAC addresses!), in bridge
722: mode. Two disks are provided, and they are both writable; the bits
723: are stored in files and Xen attaches them to a vnd(4) device in the
724: dom0 on domain creation. The system treats xbd0 as the boot device
725: without needing explicit configuration.
727: By default xm looks for domain config files in /usr/pkg/etc/xen. Note
728: that "xm create" takes the name of a config file, while other commands
729: take the name of a domain. To create the domain, connect to the
730: console, create the domain while attaching the console, shutdown the
731: domain, and see if it has finished stopping, do (or xl with Xen >=
734: xm create foo
735: xm console foo
736: xm create -c foo
737: xm shutdown foo
738: xm list
740: Typing ^] will exit the console session. Shutting down a domain is
741: equivalent to pushing the power button; a NetBSD domU will receive a
742: power-press event and do a clean shutdown. Shutting down the dom0
743: will trigger controlled shutdowns of all configured domUs.
745: domU kernels
748: On a physical computer, the BIOS reads sector 0, and a chain of boot
749: loaders finds and loads a kernel. Normally this comes from the root
750: file system. With Xen domUs, the process is totally different. The
751: normal path is for the domU kernel to be a file in the dom0's
752: file system. At the request of the dom0, Xen loads that kernel into a
753: new domU instance and starts execution. While domU kernels can be
754: anyplace, reasonable places to store domU kernels on the dom0 are in /
755: (so they are near the dom0 kernel), in /usr/pkg/etc/xen (near the
756: config files), or in /u0/xen (where the vdisks are).
758: Note that loading the domU kernel from the dom0 implies that boot
759: blocks, /boot, /boot.cfg, and so on are all ignored in the domU.
760: See the VPS section near the end for discussion of alternate ways to
761: obtain domU kernels.
763: CPU and memory
766: A domain is provided with some number of vcpus, less than the number
767: of CPUs seen by the hypervisor. (For a dom0, this is controlled by
768: the boot argument "dom0_max_vcpus=1".) For a domU, it is controlled
769: from the config file by the "vcpus = N" directive.
771: A domain is provided with memory; this is controlled in the config
772: file by "memory = N" (in megabytes). In the straightforward case, the
773: sum of the the memory allocated to the dom0 and all domUs must be less
774: than the available memory.
776: Xen also provides a "balloon" driver, which can be used to let domains
777: use more memory temporarily. TODO: Explain better, and explain how
778: well it works with NetBSD.
780: Virtual disks
783: With the file/vnd style, typically one creates a directory,
784: e.g. /u0/xen, on a disk large enough to hold virtual disks for all
785: domUs. Then, for each domU disk, one writes zeros to a file that then
786: serves to hold the virtual disk's bits; a suggested name is foo-xbd0
787: for the first virtual disk for the domU called foo. Writing zeros to
788: the file serves two purposes. One is that preallocating the contents
789: improves performance. The other is that vnd on sparse files has
790: failed to work. TODO: give working/notworking NetBSD versions for
791: sparse vnd and gnats reference. Note that the use of file/vnd for Xen
792: is not really different than creating a file-backed virtual disk for
793: some other purpose, except that xentools handles the vnconfig
794: commands. To create an empty 4G virtual disk, simply do
796: dd if=/dev/zero of=foo-xbd0 bs=1m count=4096
798: Do not use qemu-img-xen, because this will create sparse file. There
799: have been recent (2015) reports of sparse vnd(4) devices causing
800: lockups, but there is apparently no PR.
802: With the lvm style, one creates logical devices. They are then used
803: similarly to vnds. TODO: Add an example with lvm.
805: In domU config files, the disks are defined as a sequence of 3-tuples.
806: The first element is "method:/path/to/disk". Common methods are
807: "file:" for file-backed vnd. and "phy:" for something that is already
808: a (TODO: character or block) device.
810: The second element is an artifact of how virtual disks are passed to
811: Linux, and a source of confusion with NetBSD Xen usage. Linux domUs
812: are given a device name to associate with the disk, and values like
813: "hda1" or "sda1" are common. In a NetBSD domU, the first disk appears
814: as xbd0, the second as xbd1, and so on. However, xm/xl demand a
815: second argument. The name given is converted to a major/minor by
816: calling stat(2) on the name in /dev and this is passed to the domU.
817: In the general case, the dom0 and domU can be different operating
818: systems, and it is an unwarranted assumption that they have consistent
819: numbering in /dev, or even that the dom0 OS has a /dev. With NetBSD
820: as both dom0 and domU, using values of 0x0 for the first disk and 0x1
821: for the second works fine and avoids this issue. For a GNU/Linux
822: guest, one can create /dev/hda1 in /dev, or to pass 0x301 for
825: The third element is "w" for writable disks, and "r" for read-only
828: Note that NetBSD by default creates only vnd. If you need more
829: than 4 total virtual disks at a time, run e.g. "./MAKEDEV vnd4" in the
832: Note that NetBSD by default creates only xbd. If you need more
833: virtual disks in a domU, run e.g. "./MAKEDEV xbd4" in the domU.
835: Virtual Networking
838: Xen provides virtual Ethernets, each of which connects the dom0 and a
839: domU. For each virtual network, there is an interface "xvifN.M" in
840: the dom0, and in domU index N, a matching interface xennetM (NetBSD
841: name). The interfaces behave as if there is an Ethernet with two
842: adapters connected. From this primitive, one can construct various
843: configurations. We focus on two common and useful cases for which
844: there are existing scripts: bridging and NAT.
846: With bridging (in the example above), the domU perceives itself to be
847: on the same network as the dom0. For server virtualization, this is
848: usually best. Bridging is accomplished by creating a bridge(4) device
849: and adding the dom0's physical interface and the various xvifN.0
850: interfaces to the bridge. One specifies "bridge=bridge0" in the domU
851: config file. The bridge must be set up already in the dom0; an
852: example /etc/ifconfig.bridge0 is:
856: !brconfig bridge0 add wm0
858: With NAT, the domU perceives itself to be behind a NAT running on the
859: dom0. This is often appropriate when running Xen on a workstation.
860: TODO: NAT appears to be configured by "vif = [ '' ]".
862: The MAC address specified is the one used for the interface in the new
863: domain. The interface in dom0 will use this address XOR'd with
864: 00:00:00:01:00:00. Random MAC addresses are assigned if not given.
866: Sizing domains
869: Modern x86 hardware has vast amounts of resources. However, many
870: virtual servers can function just fine on far less. A system with
871: 256M of RAM and a 4G disk can be a reasonable choice. Note that it is
872: far easier to adjust virtual resources than physical ones. For
873: memory, it's just a config file edit and a reboot. For disk, one can
874: create a new file and vnconfig it (or lvm), and then dump/restore,
875: just like updating physical disks, but without having to be there and
876: without those pesky connectors.
878: Starting domains automatically
881: To start domains foo at bar at boot and shut them down cleanly on dom0
882: shutdown, in rc.conf add:
884: xendomains="foo bar"
886: Note that earlier versions of the xentools41 xendomains rc.d script
887: used xl, when one should use xm with 4.1.
889: Creating specific unprivileged domains (domU)
892: Creating domUs is almost entirely independent of operating system. We
893: have already presented the basics of config files. Note that you must
894: have already completed the dom0 setup so that "xl list" (or "xm list")
897: Creating an unprivileged NetBSD domain (domU)
900: See the earlier config file, and adjust memory. Decide on how much
901: storage you will provide, and prepare it (file or lvm).
903: While the kernel will be obtained from the dom0 file system, the same
904: file should be present in the domU as /netbsd so that tools like
905: savecore(8) can work. (This is helpful but not necessary.)
907: The kernel must be specifically for Xen and for use as a domU. The
908: i386 and amd64 provide the following kernels:
910: i386 XEN3_DOMU
911: i386 XEN3PAE_DOMU
912: amd64 XEN3_DOMU
914: Unless using Xen 3.1 (and you shouldn't) with i386-mode Xen, you must
915: use the PAE version of the i386 kernel.
917: This will boot NetBSD, but this is not that useful if the disk is
918: empty. One approach is to unpack sets onto the disk outside of xen
919: (by mounting it, just as you would prepare a physical disk for a
920: system you can't run the installer on).
922: A second approach is to run an INSTALL kernel, which has a miniroot
923: and can load sets from the network. To do this, copy the INSTALL
924: kernel to / and change the kernel line in the config file to:
926: kernel = "/home/bouyer/netbsd-INSTALL_XEN3_DOMU"
928: Then, start the domain as "xl create -c configname".
930: Alternatively, if you want to install NetBSD/Xen with a CDROM image, the following
931: line should be used in the config file.
933: disk = [ 'phy:/dev/wd0e,0x1,w', 'phy:/dev/cd0a,0x2,r' ]
935: After booting the domain, the option to install via CDROM may be
936: selected. The CDROM device should be changed to `xbd1d`.
938: Once done installing, "halt -p" the new domain (don't reboot or halt,
939: it would reload the INSTALL_XEN3_DOMU kernel even if you changed the
940: config file), switch the config file back to the XEN3_DOMU kernel,
941: and start the new domain again. Now it should be able to use "root on
942: xbd0a" and you should have a, functional NetBSD domU.
944: TODO: check if this is still accurate.
945: When the new domain is booting you'll see some warnings about *wscons*
946: and the pseudo-terminals. These can be fixed by editing the files
947: `/etc/ttys` and `/etc/wscons.conf`. You must disable all terminals in
948: `/etc/ttys`, except *console*, like this:
950: console "/usr/libexec/getty Pc" vt100 on secure
951: ttyE0 "/usr/libexec/getty Pc" vt220 off secure
952: ttyE1 "/usr/libexec/getty Pc" vt220 off secure
953: ttyE2 "/usr/libexec/getty Pc" vt220 off secure
954: ttyE3 "/usr/libexec/getty Pc" vt220 off secure
956: Finally, all screens must be commented out from `/etc/wscons.conf`.
958: It is also desirable to add
962: in rc.conf. This way, the domain will be properly shut down if
963: `xm shutdown -R` or `xm shutdown -H` is used on the dom0.
965: It is not strictly necessary to have a kernel (as /netbsd) in the domU
966: file system. However, various programs (e.g. netstat) will use that
967: kernel to look up symbols to read from kernel virtual memory. If
968: /netbsd is not the running kernel, those lookups will fail. (This is
969: not really a Xen-specific issue, but because the domU kernel is
970: obtained from the dom0, it is far more likely to be out of sync or
971: missing with Xen.)
973: Creating an unprivileged Linux domain (domU)
976: Creating unprivileged Linux domains isn't much different from
977: unprivileged NetBSD domains, but there are some details to know.
979: First, the second parameter passed to the disk declaration (the '0x1' in
980: the example below)
982: disk = [ 'phy:/dev/wd0e,0x1,w' ]
984: does matter to Linux. It wants a Linux device number here (e.g. 0x300
985: for hda). Linux builds device numbers as: (major \<\< 8 + minor).
986: So, hda1 which has major 3 and minor 1 on a Linux system will have
987: device number 0x301. Alternatively, devices names can be used (hda,
988: hdb, ...) as xentools has a table to map these names to devices
989: numbers. To export a partition to a Linux guest we can use:
991: disk = [ 'phy:/dev/wd0e,0x300,w' ]
992: root = "/dev/hda1 ro"
994: and it will appear as /dev/hda on the Linux system, and be used as root
997: To install the Linux system on the partition to be exported to the
998: guest domain, the following method can be used: install
999: sysutils/e2fsprogs from pkgsrc. Use mke2fs to format the partition
1000: that will be the root partition of your Linux domain, and mount it.
1001: Then copy the files from a working Linux system, make adjustments in
1002: `/etc` (fstab, network config). It should also be possible to extract
1003: binary packages such as .rpm or .deb directly to the mounted partition
1004: using the appropriate tool, possibly running under NetBSD's Linux
1005: emulation. Once the file system has been populated, umount it. If
1006: desirable, the file system can be converted to ext3 using tune2fs -j.
1007: It should now be possible to boot the Linux guest domain, using one of
1008: the vmlinuz-\*-xenU kernels available in the Xen binary distribution.
1010: To get the Linux console right, you need to add:
1012: extra = "xencons=tty1"
1014: to your configuration since not all Linux distributions auto-attach a
1015: tty to the xen console.
1017: Creating an unprivileged Solaris domain (domU)
1020: See possibly outdated
1021: [Solaris domU instructions](/ports/xen/howto-solaris/).
1024: PCI passthrough: Using PCI devices in guest domains
1027: The dom0 can give other domains access to selected PCI
1028: devices. This can allow, for example, a non-privileged domain to have
1029: access to a physical network interface or disk controller. However,
1030: keep in mind that giving a domain access to a PCI device most likely
1031: will give the domain read/write access to the whole physical memory,
1032: as PCs don't have an IOMMU to restrict memory access to DMA-capable
1033: device. Also, it's not possible to export ISA devices to non-dom0
1034: domains, which means that the primary VGA adapter can't be exported.
1035: A guest domain trying to access the VGA registers will panic.
1037: If the dom0 is NetBSD, it has to be running Xen 3.1, as support has
1038: not been ported to later versions at this time.
1040: For a PCI device to be exported to a domU, is has to be attached to
1041: the "pciback" driver in dom0. Devices passed to the dom0 via the
1042: pciback.hide boot parameter will attach to "pciback" instead of the
1043: usual driver. The list of devices is specified as "(bus:dev.func)",
1044: where bus and dev are 2-digit hexadecimal numbers, and func a
1045: single-digit number:
1049: pciback devices should show up in the dom0's boot messages, and the
1050: devices should be listed in the `/kern/xen/pci` directory.
1052: PCI devices to be exported to a domU are listed in the "pci" array of
1053: the domU's config file, with the format "0000:bus:dev.func".
1055: pci = [ '0000:00:06.0', '0000:00:0a.0' ]
1057: In the domU an "xpci" device will show up, to which one or more pci
1058: buses will attach. Then the PCI drivers will attach to PCI buses as
1059: usual. Note that the default NetBSD DOMU kernels do not have "xpci"
1060: or any PCI drivers built in by default; you have to build your own
1061: kernel to use PCI devices in a domU. Here's a kernel config example;
1062: note that only the "xpci" lines are unusual.
1064: include "arch/i386/conf/XEN3_DOMU"
1066: # Add support for PCI buses to the XEN3_DOMU kernel
1067: xpci* at xenbus ?
1068: pci* at xpci ?
1070: # PCI USB controllers
1071: uhci* at pci? dev ? function ? # Universal Host Controller (Intel)
1073: # USB bus support
1074: usb* at uhci?
1076: # USB Hubs
1077: uhub* at usb?
1078: uhub* at uhub? port ? configuration ? interface ?
1080: # USB Mass Storage
1081: umass* at uhub? port ? configuration ? interface ?
1082: wd* at umass?
1083: # SCSI controllers
1084: ahc* at pci? dev ? function ? # Adaptec 94x, aic78x0 SCSI
1086: # SCSI bus support (for both ahc and umass)
1087: scsibus* at scsi?
1089: # SCSI devices
1090: sd* at scsibus? target ? lun ? # SCSI disk drives
1091: cd* at scsibus? target ? lun ? # SCSI CD-ROM drives
1094: NetBSD as a domU in a VPS
1097: The bulk of the HOWTO is about using NetBSD as a dom0 on your own
1098: hardware. This section explains how to deal with Xen in a domU as a
1099: virtual private server where you do not control or have access to the
1100: dom0. This is not intended to be an exhaustive list of VPS providers;
1101: only a few are mentioned that specifically support NetBSD.
1103: VPS operators provide varying degrees of access and mechanisms for
1104: configuration. The big issue is usually how one controls which kernel
1105: is booted, because the kernel is nominally in the dom0 file system (to
1106: which VPS users do not normally have access). A second issue is how
1107: to install NetBSD.
1108: A VPS user may want to compile a kernel for security updates, to run
1109: npf, run IPsec, or any other reason why someone would want to change
1110: their kernel.
1112: One approach is to have an administrative interface to upload a kernel,
1113: or to select from a prepopulated list. Other approaches are pygrub
1114: (deprecated) and pvgrub, which are ways to have a bootloader obtain a
1115: kernel from the domU file system. This is closer to a regular physical
1116: computer, where someone who controls a machine can replace the kernel.
1118: A second issue is multiple CPUs. With NetBSD 6, domUs support
1119: multiple vcpus, and it is typical for VPS providers to enable multiple
1120: CPUs for NetBSD domUs.
1125: pygrub runs in the dom0 and looks into the domU file system. This
1126: implies that the domU must have a kernel in a file system in a format
1127: known to pygrub. As of 2014, pygrub seems to be of mostly historical
1133: pvgrub is a version of grub that uses PV operations instead of BIOS
1134: calls. It is booted from the dom0 as the domU kernel, and then reads
1135: /grub/menu.lst and loads a kernel from the domU file system.
1137: [Panix](http://www.panix.com/) lets users use pvgrub. Panix reports
1138: that pvgrub works with FFsv2 with 16K/2K and 32K/4K block/frag sizes
1139: (and hence with defaults from "newfs -O 2"). See [Panix's pvgrub
1140: page](http://www.panix.com/v-colo/grub.html), which describes only
1141: Linux but should be updated to cover NetBSD :-).
1143: [prgmr.com](http://prgmr.com/) also lets users with pvgrub to boot
1144: their own kernel. See then [prgmr.com NetBSD
1146: (which is in need of updating).
1148: It appears that [grub's FFS
1150: does not support all aspects of modern FFS, but there are also reports
1151: that FFSv2 works fine. At prgmr, typically one has an ext2 or FAT
1152: partition for the kernel with the intent that grub can understand it,
1153: which leads to /netbsd not being the actual kernel. One must remember
1154: to update the special boot partition.
1159: See the [Amazon EC2 page](../amazon_ec2/).
1161: Using npf
1164: In standard kernels, npf is a module, and thus cannot be loaded in a
1165: DOMU kernel.
1167: TODO: Explain how to compile npf into a custom kernel, answering (but
1168: note that the problem was caused by not booting the right kernel)
1169: [this email to
1172: TODO items for improving NetBSD/xen
1175: * Make the NetBSD dom0 kernel work with SMP.
1176: * Test the Xen 4.5 packages adequately to be able to recommend them as
1177: the standard approach.
1178: * Get PCI passthrough working on Xen 4.5
1179: * Get pvgrub into pkgsrc, either via xentools or separately.
1180: * grub
1181: * Check/add support to pkgsrc grub2 for UFS2 and arbitrary
1182: fragsize/blocksize (UFS2 support may be present; the point is to
1183: make it so that with any UFS1/UFS2 file system setup that works
1184: with NetBSD grub will also work).
1185: See [pkg/40258](http://gnats.netbsd.org/40258).
1186: * Push patches upstream.
1187: * Get UFS2 patches into pvgrub.
1188: * Add support for PV ops to a version of /boot, and make it usable as
1189: a kernel in Xen, similar to pvgrub.
1190: * Solve somehow the issue with modules for GENERIC not being loadable
1191: in a Xen dom0 or domU kernel.
1193: Random pointers
1196: This section contains links from elsewhere not yet integrated into the
1197: HOWTO, and other guides.
1199: * http://www.lumbercartel.ca/library/xen/
1200: * http://pbraun.nethence.com/doc/sysutils/xen_netbsd_dom0.html
1201: * https://gmplib.org/~tege/xen.html
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