File:  [NetBSD Developer Wiki] / wikisrc / ports / xen / howto.mdwn
Revision 1.151: download - view: text, annotated - select for diffs
Thu Jul 26 11:16:22 2018 UTC (16 months, 2 weeks ago) by maxv
Branches: MAIN
CVS tags: HEAD
Remove the noise surrounding PAE.

    1: [[!meta title="Xen HowTo"]]
    2: 
    3: Introduction
    4: ============
    5: 
    6: [![[Xen
    7: screenshot]](https://www.netbsd.org/gallery/in-Action/hubertf-xens.png)](https://www.netbsd.org/gallery/in-Action/hubertf-xen.png)
    8: 
    9: Xen is a hypervisor for x86 hardware, which supports running multiple guest
   10: operating systems on a single physical machine.  Xen is a Type 1 or
   11: bare-metal hypervisor; one uses the Xen kernel to control the CPU,
   12: memory and console, a dom0 operating system which mediates access to
   13: other hardware (e.g., disks, network, USB), and one or more domU
   14: operating systems which operate in an unprivileged virtualized
   15: environment.  IO requests from the domU systems are forwarded by the
   16: Xen hypervisor to the dom0 to be fulfilled.
   17: 
   18: Xen supports different styles of guest:
   19: 
   20: [[!table data="""
   21: Style of guest	|Supported by NetBSD
   22: PV		|Yes (dom0, domU)
   23: HVM		|Yes (domU)
   24: PVHVM		|No
   25: PVH		|No
   26: """]]
   27: 
   28: In Para-Virtualized (PV) mode, the guest OS does not attempt to access
   29: hardware directly, but instead makes hypercalls to the hypervisor; PV
   30: guests must be specifically coded for Xen. In HVM mode, no guest
   31: modification is required; however, hardware support is required, such
   32: as VT-x on Intel CPUs and SVM on AMD CPUs.
   33: 
   34: There are further features for IOMMU virtualization, Intel's VT-d and
   35: AMD's AMD-Vi.  TODO: Explain whether Xen on NetBSD makes use of these
   36: features.  TODO: Review by someone who really understands this.
   37: 
   38: At boot, the dom0 kernel is loaded as a module with Xen as the kernel.
   39: The dom0 can start one or more domUs.  (Booting is explained in detail
   40: in the dom0 section.)
   41: 
   42: This HOWTO presumes a basic familiarity with the Xen system
   43: architecture, with installing NetBSD on i386/amd64 hardware, and with
   44: installing software from pkgsrc.  See also the [Xen
   45: website](http://www.xenproject.org/).
   46: 
   47: This HOWTO attempts to address both the case of running a NetBSD dom0
   48: on hardware and running domUs under it (NetBSD and other), and also
   49: running NetBSD as a domU in a VPS.
   50: 
   51: Versions of Xen and NetBSD
   52: ==========================
   53: 
   54: Most of the installation concepts and instructions are independent
   55: of Xen version and NetBSD version.  This section gives advice on
   56: which version to choose.  Versions not in pkgsrc and older unsupported
   57: versions of NetBSD are intentionally ignored.
   58: 
   59: The term "amd64" is used to refer to both the NetBSD port and to the
   60: hardware architecture on which it runs.  Such hardware is generally
   61: made by both Intel and AMD, and common on PC computers.
   62: 
   63: Xen versions
   64: ------------
   65: 
   66: In NetBSD, Xen is provided in pkgsrc, via matching pairs of packages
   67: xenkernel and xentools.  We will refer only to the kernel versions,
   68: but note that both packages must be installed together and must have
   69: matching versions.
   70: 
   71: Versions available in pkgsrc:
   72: 
   73: [[!table data="""
   74: Xen Version	|Package Name	|Xen CPU Support	|EOL'ed By Upstream
   75: 4.2		|xenkernel42	|32bit, 64bit		|Yes
   76: 4.5		|xenkernel45	|64bit			|Yes
   77: 4.6		|xenkernel46	|64bit			|Partially
   78: 4.8		|xenkernel48	|64bit			|No
   79: 4.11		|xenkernel411	|64bit			|No
   80: """]]
   81: 
   82: See also the [Xen Security Advisory page](http://xenbits.xen.org/xsa/).
   83: 
   84: Note: Xen 4.2 was the last version to support 32bit CPUs.
   85: 
   86: Xen command program
   87: -------------------
   88: 
   89: Early Xen used a program called xm to manipulate the system from the
   90: dom0.  Starting in 4.1, a replacement program with similar behavior
   91: called xl is provided, but it does not work well in 4.1.  In 4.2, both
   92: xm and xl work fine.  4.4 is the last version that has xm.
   93: 
   94: You must make a global choice to use xm or xl, because it affects not
   95: only which command you use, but the command used by rc.d scripts
   96: (specifically xendomains) and which daemons should be run.  The
   97: xentools packages provide xl for 4.2 and up.
   98: 
   99: In 4.2, you can choose to use xm by simply changing the ctl_command
  100: variable and setting xend=YES in rc.conf.
  101: 
  102: With xl, virtual devices are configured in parallel, which can cause
  103: problems if they are written assuming serial operation (e.g., updating
  104: firewall rules without explicit locking).  There is now locking for
  105: the provided scripts, which works for normal casses (e.g, file-backed
  106: xbd, where a vnd must be allocated).  But, as of 201612, it has not
  107: been adequately tested for a complex custom setup with a large number
  108: of interfaces.
  109: 
  110: NetBSD versions
  111: ---------------
  112: 
  113: The netbsd-7, netbsd-8, and -current branches are all reasonable
  114: choices, with more or less the same considerations for non-Xen use.
  115: NetBSD 8 is recommended as the stable version of the most recent
  116: release for production use.
  117: 
  118: For developing Xen, netbsd-current may be appropriate.
  119: 
  120: As of NetBSD 6, a NetBSD domU will support multiple vcpus.  There is
  121: no SMP support for NetBSD as dom0.  (The dom0 itself doesn't really
  122: need SMP for dom0 functions; the lack of support is really a problem
  123: when using a dom0 as a normal computer.)
  124: 
  125: Note: NetBSD support is called XEN3. However, it does support Xen 4,
  126: because the hypercall interface has remained identical.
  127: 
  128: Architecture
  129: ------------
  130: 
  131: Xen itself can run on i386 (Xen < 4.2) or amd64 hardware (all Xen
  132: versions).  Practically, almost any computer where one would want to
  133: run Xen today supports amd64.
  134: 
  135: Xen, the dom0 system, and each domU system can be either i386 or
  136: amd64.  When building a xenkernel package, one obtains an i386 Xen
  137: kernel on an i386 host, and an amd64 Xen kernel on an amd64 host.  If
  138: the Xen kernel is i386, then the dom0 kernel and all domU kernels must
  139: be i386.  With an amd64 Xen kernel, an amd64 dom0 kernel is known to
  140: work, and an i386 dom0 kernel should in theory work.  An amd64
  141: Xen/dom0 is known to support both i386 and amd64 domUs.
  142: 
  143: i386 dom0 and domU kernels must be PAE. PAE kernels are included in
  144: the NetBSD default build.
  145: 
  146: Because of the above, the standard approach is to use an amd64 Xen
  147: kernel and NetBSD/amd64 for the dom0.  For domUs, NetBSD/i386 (with
  148: the PAE kernel) and NetBSD/amd64 are in widespread use, and there is
  149: little to no Xen-specific reason to prefer one over the other.
  150: 
  151: Note that to use an i386 dom0 with Xen 4.5 or higher, one must build
  152: (or obtain from pre-built packages) an amd64 Xen kernel and install
  153: that on the system.  (One must also use a PAE i386 kernel, but this is
  154: also required with an i386 Xen kernel.).  Almost no one in the
  155: NetBSD/Xen community does this, and the standard, well-tested,
  156: approach is to use an amd64 dom0.
  157: 
  158: A [posting on
  159: xen-devel](https://lists.xen.org/archives/html/xen-devel/2012-07/msg00085.html)
  160: explained that PV system call overhead was higher on amd64, and thus
  161: there is some notion that i386 guests are faster.  It goes on to
  162: caution that the total situation is complex and not entirely
  163: understood. On top of that caution, the post is about Linux, not
  164: NetBSD.  TODO: Include link to benchmarks, if someone posts them.
  165: 
  166: NetBSD as a dom0
  167: ================
  168: 
  169: NetBSD can be used as a dom0 and works very well.  The following
  170: sections address installation, updating NetBSD, and updating Xen.
  171: Note that it doesn't make sense to talk about installing a dom0 OS
  172: without also installing Xen itself.  We first address installing
  173: NetBSD, which is not yet a dom0, and then adding Xen, pivoting the
  174: NetBSD install to a dom0 install by just changing the kernel and boot
  175: configuration.
  176: 
  177: For experimenting with Xen, a machine with as little as 1G of RAM and
  178: 100G of disk can work.  For running many domUs in productions, far
  179: more will be needed; e.g. 4-8G and 1T of disk is reasonable for a
  180: half-dozen domUs of 512M and 32G each.  Basically, the RAM and disk
  181: have to be bigger than the sum of the RAM/disk needs of the dom0 and
  182: all the domUs.
  183: 
  184: In 2018-05, trouble booting a dom0 was reported with 256M of RAM: with
  185: 512M it worked reliably.  This does not make sense, but if you see
  186: "not ELF" after Xen boots, try increasing dom0 RAM.
  187: 
  188: Styles of dom0 operation
  189: ------------------------
  190: 
  191: There are two basic ways to use Xen.  The traditional method is for
  192: the dom0 to do absolutely nothing other than providing support to some
  193: number of domUs.  Such a system was probably installed for the sole
  194: purpose of hosting domUs, and sits in a server room on a UPS.
  195: 
  196: The other way is to put Xen under a normal-usage computer, so that the
  197: dom0 is what the computer would have been without Xen, perhaps a
  198: desktop or laptop.  Then, one can run domUs at will.  Purists will
  199: deride this as less secure than the previous approach, and for a
  200: computer whose purpose is to run domUs, they are right.  But Xen and a
  201: dom0 (without domUs) is not meaningfully less secure than the same
  202: things running without Xen.  One can boot Xen or boot regular NetBSD
  203: alternately with little problems, simply refraining from starting the
  204: Xen daemons when not running Xen.
  205: 
  206: Note that NetBSD as dom0 does not support multiple CPUs.  This will
  207: limit the performance of the Xen/dom0 workstation approach.  In theory
  208: the only issue is that the "backend drivers" are not yet MPSAFE:
  209:   https://mail-index.netbsd.org/netbsd-users/2014/08/29/msg015195.html
  210: 
  211: Installation of NetBSD
  212: ----------------------
  213: 
  214: First,
  215: [install NetBSD/amd64](/guide/inst/)
  216: just as you would if you were not using Xen.
  217: However, the partitioning approach is very important.
  218: 
  219: If you want to use RAIDframe for the dom0, there are no special issues
  220: for Xen.  Typically one provides RAID storage for the dom0, and the
  221: domU systems are unaware of RAID.  The 2nd-stage loader bootxx_* skips
  222: over a RAID1 header to find /boot from a file system within a RAID
  223: partition; this is no different when booting Xen.
  224: 
  225: There are 4 styles of providing backing storage for the virtual disks
  226: used by domUs: raw partitions, LVM, file-backed vnd(4), and SAN.
  227: 
  228: With raw partitions, one has a disklabel (or gpt) partition sized for
  229: each virtual disk to be used by the domU.  (If you are able to predict
  230: how domU usage will evolve, please add an explanation to the HOWTO.
  231: Seriously, needs tend to change over time.)
  232: 
  233: One can use [lvm(8)](/guide/lvm/) to create logical devices to use
  234: for domU disks.  This is almost as efficient as raw disk partitions
  235: and more flexible.  Hence raw disk partitions should typically not
  236: be used.
  237: 
  238: One can use files in the dom0 file system, typically created by dd'ing
  239: /dev/zero to create a specific size.  This is somewhat less efficient,
  240: but very convenient, as one can cp the files for backup, or move them
  241: between dom0 hosts.
  242: 
  243: Finally, in theory one can place the files backing the domU disks in a
  244: SAN.  (This is an invitation for someone who has done this to add a
  245: HOWTO page.)
  246: 
  247: Installation of Xen
  248: -------------------
  249: 
  250: In the dom0, install sysutils/xenkernel42 and sysutils/xentools42 from
  251: pkgsrc (or another matching pair).  See [the pkgsrc
  252: documentation](https://www.NetBSD.org/docs/pkgsrc/) for help with
  253: pkgsrc.  Ensure that your packages are recent; the HOWTO does not
  254: contemplate old builds.
  255: 
  256: 
  257: For Xen 3.1, support for HVM guests is in sysutils/xentool3-hvm.  More
  258: recent versions have HVM support integrated in the main xentools
  259: package.  It is entirely reasonable to run only PV guests.
  260: 
  261: Next you need to install the selected Xen kernel itself, which is
  262: installed by pkgsrc as "/usr/pkg/xen*-kernel/xen.gz".  Copy it to /.
  263: For debugging, one may copy xen-debug.gz; this is conceptually similar
  264: to DIAGNOSTIC and DEBUG in NetBSD.  xen-debug.gz is basically only
  265: useful with a serial console.  Then, place a NetBSD XEN3_DOM0 kernel
  266: in /, copied from releasedir/amd64/binary/kernel/netbsd-XEN3_DOM0.gz
  267: of a NetBSD build.  If using i386, use
  268: releasedir/i386/binary/kernel/netbsd-XEN3PAE_DOM0.gz.  (If using Xen
  269: 3.1 and i386, you may use XEN3_DOM0 with the non-PAE Xen.  But you
  270: should not use Xen 3.1.)  Both xen and the NetBSD kernel may be (and
  271: typically are) left compressed.
  272: 
  273: In a dom0, kernfs is mandatory for xend to communicate with the
  274: kernel, so ensure that /kern is in fstab.  (A standard NetBSD install
  275: should already mount /kern.)
  276: 
  277: Because you already installed NetBSD, you have a working boot setup
  278: with an MBR bootblock, either bootxx_ffsv1 or bootxx_ffsv2 at the
  279: beginning of your root file system, have /boot, and likely also
  280: /boot.cfg.  (If not, fix before continuing!)
  281: 
  282: Add a line to to /boot.cfg to boot Xen.  See boot.cfg(5) for an
  283: example.  The basic line is
  284: 
  285:         menu=Xen:load /netbsd-XEN3_DOM0.gz console=pc;multiboot /xen.gz dom0_mem=512M
  286: 
  287: which specifies that the dom0 should have 512M, leaving the rest to be
  288: allocated for domUs.  To use a serial console, use
  289: 
  290:         menu=Xen:load /netbsd-XEN3_DOM0.gz;multiboot /xen.gz dom0_mem=512M console=com1 com1=9600,8n1
  291: 
  292: which will use the first serial port for Xen (which counts starting
  293: from 1, unlike NetBSD which counts starting from 0), forcing
  294: speed/parity.  Because the NetBSD command line lacks a
  295: "console=pc" argument, it will use the default "xencons" console device,
  296: which directs the console I/O through Xen to the same console device Xen
  297: itself uses (in this case, the serial port).
  298: 
  299: In an attempt to add performance, one can also add
  300: 
  301:         dom0_max_vcpus=1 dom0_vcpus_pin
  302: 
  303: to force only one vcpu to be provided (since NetBSD dom0 can't use
  304: more) and to pin that vcpu to a physical CPU.  TODO: benchmark this.
  305: 
  306: Xen has [many boot
  307: options](http://xenbits.xenproject.org/docs/4.5-testing/misc/xen-command-line.html),
  308: and other than dom0 memory and max_vcpus, they are generally not
  309: necessary.
  310: 
  311: As with non-Xen systems, you should have a line to boot /netbsd (a
  312: kernel that works without Xen).  Consider a line to boot /netbsd.ok (a
  313: fallback version of the non-Xen kernel, updated manually when you are
  314: sure /netbsd is ok).  Consider also a line to boot fallback versions
  315: of Xen and the dom0 kernel, but note that non-Xen NetBSD can be used
  316: to resolve Xen booting issues.
  317: 
  318: Probably you want a default=N line to choose Xen in the absence of
  319: intervention.
  320: 
  321: Now, reboot so that you are running a DOM0 kernel under Xen, rather
  322: than GENERIC without Xen.
  323: 
  324: Using grub (historic)
  325: ---------------------
  326: 
  327: Before NetBSD's native bootloader could support Xen, the use of
  328: grub was recommended.  If necessary, see the
  329: [old grub information](/ports/xen/howto-grub).
  330: 
  331: The [HowTo on Installing into
  332: RAID-1](https://mail-index.NetBSD.org/port-xen/2006/03/01/0010.html)
  333: explains how to set up booting a dom0 with Xen using grub with
  334: NetBSD's RAIDframe.  (This is obsolete with the use of NetBSD's native
  335: boot.  Now, just create a system with RAID-1, and alter /boot.cfg as
  336: described above.)
  337: 
  338: Configuring Xen
  339: ---------------
  340: 
  341: Xen logs will be in /var/log/xen.
  342: 
  343: Now, you have a system that will boot Xen and the dom0 kernel, but not
  344: do anything else special.  Make sure that you have rebooted into Xen.
  345: There will be no domUs, and none can be started because you still have
  346: to configure the dom0 daemons.
  347: 
  348: The daemons which should be run vary with Xen version and with whether
  349: one is using xm or xl.  The Xen 3.1, 3.3 and 4.1 packages use xm.  Xen
  350: 4.2 and up packages use xl.  To use xm with 4.2, edit xendomains to
  351: use xm instead.
  352: 
  353: For 3.1 and 3.3, you should enable xend and xenbackendd:
  354: 
  355:         xend=YES
  356:         xenbackendd=YES
  357: 
  358: For 4.1 and up, you should enable xencommons.  Not enabling xencommons
  359: will result in a hang; it is necessary to hit ^C on the console to let
  360: the machine finish booting.  If you are using xm (default in 4.1, or
  361: if you changed xendomains in 4.2), you should also enable xend:
  362: 
  363:         xend=YES # only if using xm, and only installed <= 4.2
  364:         xencommons=YES
  365: 
  366: TODO: Recommend for/against xen-watchdog.
  367: 
  368: After you have configured the daemons and either started them (in the
  369: order given) or rebooted, use xm or xl to inspect Xen's boot messages,
  370: available resources, and running domains.  An example with xl follows:
  371: 
  372:         # xl dmesg
  373: 	[xen's boot info]
  374:         # xl info
  375: 	[available memory, etc.]
  376:         # xl list
  377:         Name              Id  Mem(MB)  CPU  State  Time(s)  Console
  378:         Domain-0           0       64    0  r----     58.1
  379: 
  380: ### Issues with xencommons
  381: 
  382: xencommons starts xenstored, which stores data on behalf of dom0 and
  383: domUs.  It does not currently work to stop and start xenstored.
  384: Certainly all domUs should be shutdown first, following the sort order
  385: of the rc.d scripts.  However, the dom0 sets up state with xenstored,
  386: and is not notified when xenstored exits, leading to not recreating
  387: the state when the new xenstored starts.  Until there's a mechanism to
  388: make this work, one should not expect to be able to restart xenstored
  389: (and thus xencommons).  There is currently no reason to expect that
  390: this will get fixed any time soon.
  391: 
  392: ### No-longer needed advice about devices
  393: 
  394: The installation of NetBSD should already have created devices for xen
  395: (xencons, xenevt, xsd_kva), but if they are not present, create them:
  396: 
  397:         cd /dev && sh MAKEDEV xen
  398: 
  399: anita (for testing NetBSD)
  400: --------------------------
  401: 
  402: With the setup so far (assuming 4.2/xl), one should be able to run
  403: anita (see pkgsrc/misc/py-anita) to test NetBSD releases, by doing (as
  404: root, because anita must create a domU):
  405: 
  406:         anita --vmm=xl test file:///usr/obj/i386/
  407: 
  408: Alternatively, one can use --vmm=xm to use xm-based domU creation
  409: instead (and must, on Xen <= 4.1).   TODO: confirm that anita xl really works.
  410:     
  411: Xen-specific NetBSD issues
  412: --------------------------
  413: 
  414: There are (at least) two additional things different about NetBSD as a
  415: dom0 kernel compared to hardware.
  416: 
  417: One is that the module ABI is different because some of the #defines
  418: change, so one must build modules for Xen.  As of netbsd-7, the build
  419: system does this automatically.  TODO: check this.  (Before building
  420: Xen modules was added, it was awkward to use modules to the point
  421: where it was considered that it did not work.)
  422: 
  423: The other difference is that XEN3_DOM0 does not have exactly the same
  424: options as GENERIC.  While it is debatable whether or not this is a
  425: bug, users should be aware of this and can simply add missing config
  426: items if desired.
  427: 
  428: Updating NetBSD in a dom0
  429: -------------------------
  430: 
  431: This is just like updating NetBSD on bare hardware, assuming the new
  432: version supports the version of Xen you are running.  Generally, one
  433: replaces the kernel and reboots, and then overlays userland binaries
  434: and adjusts /etc.
  435: 
  436: Note that one must update both the non-Xen kernel typically used for
  437: rescue purposes and the DOM0 kernel used with Xen.
  438: 
  439: Converting from grub to /boot
  440: -----------------------------
  441: 
  442: These instructions were [TODO: will be] used to convert a system from
  443: grub to /boot.  The system was originally installed in February of
  444: 2006 with a RAID1 setup and grub to boot Xen 2, and has been updated
  445: over time.  Before these commands, it was running NetBSD 6 i386, Xen
  446: 4.1 and grub, much like the message linked earlier in the grub
  447: section.
  448: 
  449:         # Install MBR bootblocks on both disks. 
  450:         fdisk -i /dev/rwd0d
  451:         fdisk -i /dev/rwd1d
  452:         # Install NetBSD primary boot loader (/ is FFSv1) into RAID1 components.
  453:         installboot -v /dev/rwd0d /usr/mdec/bootxx_ffsv1
  454:         installboot -v /dev/rwd1d /usr/mdec/bootxx_ffsv1
  455:         # Install secondary boot loader
  456:         cp -p /usr/mdec/boot /
  457:         # Create boot.cfg following earlier guidance:
  458:         menu=Xen:load /netbsd-XEN3PAE_DOM0.gz console=pc;multiboot /xen.gz dom0_mem=512M
  459:         menu=Xen.ok:load /netbsd-XEN3PAE_DOM0.ok.gz console=pc;multiboot /xen.ok.gz dom0_mem=512M
  460:         menu=GENERIC:boot
  461:         menu=GENERIC single-user:boot -s
  462:         menu=GENERIC.ok:boot netbsd.ok
  463:         menu=GENERIC.ok single-user:boot netbsd.ok -s
  464:         menu=Drop to boot prompt:prompt
  465:         default=1
  466:         timeout=30
  467: 
  468: TODO: actually do this and fix it if necessary.
  469: 
  470: Upgrading Xen versions
  471: ---------------------
  472: 
  473: Minor version upgrades are trivial.  Just rebuild/replace the
  474: xenkernel version and copy the new xen.gz to / (where /boot.cfg
  475: references it), and reboot.
  476: 
  477: Major version upgrades are conceptually not difficult, but can run
  478: into all the issues found when installing Xen.  Assuming migration
  479: from 4.1 to 4.2, remove the xenkernel41 and xentools41 packages and
  480: install the xenkernel42 and xentools42 packages.  Copy the 4.2 xen.gz
  481: to /.
  482: 
  483: Ensure that the contents of /etc/rc.d/xen* are correct.  Specifically,
  484: they must match the package you just installed and not be left over
  485: from some previous installation.
  486: 
  487: Enable the correct set of daemons; see the configuring section above.
  488: (Upgrading from 3.x to 4.x without doing this will result in a hang.)
  489: 
  490: Ensure that the domU config files are valid for the new version.
  491: Specifically, for 4.x remove autorestart=True, and ensure that disks
  492: are specified with numbers as the second argument, as the examples
  493: above show, and not NetBSD device names.
  494: 
  495: Hardware known to work
  496: ----------------------
  497: 
  498: Arguably, this section is misplaced, and there should be a page of
  499: hardware that runs NetBSD/amd64 well, with the mostly-well-founded
  500: assumption that NetBSD/xen runs fine on any modern hardware that
  501: NetBSD/amd64 runs well on.  Until then, we give motherboard/CPU (and
  502: sometimes RAM) pairs/triples to aid those choosing a motherboard.
  503: Note that Xen systems usually do not run X, so a listing here does not
  504: imply that X works at all.
  505: 
  506:         Supermicro X9SRL-F, Xeon E5-1650 v2, 96 GiB ECC
  507:         Supermicro ??, Atom C2758 (8 core), 32 GiB ECC
  508:         ASUS M5A78L-M/USB3 AM3+ microATX, AMD Piledriver X8 4000MHz, 16 GiB ECC
  509: 
  510: Older hardware:
  511: 
  512:         Intel D915GEV, Pentium4 CPU 3.40GHz, 4GB 533MHz Synchronous DDR2
  513:         INTEL DG33FB, "Intel(R) Core(TM)2 Duo CPU     E6850  @ 3.00GHz"
  514:         INTEL DG33FB, "Intel(R) Core(TM)2 Duo CPU     E8400  @ 3.00GHz"
  515: 
  516: Running Xen under qemu
  517: ----------------------
  518: 
  519: The astute reader will note that this section is somewhat twisted.
  520: However, it can be useful to run Xen under qemu either because the
  521: version of NetBSD as a dom0 does not run on the hardware in use, or to
  522: generate automated test cases involving Xen.
  523: 
  524: In 2015-01, the following combination was reported to mostly work:
  525: 
  526:         host OS: NetBSD/amd64 6.1.4
  527:         qemu: 2.2.0 from pkgsrc
  528:         Xen kernel: xenkernel42-4.2.5nb1 from pkgsrc
  529:         dom0 kernel: NetBSD/amd64 6.1.5
  530:         Xen tools: xentools42-4.2.5 from pkgsrc
  531: 
  532: See [PR 47720](https://gnats.netbsd.org/47720) for a problem with dom0
  533: shutdown.
  534: 
  535: Unprivileged domains (domU)
  536: ===========================
  537: 
  538: This section describes general concepts about domUs.  It does not
  539: address specific domU operating systems or how to install them.  The
  540: config files for domUs are typically in /usr/pkg/etc/xen, and are
  541: typically named so that the file name, domU name and the domU's host
  542: name match.
  543: 
  544: The domU is provided with CPU and memory by Xen, configured by the
  545: dom0.  The domU is provided with disk and network by the dom0,
  546: mediated by Xen, and configured in the dom0.
  547: 
  548: Entropy in domUs can be an issue; physical disks and network are on
  549: the dom0.  NetBSD's /dev/random system works, but is often challenged.
  550: 
  551: Config files
  552: ------------
  553: 
  554: There is no good order to present config files and the concepts
  555: surrounding what is being configured.  We first show an example config
  556: file, and then in the various sections give details.
  557: 
  558: See (at least in xentools41) /usr/pkg/share/examples/xen/xmexample*,
  559: for a large number of well-commented examples, mostly for running
  560: GNU/Linux.
  561: 
  562: The following is an example minimal domain configuration file
  563: "/usr/pkg/etc/xen/foo".  It is (with only a name change) an actual
  564: known working config file on Xen 4.1 (NetBSD 5 amd64 dom0 and NetBSD 5
  565: i386 domU).  The domU serves as a network file server.
  566: 
  567:         # -*- mode: python; -*-
  568: 
  569:         kernel = "/netbsd-XEN3PAE_DOMU-i386-foo.gz"
  570:         memory = 1024
  571:         vif = [ 'mac=aa:00:00:d1:00:09,bridge=bridge0' ]
  572:         disk = [ 'file:/n0/xen/foo-wd0,0x0,w',
  573:                  'file:/n0/xen/foo-wd1,0x1,w' ]
  574: 
  575: The domain will have the same name as the file.  The kernel has the
  576: host/domU name in it, so that on the dom0 one can update the various
  577: domUs independently.  The vif line causes an interface to be provided,
  578: with a specific mac address (do not reuse MAC addresses!), in bridge
  579: mode.  Two disks are provided, and they are both writable; the bits
  580: are stored in files and Xen attaches them to a vnd(4) device in the
  581: dom0 on domain creation.  The system treats xbd0 as the boot device
  582: without needing explicit configuration.
  583: 
  584: By default xm looks for domain config files in /usr/pkg/etc/xen.  Note
  585: that "xm create" takes the name of a config file, while other commands
  586: take the name of a domain.  To create the domain, connect to the
  587: console, create the domain while attaching the console, shutdown the
  588: domain, and see if it has finished stopping, do (or xl with Xen >=
  589: 4.2):
  590: 
  591:         xm create foo
  592:         xm console foo
  593:         xm create -c foo
  594:         xm shutdown foo
  595:         xm list
  596: 
  597: Typing ^] will exit the console session.  Shutting down a domain is
  598: equivalent to pushing the power button; a NetBSD domU will receive a
  599: power-press event and do a clean shutdown.  Shutting down the dom0
  600: will trigger controlled shutdowns of all configured domUs.
  601: 
  602: domU kernels
  603: ------------
  604: 
  605: On a physical computer, the BIOS reads sector 0, and a chain of boot
  606: loaders finds and loads a kernel.  Normally this comes from the root
  607: file system.  With Xen domUs, the process is totally different.  The
  608: normal path is for the domU kernel to be a file in the dom0's
  609: file system.  At the request of the dom0, Xen loads that kernel into a
  610: new domU instance and starts execution.  While domU kernels can be
  611: anyplace, reasonable places to store domU kernels on the dom0 are in /
  612: (so they are near the dom0 kernel), in /usr/pkg/etc/xen (near the
  613: config files), or in /u0/xen (where the vdisks are).
  614: 
  615: Note that loading the domU kernel from the dom0 implies that boot
  616: blocks, /boot, /boot.cfg, and so on are all ignored in the domU.
  617: See the VPS section near the end for discussion of alternate ways to
  618: obtain domU kernels.
  619: 
  620: CPU and memory
  621: --------------
  622: 
  623: A domain is provided with some number of vcpus, less than the number
  624: of CPUs seen by the hypervisor.  (For a dom0, this is controlled by
  625: the boot argument "dom0_max_vcpus=1".)  For a domU, it is controlled
  626: from the config file by the "vcpus = N" directive.
  627: 
  628: A domain is provided with memory; this is controlled in the config
  629: file by "memory = N" (in megabytes).  In the straightforward case, the
  630: sum of the the memory allocated to the dom0 and all domUs must be less
  631: than the available memory.
  632: 
  633: Xen also provides a "balloon" driver, which can be used to let domains
  634: use more memory temporarily.  TODO: Explain better, and explain how
  635: well it works with NetBSD.
  636: 
  637: Virtual disks
  638: -------------
  639: 
  640: With the file/vnd style, typically one creates a directory,
  641: e.g. /u0/xen, on a disk large enough to hold virtual disks for all
  642: domUs.  Then, for each domU disk, one writes zeros to a file that then
  643: serves to hold the virtual disk's bits; a suggested name is foo-xbd0
  644: for the first virtual disk for the domU called foo.  Writing zeros to
  645: the file serves two purposes.  One is that preallocating the contents
  646: improves performance.  The other is that vnd on sparse files has
  647: failed to work.  TODO: give working/notworking NetBSD versions for
  648: sparse vnd and gnats reference.  Note that the use of file/vnd for Xen
  649: is not really different than creating a file-backed virtual disk for
  650: some other purpose, except that xentools handles the vnconfig
  651: commands.  To create an empty 4G virtual disk, simply do
  652: 
  653:         dd if=/dev/zero of=foo-xbd0 bs=1m count=4096
  654: 
  655: Do not use qemu-img-xen, because this will create sparse file.  There
  656: have been recent (2015) reports of sparse vnd(4) devices causing
  657: lockups, but there is apparently no PR.
  658: 
  659: With the lvm style, one creates logical devices.  They are then used
  660: similarly to vnds.  TODO: Add an example with lvm.
  661: 
  662: In domU config files, the disks are defined as a sequence of 3-tuples.
  663: The first element is "method:/path/to/disk".  Common methods are
  664: "file:" for file-backed vnd. and "phy:" for something that is already
  665: a (TODO: character or block) device.
  666: 
  667: The second element is an artifact of how virtual disks are passed to
  668: Linux, and a source of confusion with NetBSD Xen usage.  Linux domUs
  669: are given a device name to associate with the disk, and values like
  670: "hda1" or "sda1" are common.  In a NetBSD domU, the first disk appears
  671: as xbd0, the second as xbd1, and so on.  However, xm/xl demand a
  672: second argument.  The name given is converted to a major/minor by
  673: calling stat(2) on the name in /dev and this is passed to the domU.
  674: In the general case, the dom0 and domU can be different operating
  675: systems, and it is an unwarranted assumption that they have consistent
  676: numbering in /dev, or even that the dom0 OS has a /dev.  With NetBSD
  677: as both dom0 and domU, using values of 0x0 for the first disk and 0x1
  678: for the second works fine and avoids this issue.  For a GNU/Linux
  679: guest, one can create /dev/hda1 in /dev, or to pass 0x301 for
  680: /dev/hda1.
  681: 
  682: The third element is "w" for writable disks, and "r" for read-only
  683: disks.
  684: 
  685: Note that NetBSD by default creates only vnd[0123].  If you need more
  686: than 4 total virtual disks at a time, run e.g. "./MAKEDEV vnd4" in the
  687: dom0.
  688: 
  689: Note that NetBSD by default creates only xbd[0123].  If you need more
  690: virtual disks in a domU, run e.g. "./MAKEDEV xbd4" in the domU.
  691: 
  692: Virtual Networking
  693: ------------------
  694: 
  695: Xen provides virtual Ethernets, each of which connects the dom0 and a
  696: domU.  For each virtual network, there is an interface "xvifN.M" in
  697: the dom0, and in domU index N, a matching interface xennetM (NetBSD
  698: name).  The interfaces behave as if there is an Ethernet with two
  699: adapters connected.  From this primitive, one can construct various
  700: configurations.  We focus on two common and useful cases for which
  701: there are existing scripts: bridging and NAT.
  702: 
  703: With bridging (in the example above), the domU perceives itself to be
  704: on the same network as the dom0.  For server virtualization, this is
  705: usually best.  Bridging is accomplished by creating a bridge(4) device
  706: and adding the dom0's physical interface and the various xvifN.0
  707: interfaces to the bridge.  One specifies "bridge=bridge0" in the domU
  708: config file.  The bridge must be set up already in the dom0; an
  709: example /etc/ifconfig.bridge0 is:
  710: 
  711:         create
  712:         up
  713:         !brconfig bridge0 add wm0
  714: 
  715: With NAT, the domU perceives itself to be behind a NAT running on the
  716: dom0.  This is often appropriate when running Xen on a workstation.
  717: TODO: NAT appears to be configured by "vif = [ '' ]".
  718: 
  719: The MAC address specified is the one used for the interface in the new
  720: domain.  The interface in dom0 will use this address XOR'd with
  721: 00:00:00:01:00:00.  Random MAC addresses are assigned if not given.
  722: 
  723: Sizing domains
  724: --------------
  725: 
  726: Modern x86 hardware has vast amounts of resources.  However, many
  727: virtual servers can function just fine on far less.  A system with
  728: 512M of RAM and a 4G disk can be a reasonable choice.  Note that it is
  729: far easier to adjust virtual resources than physical ones.  For
  730: memory, it's just a config file edit and a reboot.  For disk, one can
  731: create a new file and vnconfig it (or lvm), and then dump/restore,
  732: just like updating physical disks, but without having to be there and
  733: without those pesky connectors.
  734: 
  735: Starting domains automatically
  736: ------------------------------
  737: 
  738: To start domains foo at bar at boot and shut them down cleanly on dom0
  739: shutdown, in rc.conf add:
  740: 
  741:         xendomains="foo bar"
  742: 
  743: Note that earlier versions of the xentools41 xendomains rc.d script
  744: used xl, when one should use xm with 4.1.
  745: 
  746: Creating specific unprivileged domains (domU)
  747: =============================================
  748: 
  749: Creating domUs is almost entirely independent of operating system.  We
  750: have already presented the basics of config files.  Note that you must
  751: have already completed the dom0 setup so that "xl list" (or "xm list")
  752: works.
  753: 
  754: Creating an unprivileged NetBSD domain (domU)
  755: ---------------------------------------------
  756: 
  757: See the earlier config file, and adjust memory.  Decide on how much
  758: storage you will provide, and prepare it (file or lvm).
  759: 
  760: While the kernel will be obtained from the dom0 file system, the same
  761: file should be present in the domU as /netbsd so that tools like
  762: savecore(8) can work.   (This is helpful but not necessary.)
  763: 
  764: The kernel must be specifically for Xen and for use as a domU.  The
  765: i386 and amd64 provide the following kernels:
  766: 
  767:         i386 XEN3_DOMU
  768:         i386 XEN3PAE_DOMU
  769:         amd64 XEN3_DOMU
  770: 
  771: Unless using Xen 3.1 (and you shouldn't) with i386-mode Xen, you must
  772: use the PAE version of the i386 kernel.
  773: 
  774: This will boot NetBSD, but this is not that useful if the disk is
  775: empty.  One approach is to unpack sets onto the disk outside of xen
  776: (by mounting it, just as you would prepare a physical disk for a
  777: system you can't run the installer on).
  778: 
  779: A second approach is to run an INSTALL kernel, which has a miniroot
  780: and can load sets from the network.  To do this, copy the INSTALL
  781: kernel to / and change the kernel line in the config file to:
  782: 
  783:         kernel = "/home/bouyer/netbsd-INSTALL_XEN3_DOMU"
  784: 
  785: Then, start the domain as "xl create -c configname".
  786: 
  787: Alternatively, if you want to install NetBSD/Xen with a CDROM image, the following
  788: line should be used in the config file.
  789: 
  790:     disk = [ 'phy:/dev/wd0e,0x1,w', 'phy:/dev/cd0a,0x2,r' ]
  791: 
  792: After booting the domain, the option to install via CDROM may be
  793: selected.  The CDROM device should be changed to `xbd1d`.
  794: 
  795: Once done installing, "halt -p" the new domain (don't reboot or halt,
  796: it would reload the INSTALL_XEN3_DOMU kernel even if you changed the
  797: config file), switch the config file back to the XEN3_DOMU kernel,
  798: and start the new domain again. Now it should be able to use "root on
  799: xbd0a" and you should have a, functional NetBSD domU.
  800: 
  801: TODO: check if this is still accurate.
  802: When the new domain is booting you'll see some warnings about *wscons*
  803: and the pseudo-terminals. These can be fixed by editing the files
  804: `/etc/ttys` and `/etc/wscons.conf`. You must disable all terminals in
  805: `/etc/ttys`, except *console*, like this:
  806: 
  807:     console "/usr/libexec/getty Pc"         vt100   on secure
  808:     ttyE0   "/usr/libexec/getty Pc"         vt220   off secure
  809:     ttyE1   "/usr/libexec/getty Pc"         vt220   off secure
  810:     ttyE2   "/usr/libexec/getty Pc"         vt220   off secure
  811:     ttyE3   "/usr/libexec/getty Pc"         vt220   off secure
  812: 
  813: Finally, all screens must be commented out from `/etc/wscons.conf`.
  814: 
  815: It is also desirable to add
  816: 
  817:         powerd=YES
  818: 
  819: in rc.conf. This way, the domain will be properly shut down if
  820: `xm shutdown -R` or `xm shutdown -H` is used on the dom0.
  821: 
  822: It is not strictly necessary to have a kernel (as /netbsd) in the domU
  823: file system.  However, various programs (e.g. netstat) will use that
  824: kernel to look up symbols to read from kernel virtual memory.  If
  825: /netbsd is not the running kernel, those lookups will fail.  (This is
  826: not really a Xen-specific issue, but because the domU kernel is
  827: obtained from the dom0, it is far more likely to be out of sync or
  828: missing with Xen.)
  829: 
  830: Creating an unprivileged Linux domain (domU)
  831: --------------------------------------------
  832: 
  833: Creating unprivileged Linux domains isn't much different from
  834: unprivileged NetBSD domains, but there are some details to know.
  835: 
  836: First, the second parameter passed to the disk declaration (the '0x1' in
  837: the example below)
  838: 
  839:     disk = [ 'phy:/dev/wd0e,0x1,w' ]
  840: 
  841: does matter to Linux. It wants a Linux device number here (e.g. 0x300
  842: for hda).  Linux builds device numbers as: (major \<\< 8 + minor).
  843: So, hda1 which has major 3 and minor 1 on a Linux system will have
  844: device number 0x301.  Alternatively, devices names can be used (hda,
  845: hdb, ...)  as xentools has a table to map these names to devices
  846: numbers.  To export a partition to a Linux guest we can use:
  847: 
  848:         disk = [ 'phy:/dev/wd0e,0x300,w' ]
  849:         root = "/dev/hda1 ro"
  850: 
  851: and it will appear as /dev/hda on the Linux system, and be used as root
  852: partition.
  853: 
  854: To install the Linux system on the partition to be exported to the
  855: guest domain, the following method can be used: install
  856: sysutils/e2fsprogs from pkgsrc.  Use mke2fs to format the partition
  857: that will be the root partition of your Linux domain, and mount it.
  858: Then copy the files from a working Linux system, make adjustments in
  859: `/etc` (fstab, network config).  It should also be possible to extract
  860: binary packages such as .rpm or .deb directly to the mounted partition
  861: using the appropriate tool, possibly running under NetBSD's Linux
  862: emulation.  Once the file system has been populated, umount it.  If
  863: desirable, the file system can be converted to ext3 using tune2fs -j.
  864: It should now be possible to boot the Linux guest domain, using one of
  865: the vmlinuz-\*-xenU kernels available in the Xen binary distribution.
  866: 
  867: To get the Linux console right, you need to add:
  868: 
  869:     extra = "xencons=tty1"
  870: 
  871: to your configuration since not all Linux distributions auto-attach a
  872: tty to the xen console.
  873: 
  874: Creating an unprivileged Solaris domain (domU)
  875: ----------------------------------------------
  876: 
  877: See possibly outdated
  878: [Solaris domU instructions](/ports/xen/howto-solaris/).
  879: 
  880: 
  881: PCI passthrough: Using PCI devices in guest domains
  882: ---------------------------------------------------
  883: 
  884: The dom0 can give other domains access to selected PCI
  885: devices. This can allow, for example, a non-privileged domain to have
  886: access to a physical network interface or disk controller.  However,
  887: keep in mind that giving a domain access to a PCI device most likely
  888: will give the domain read/write access to the whole physical memory,
  889: as PCs don't have an IOMMU to restrict memory access to DMA-capable
  890: device.  Also, it's not possible to export ISA devices to non-dom0
  891: domains, which means that the primary VGA adapter can't be exported.
  892: A guest domain trying to access the VGA registers will panic.
  893: 
  894: If the dom0 is NetBSD, it has to be running Xen 3.1, as support has
  895: not been ported to later versions at this time.
  896: 
  897: For a PCI device to be exported to a domU, is has to be attached to
  898: the "pciback" driver in dom0.  Devices passed to the dom0 via the
  899: pciback.hide boot parameter will attach to "pciback" instead of the
  900: usual driver.  The list of devices is specified as "(bus:dev.func)",
  901: where bus and dev are 2-digit hexadecimal numbers, and func a
  902: single-digit number:
  903: 
  904:         pciback.hide=(00:0a.0)(00:06.0)
  905: 
  906: pciback devices should show up in the dom0's boot messages, and the
  907: devices should be listed in the `/kern/xen/pci` directory.
  908: 
  909: PCI devices to be exported to a domU are listed in the "pci" array of
  910: the domU's config file, with the format "0000:bus:dev.func".
  911: 
  912:         pci = [ '0000:00:06.0', '0000:00:0a.0' ]
  913: 
  914: In the domU an "xpci" device will show up, to which one or more pci
  915: buses will attach.  Then the PCI drivers will attach to PCI buses as
  916: usual.  Note that the default NetBSD DOMU kernels do not have "xpci"
  917: or any PCI drivers built in by default; you have to build your own
  918: kernel to use PCI devices in a domU.  Here's a kernel config example;
  919: note that only the "xpci" lines are unusual.
  920: 
  921:         include         "arch/i386/conf/XEN3_DOMU"
  922: 
  923:         # Add support for PCI buses to the XEN3_DOMU kernel
  924:         xpci* at xenbus ?
  925:         pci* at xpci ?
  926: 
  927:         # PCI USB controllers
  928:         uhci*   at pci? dev ? function ?        # Universal Host Controller (Intel)
  929: 
  930:         # USB bus support
  931:         usb*    at uhci?
  932: 
  933:         # USB Hubs
  934:         uhub*   at usb?
  935:         uhub*   at uhub? port ? configuration ? interface ?
  936: 
  937:         # USB Mass Storage
  938:         umass*  at uhub? port ? configuration ? interface ?
  939:         wd*     at umass?
  940:         # SCSI controllers
  941:         ahc*    at pci? dev ? function ?        # Adaptec [23]94x, aic78x0 SCSI
  942: 
  943:         # SCSI bus support (for both ahc and umass)
  944:         scsibus* at scsi?
  945: 
  946:         # SCSI devices
  947:         sd*     at scsibus? target ? lun ?      # SCSI disk drives
  948:         cd*     at scsibus? target ? lun ?      # SCSI CD-ROM drives
  949: 
  950: 
  951: NetBSD as a domU in a VPS
  952: =========================
  953: 
  954: The bulk of the HOWTO is about using NetBSD as a dom0 on your own
  955: hardware.  This section explains how to deal with Xen in a domU as a
  956: virtual private server where you do not control or have access to the
  957: dom0.  This is not intended to be an exhaustive list of VPS providers;
  958: only a few are mentioned that specifically support NetBSD.
  959: 
  960: VPS operators provide varying degrees of access and mechanisms for
  961: configuration.  The big issue is usually how one controls which kernel
  962: is booted, because the kernel is nominally in the dom0 file system (to
  963: which VPS users do not normally have access).  A second issue is how
  964: to install NetBSD.
  965: A VPS user may want to compile a kernel for security updates, to run
  966: npf, run IPsec, or any other reason why someone would want to change
  967: their kernel.
  968: 
  969: One approach is to have an administrative interface to upload a kernel,
  970: or to select from a prepopulated list.  Other approaches are pygrub
  971: (deprecated) and pvgrub, which are ways to have a bootloader obtain a
  972: kernel from the domU file system.  This is closer to a regular physical
  973: computer, where someone who controls a machine can replace the kernel.
  974: 
  975: A second issue is multiple CPUs.  With NetBSD 6, domUs support
  976: multiple vcpus, and it is typical for VPS providers to enable multiple
  977: CPUs for NetBSD domUs.
  978: 
  979: pygrub
  980: -------
  981: 
  982: pygrub runs in the dom0 and looks into the domU file system.  This
  983: implies that the domU must have a kernel in a file system in a format
  984: known to pygrub.  As of 2014, pygrub seems to be of mostly historical
  985: interest.
  986: 
  987: pvgrub
  988: ------
  989: 
  990: pvgrub is a version of grub that uses PV operations instead of BIOS
  991: calls.  It is booted from the dom0 as the domU kernel, and then reads
  992: /grub/menu.lst and loads a kernel from the domU file system.
  993: 
  994: [Panix](http://www.panix.com/) lets users use pvgrub.  Panix reports
  995: that pvgrub works with FFsv2 with 16K/2K and 32K/4K block/frag sizes
  996: (and hence with defaults from "newfs -O 2").  See [Panix's pvgrub
  997: page](http://www.panix.com/v-colo/grub.html), which describes only
  998: Linux but should be updated to cover NetBSD :-).
  999: 
 1000: [prgmr.com](http://prgmr.com/) also lets users with pvgrub to boot
 1001: their own kernel.  See then [prgmr.com NetBSD
 1002: HOWTO](http://wiki.prgmr.com/mediawiki/index.php/NetBSD_as_a_DomU)
 1003: (which is in need of updating).
 1004: 
 1005: It appears that [grub's FFS
 1006: code](http://xenbits.xensource.com/hg/xen-unstable.hg/file/bca284f67702/tools/libfsimage/ufs/fsys_ufs.c)
 1007: does not support all aspects of modern FFS, but there are also reports
 1008: that FFSv2 works fine.  At prgmr, typically one has an ext2 or FAT
 1009: partition for the kernel with the intent that grub can understand it,
 1010: which leads to /netbsd not being the actual kernel.  One must remember
 1011: to update the special boot partition.
 1012: 
 1013: Amazon
 1014: ------
 1015: 
 1016: See the [Amazon EC2 page](/amazon_ec2/).
 1017: 
 1018: TODO items for improving NetBSD/xen
 1019: ===================================
 1020: 
 1021: * Make the NetBSD dom0 kernel work with SMP.
 1022: * Test the Xen 4.5 packages adequately to be able to recommend them as
 1023:   the standard approach.
 1024: * Get PCI passthrough working on Xen 4.5
 1025: * Get pvgrub into pkgsrc, either via xentools or separately.
 1026: * grub
 1027:   * Check/add support to pkgsrc grub2 for UFS2 and arbitrary
 1028:     fragsize/blocksize (UFS2 support may be present; the point is to
 1029:     make it so that with any UFS1/UFS2 file system setup that works
 1030:     with NetBSD grub will also work).
 1031:     See [pkg/40258](https://gnats.netbsd.org/40258).
 1032:   * Push patches upstream.
 1033:   * Get UFS2 patches into pvgrub.
 1034: * Add support for PV ops to a version of /boot, and make it usable as
 1035:   a kernel in Xen, similar to pvgrub.
 1036: 
 1037: Random pointers
 1038: ===============
 1039: 
 1040: This section contains links from elsewhere not yet integrated into the
 1041: HOWTO, and other guides.
 1042: 
 1043: * http://www.lumbercartel.ca/library/xen/
 1044: * http://pbraun.nethence.com/doc/sysutils/xen_netbsd_dom0.html
 1045: * https://gmplib.org/~tege/xen.html

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