Xen dom0, hypervisor

/!\ Now that GNU Mach handles PAE you can use a PAE-enabled hypervisor.

You can either get binaries at http://youpibouh.thefreecat.org/hurd-xen/ or build them yourself.

  • Copy gnumach-xen-pae and hurd-modules to your dom0 /boot. If you still have a non-PAE hypervisor, use gnumach-xen-nonpae instead.
  • Copy hurd into /etc/xen, edit it for fixing access to your hurd / and swap

GNU/Hurd system

/!\ You need an already installed GNU/Hurd system.

If you have a free partition, you can fdisk to type 0x83, create a filesystem using:

sudo mke2fs -b 4096 -I 128 -o hurd /dev/sda4 

Replace /dev/sda4 with your partition. Install and use crosshurd to setup a GNU/Hurd system on this partition.

/etc/xen/hurd configuration

There are two ways to boot a Hurd system: either directly boot gnumach, or boot it through PV-Grub. The former is a bit more complex.

Directly booting gnumach

Here is a sample /etc/xen/hurd configuration

kernel = "/boot/gnumach-xen-pae"
memory = 512
disk = ['phy:sda4,hda,w']
extra = "root=device:hd0"
vif = [ '' ]
ramdisk = "/boot/hurd-modules"

hurd-modules from http://youpibouh.thefreecat.org/hurd-xen/ was built from a specific libc version, /!\ This means that when using this image, your GNU/Hurd system also needs to have the same version!

It is preferrable to rebuild your own hurd-modules, using your own libc version, by using the http://youpibouh.thefreecat.org/hurd-xen/build_hurd-modules script.

Suggestions about networking configuration are available.

If you need stable MAC addresses, use a syntax like vif = [ 'mac=00:16:3e:XX:XX:XX, bridge=br0' ].

Booting through pv-grub


Starting from Xen 4.0, you can run the GNU Hurd using pv-grub.

Download http://youpibouh.thefreecat.org/hurd-xen/pv-grub.gz into /boot, and use the following for instance:

kernel = "/boot/pv-grub.gz"
memory = 512
disk = ['phy:sda4,hda,w']
extra = "(hd0)/boot/grub/menu.lst"
vif = [ '' ]

extra is now the path to the grub config file, which must contain the usual grub command to boot a hurd system.

Running Hurd with Xen

To run Hurd with Xen, use:

xm create -c hurd

and gnumach should get started. Proceed with native-install.

export TERM=mach
  • If xm complains about networking (vif could not be connected), it's Xen scripts' fault, see Xen documentation for how to configure the network. The simplest way is network-bridge with fixed IPs (note that you need the bridge-utils package for this). You can also just disable networking by commenting the vif line in the config.
  • If xm complains Error: (2, 'Invalid kernel', 'xc_dom_compat_check: guest type xen-3.0-x86_32 not supported by xen kernel, sorry\n'), you most probably have a PAE-enabled hypervisor and a non-PAE gnumach. Either install and boot non-PAE hypervisor and kernel, or rebuilt gnumach in PAE mode.


You will need the following notation for the gnumach root= parameter:


to access the second partition of hd0, for instance.

You will also need to use the parted storeio module for the /dev entries, for instance:

settrans -fgap /dev/hd0s1 /hurd/storeio -T typed part:1:device:hd0



GNU Savannah task #5468, GNU Savannah task #6584.

Building from sources

If you want to generate your own gnumach kernel, see building, and use

./configure --enable-platform=xen

IRC, freenode, #hurd, 2013-11-09

<phcoder> youpi: would a limitation of 32 modules to hurd in pvgrub2 be a
<phcoder> *31
<youpi> phcoder: probably not
<phcoder> youpi: ok

<phcoder> youpi: gnumach goes into infinite loop with "warning: nsec
  0x000096dae65d2697 < lastnsec 0x000096db11dee20d". Second value stays
  constant, first value loops from 0x000096da14968a59 to
  0x000096db08bf359e. Not sure if the problem is on GRUB or gnumach ide
<youpi> loops?!
<youpi> that's the time coming from the hypervisor
<youpi> not a problem from GRUB anyway
<phcoder> Yes, loops in steps of around 0x40 and comes back regularly.
<youpi> Mmm, maybe it could be grub not properly setting up
  hyp_shared_info.vcpu_info[], actually
<youpi> i.e. the mfn in boot_info.shared_info
<phcoder> I don't think we write to shared page at all
<phcoder> could gnumach suffer from overflow on fast CPU?
<phcoder>   next_start.shared_info = grub_xen_start_page_addr->shared_info;
<phcoder> And shared_info is machine address, so no need to adjust it
<phcoder> tsc_shift can be negative. Does gnumach handle this?
<youpi> yes
<youpi> here it's the base which doesn't change, actually
<phcoder> Do you mean this:         system_time =
  time->system_time; ?
<phcoder> But wait: ((delta * (unsigned64_t) mul) >> 32)
<phcoder> this overflows after 2^32 nanoseconds
<phcoder> which is about 4 seconds
<phcoder> I think this is the mistake
<phcoder> which is more or less what I see
<phcoder> Let me make a patch
<youpi> does xen have some tickless  feature now?
<youpi> I'd expect the clock to get updated at least sometimes during 4
  seconds :)
<phcoder> Hm, can't compile master:
<phcoder> ./include/mach/xen.h:52:18: error: ‘MACH2PHYS_VIRT_START_PAE’
  undeclared (first use in this function)
<phcoder> Here is the patch: http://paste.debian.net/64857/
<youpi> it's defined in xen/public/arch-x86/xen-x86_32.h
<phcoder> yes it is. Let's see why it's not included
<phcoder> Hm, for some reason it pulls 64-bit headers in
<youpi> how do you cross-compile?
<youpi> I use
<youpi> ./configure --host=i686-gnu CC='gcc -m32' LD='ld -melf_i386' 
<phcoder> Yes. GRUB adds those itself
<phcoder> youpi: confirmed: my patch solves the problem
<phcoder> any yes: I tried with unpatched master and it fails
<youpi> ok
<youpi> phcoder: thanks!

<phcoder> Now I get plenty of "getcwd: cannot access parent directories:
  Inappropriate file type or format". But I don't think it's grub-related
<youpi> what do you get before that?
<youpi> perhaps ext2fs doesn't get properly initialized
<youpi> which module commande line do you get in the boot log?
<youpi> perhaps it's simply a typo in there
<phcoder> http://paste.debian.net/64865/
<youpi>  $(task-create) $(task-resume) is missing at the end of the ext2fs
  line indeed
<youpi> in your paste it stops at $(
<phcoder> this is at the end of my console. I believe it to be
  cosmetic. Let me reset console to some sane state
<youpi> ok
<youpi> the spurious event at the start is probably worth checking up
<youpi> it looks like events that pvgrub2 should have eaten
<youpi> (in its own drivers, before finishing shutting them down)
<phcoder> when redirecting console to file: http://paste.debian.net/64868/
<phcoder> could swapon have sth to do with it?
<youpi> I'd be surprised
<phcoder> my guess it's because I use older userland (debian about May) and
  new kernel (fresh from master)
<youpi> the kernel hasn't really changed
<youpi> you could rebuild the may-debian kernel with your patch to make
<youpi> but probably better trying to fix swapon first, at least
<youpi> (even if that'd surprise me)
<youpi> "trying fixiing* swapon", actually
<youpi> it makes a difference :)
<phcoder> We actually never eat event on evtchn, we look into buffers to
  check for response
<youpi> ah, that's why
<youpi> you should really eat the events too
<youpi> in principle it wouldn't hurt not to, but you'd probably get

<phcoder> youpi: would doing EVTCHNOP_reset at the end be enough?
<youpi> possibly, I don't know that one
<youpi> looks like a good thing to do before handing control, indeed
<youpi>     /* Clear pending event to avoid unexpected behavior on
  re-bind. */
<youpi>     evtchn_port_clear_pending(d1, chn1);
<youpi> yes, it does clear the pending events
<phcoder> http://paste.debian.net/64870/
<phcoder> I did this: http://paste.debian.net/64871/
<youpi> well, closing the event channels would be a good idea too
<youpi> (reset does not only clear pending events, it also closes the event
<phcoder> well we can't close console one. So it leave to close disk ones
  (the ones we allocated)
<phcoder> http://paste.debian.net/64875/
<phcoder> New log: http://paste.debian.net/64876/ (swapon fixed, given 1G
  of memory)
<youpi> ok, so it really is something else
<phcoder> looks like there is a space after $(task-resume) but can't tell
  if it's real or comes from message
<phcoder> tottally artefact

<phcoder> youpi: this happens when booted in qemu with old kernel now. Now
  my bet is on weird fs corruption or because I accessed it with Linux in
  rw. In any case I feel like it's time to call it a port and commit
<youpi> I'd say so, yes
<phcoder> Let's look what's remaining: vfb, vkbd and vif: don't need them
  for first port commit. Hm, there is an issue of default configfile. What
  is pvgrub default behaviour?
<youpi> iirc it just enters the shell
<youpi> I had implemented vfb and vkbd to get the graphical support, but
  that's optional indeed
<youpi> vif is useful for netboot only
<youpi> ah, no, by default it runs dhcp --with-configfile
<phcoder> youpi: port committed to trunk
<youpi> \o/
<youpi> I was lamenting for 5 years that that wasn't happening :)
<youpi> Citrix could have asked one of his engineers to work on it, really
<phcoder> documentation on using the port is still missing though
<youpi> amazon EC2 users will be happy to upgrade from pv-grub to pv-grub2
<youpi> I asked some amazon guy at SuperComputing whether he knew how many
  people were using pv-grub, but he told me that was customer private
<phcoder> Another interesting idea would be to switch between 64-bit and
  32-bit domains somehow
<youpi> yes, we were discussing about it at XenSource when I implemented
<youpi> that's not really an easy thing
<youpi> pvh would probably help there, again
<youpi> in the end, we considered that it was usually not hard to select a
  32bit or 64bit pv-grub depending on the userland bitness
<youpi> we considered adding a hypercall to change the bitness of a domU,
  but that's really involved
<phcoder> Well when you discussed i386 domains were still around
<phcoder> now it's only PAE and amd64 and they are very similar. Only few
  gdt differ
<youpi> well, switching from 32-PAE to 64 is not *so* hard
<youpi> since a 32bit-loaded OS can fit in 64bit
<youpi> the converse is more questionable of course
<phcoder> yes
<youpi> still, it's really not easy for the hypervisor
<youpi> it'd mean converting stuff here and there
<youpi> most probably missing things here and there :)
<phcoder> Ok, not that important anyway
<youpi> we felt it was too dangerous to promise the feature as working :)
<youpi> heh, 5000 lines patch, just like my patch adding support to Mach :)
<phcoder> BTW do you know how to check if kernel supports dom0 ? Apparently
  there is feature "privilegied" and dom0 kernels are supposed to have it
  in notes but my linux one doesn't even though I'm in xen now
<youpi> it's XENFEAT_dom0
<youpi> called dom0 in the notes
<phcoder> http://paste.debian.net/64894/
<youpi> well, maybe the hypervisor doesn't actually check it's there
<youpi> phcoder: what does grub-mkstandalone?
<phcoder> puts all modules in memdisk which is embed into core
<youpi> ah, ok
<youpi> we didn't have to care about that in grub1 indeed :)

IRC, freenode, #hurd, 2013-11-09

<phcoder> youpi: now I get "hd0: dom0's VBD 768
  (/home/phcoder/diskimg/debian-hurd-20130504.img,w) 3001MB"
<phcoder> but "start ext2fs: ext2fs: device:hd0s1: No such device or
<phcoder> disk = [
  'file:/home/phcoder/diskimg/debian-hurd-20130504.img,hda,w' ]
<phcoder> Hm, using "disk = [ 'phy:loop0,hda1,w' ]" instead worked (loop0
  is an offset loop)
<youpi> yes, xen disks don't support partitioning
<youpi> and we haven't migrated userland to userland partitioning yet


Host-side Writeback Caching

Optimization possible as it is with QEMU?

IRC, freenode, #hurd, 2011-06-08

<braunr> youpi: does xen provide disk caching options ?
<youpi> through a blktap, probably
<braunr> ok

IRC, freenode, #hurd, 2013-11-09

<phcoder> youpi: debian-hurd-20130504.img apparently has a kernel without
  xen note. Do I have to do sth special to get xen kernel?
<youpi> phcoder: there's the -xen package for that
<youpi> I haven't made the kernel hybrid
<phcoder> youpi: easiest way is probably to have different entry
  points. You could even just link both of them at different addresses and
  then glue together though it's not very efficient
<youpi> it's also about all the privileged operations that have to be
  replaced with PV operations
<youpi> PVH will help with that regard

<phcoder> youpi: btw, I recommend compiling xen kernel for 686 and drop