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# 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|hurd/running]].

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

# `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
    ./native-install

- 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.

# Partitions

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

root=part:2:device:hd0

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

# Miscellaneous

[[Internals]].

[[!GNU_Savannah_task 5468]], [[!GNU_Savannah_task 6584]].

# Building from sources

If you want to generate your own gnumach kernel, see [[microkernel/mach/gnumach/building]], and use

    ./configure --enable-platform=xen
    make

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

    <phcoder> youpi: would a limitation of 32 modules to hurd in pvgrub2 be a
      problem?
    <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>  #define PFN_LIST MACH2PHYS_VIRT_START_PAE
    <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
      sure
    <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
      surprises

    <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
      channels)
    <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
      information
    <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
      pv-grub
    <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
      address"
    <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

[[hurd/libstore/part]].


# Host-side Writeback Caching

Optimization possible as it is with
[[QEMU|hurd/running/qemu/writeback_caching]]?

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
      non-pae