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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

Here is a sample /etc/xen/hurd configuration

    kernel = "/boot/gnumach-xen-pae"
    memory = 256
    disk = ['phy:sda4,hda,w']
    extra = "root=device:hd0"
    vif = [ '' ]
    ramdisk = "/boot/hurd-modules"

Do not give more than 580MB memory (due to bootstrap limitations, it's not easy
to map more).

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' ]`.


# 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.


# Building from sources

If you want to generate these images, first get the `gnumach-1-branch-Xen-branch` branch from gnumach CVS.
Then look for "Ugly" in `kern/bootstrap.c`, how to generate `hurd-modules` is explained there, and you'll have to fix `EXT2FS_SIZE` and `LD_SO_SIZE` by hand.
Then use

    ./configure --enable-platform=xen
    make

The current `hurd-modules` was built from the debian packages `hurd 20070606-2` and `libc0.3 2.6.1-1`.
/!\ This means that when using this image, your GNU/Hurd system also needs to be a glibc version 2.6 or later-based one!

# `pv-grub`

From Xen 4.0 on you can run the GNU Hurd directly using `pv-grub`,
without the need to [prepare a special bootstrap
image](http://youpibouh.thefreecat.org/hurd-xen/build_hurd-modules) (like an
initrd).

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

    kernel = "/boot/pv-grub.gz"
    memory = 256
    disk = ['phy:sda4,hda,w']
    extra = "(hd0,1)/boot/grub/menu.lst"
    vif = [ '' ]

extra is now the path to the grub config file.

# 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]].


# Host-side Writeback Caching

Optimization possible as it is with [[QEMU|hurd/running/qemu/discussion]],
*Host-side Writeback Caching*?

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

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