RHEL 4 Xen guest HOWTO

	Rik van Riel, Oct 5 2006


Currently the RHEL4 Xen paravirtualized kernels are ready, but
there is no installer support yet.  This temporary HOWTO describes
how you can turn a fully virtualized RHEL4 guest into a paravirt
Xen guest.


WARNING: As of October 5 2006, there are a few KNOWN BUGS left
in the RHEL4 Xen kernel.  This HOWTO is FOR TESTING PURPOSES ONLY.


	Requirements:

- FC6 or RHEL5 Beta dom0 as the host OS
- a fully virtualized guest image, which can be created using
    - Xen with Intel VMX or AMD AMD-V CPU, or
    - qemu
- the RHEL4 paravirtualized kernels from
    http://people.redhat.com/~jbaron/rhel4/RPMS.kernel/


1) Install RHEL4 as a fully virtualized guest, using either
   Xen w/ HVM or qemu.

2) In your RHEL4 guest, modify /etc/modprobe.conf like this:

$ cat /etc/modprobe.conf
alias eth0 xennet
alias scsi_hostadapter xenblk

3) Install the RHEL4 paravirtualized kernel RPM for your
   architecture.

   The modprobe.conf changes above should make sure that mkinitrd
   makes sure the xenblk driver is loaded on bootup.

4) Create a config file for your RHEL4 Xen guest in
   /etc/xen.  If your fully virtualized system was a Xen
   guest too, you will want to keep the vif MAC address
   and the UUID the same as before.

$ cat /etc/xen/rhel4
# My RHEL4 Xen guest
name = "RHEL4"
memory = "256"
uuid = "13db28ea-8536-53dc-3646-1f20fcc1199b"
disk = [ 'phy:/dev/VolGroup00/RHEL4,xvda,w' ]
# disk = [ 'file:/path/to/disk/image,xvda,w' ]
vif = [ 'bridge=xenbr0,mac=00:16:3e:xx:xx:xx' ]
bootloader="/usr/bin/pygrub"
# vcpus = 2

    If your guest is file backed and not LVM backed, you will
    want to fill in the second "disk =" line and comment out
    the first one.

    If you want an SMP guest, comment out the "vcpus = 2" line,
    and set the number to however many vcpus you would like
    your guest to have.

    Please change the MAC address!  If everybody uses the same
    MAC address, there will be network problems, guaranteed...
    However, the first 3 MAC address numbers (00:16:3e) should
    stay intact.  That is the official Xen MAC address range.

5)  Start your Xen guest.  Make sure to choose the right
    kernel in the pygrub menu, because anything except the
    paravirtualized kernel will fail to boot.

    Use the following command, replacing "rhel4" with the
    name of your Xen guest configuration file.

$ xm create -c rhel4

6)  Found a bug?   No surprise, as of October 5th 2006
    there are still a few known bugs in the RHEL4 Xen kernel.
    Please check bugzilla to see if your bug has been filed
    already.  If you found something new, please let us
    know and we will try to fix it.


Gotchas:

1)  You can only run i686 guests on i686 hosts, or x86-64
    guests on x86-64 hosts.  Do not try to mix & match, 
    because that will not work.

2)  In order to get better system call and NPTL performance
    on i686, you will want to upgrade your glibc to the RPMS
    in the glibc directory.  This glibc adds the "nosegneg"
    variant, which avoids negative segment accesses for TLS.
    Use of this glibc variant has increased performance by
    double digit percentages for a number of workloads.

3)  Herbert Xu's network segmentation offload could not be
    backported to RHEL4 without destabilizing the network
    layer for non-Xen kernels too, so network performance
    in RHEL4 Xen guests will not be as good as that in RHEL5.

4)  If you use qemu to install, make sure the MAC address you
    put in your Xen config file matches what's in the guest
    in /etc/sysconfig/network-scripts/ifup-eth0, otherwise the
    initscripts will refuse to bring up the guest's network.


Community:

1)  Email:  https://www.redhat.com/mailman/listinfo/nahant-list

2)  IRC:   irc.oftc.net  #xen


Success!