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Help needed running an X11 environment inside a DomU (Re: [Xen-users] Cr

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Subject: Help needed running an X11 environment inside a DomU (Re: [Xen-users] Creating a paravirtualized guest with Xen 3.4.1 andCentos 5.3 (64bit))
From: Martin Troester <TroyMcClure@xxxxxx>
Date: Thu, 24 Sep 2009 20:38:55 +0200
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Hi all,

looks I am somewhat stuck with getting X11 to run inside a DomU. Searching for this gives me only little help. As far as I understand, it does not work out of the box because there is on real graphics hardware available. That definitely explains the output of the X server, complaining about not finding any screens to use.

One thing seems to be provide an external framebuffer, as described here: http://wiki.xensource.com/xenwiki/VirtualFramebuffer This page is dated Feb 2008, so is there any important news about this? The last line is most discouraging to me, is there any interesting updates regarding desktop size? What about performance? Well, I'll try that now anyway.

In addition, as the VM runs fine in HVM (including X), is there any way to configure an emulated graphics device for a pv DomU? Reads strange... But with graphics hardware not being virtualizable, something like this would help.

Any hints/links/documentation on my problem is really appreciated.



Martin Troester wrote:

thanks for the steps to move an existing physical image to a paravirt DomU image. However, I think this only works if both Dom0 and DomU use the same architecture (64bit e.g.). In my case, my host is a 64bit Dom0 with 32bit DomU guests, so I think it's at least necessary to get 32bit kernel and initrd images someplace. Is there an official way to get the released 32bit xen kernel versions to be used on a 64bit host? Otherwise there's no problem to get them manually, but that means not getting auto-updates...

In addition, I found some comments on the list about CentOS 5.3 initrd images coming with scsi modules, and thus not working without rebuilding or manually changing to disable scsi.

Therefore I did the following steps to get an HVM VM initially transformed to PV:

1) get a known-good Centos image for Xen, e.g. at http://stacklet.com/downloads/images/centos/5.3
2) make sure that image works for you with pygrub
3) extract kernel, initrd and /lib/modules/
3) make it work with kernel/initrd/parameters provided externally
3a) don't use a wrong option for ramdisk support ;-)
4) add /lib/modules extracted from other image to HVM image
5a) add kernel parameter "console=/dev/tty" to prove it boots up
5b) modify /etc/securetty and /etc/inittab (if not done yet by your distro), see http://xen.xensource.com/files/xensummit_4/xensummit_linux_console_slides.pdf
6) boot up the HVM VM in paravirtualized mode

Especially 5) caused me some trouble, as without this option in /etc/inittab, the VM just froze before bringing the login prompt.

This is how far I got in a few hours tonight, but now the previously automatically launching X server does not start. Well, but I'm getting closer.

Open question:
- how to get a Centos 32bit Xen DomU kernel on a 64bit host via repositories?

Ultimatively, it will probably be best to use pygrub and do the kernel management from inside the VM (this will also be easier when thinking about /lib/modules). But at least for my VM, the harddisk layout does not work with pygrup (yet).


Jeff Sturm wrote:
-----Original Message-----
From: xen-users-bounces@xxxxxxxxxxxxxxxxxxx [mailto:xen-users-
bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of Martin Troester
Sent: Tuesday, September 22, 2009 4:17 PM
To: xen-users@xxxxxxxxxxxxxxxxxxx
Subject: Re: [Xen-users] Creating a paravirtualized guest with Xen
3.4.1 andCentos
5.3 (64bit)

Now I'll find out if I can get my original problem solved, migrate the
existing fully virtualized system to a paravirtualizded kernel. Let's

It's not exactly the same, but I have done physical-to-paravirt
migrations on CentOS 5.2 with little effort.  It's probably easiest if
you install kernel-xen packages before you migrate:

1) yum install kernel-xen
2) shutdown
3) copy disk images to dom0
4) create domU config
5) start domU

Afterwards you can use either pygrub or kernel/ramdisk, whichever you


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