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Re: [Xen-users] 64bit kernel for 32bit pv DomU in 64bit Dom0

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Subject: Re: [Xen-users] 64bit kernel for 32bit pv DomU in 64bit Dom0
From: Mark Williamson <mark.williamson@xxxxxxxxxxxx>
Date: Mon, 29 Oct 2007 23:21:17 +0000
Cc: Andris <andris@xxxxxxxx>, Pierre-Alain RIVIERE <pariviere@xxxxxxxx>
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> 32 bit userland will always be compatible with 64 bit kernel.
> And as far as I know there's no problem to proceed this way  and I do
> the same.

I seem to recall certain tools (iptables being particularly notable) need to 
be 64-bit.  Of course if you don't use those tools then I guess it doesn't 
matter :-)

> This link also describes 32/64 bit compatibility between Dom0 and DomU.
> http://lists.xensource.com/archives/html/xen-users/2007-07/msg00842.html

Since you've got 3.1.1 running in dom0 you can actually boot a PV 32-bit PAE 
kernel directly as a domU.  i.e. you don't need to use a 64-bit kernel under 
the guest, just use 32-bit PAE Linux.  This may even have better performance.

Another option would be to install 32-bit paravirtualised drivers into your 
32-bit HVM guest.  This should enable you to stick to more-or-less the 
original kernel, but you'd be using the optimised drivers instead of device 
emulation.  There are sources for this in the Xen distribution, which can be 
built and inserted into an otherwise standard Linux kernel.

In all of these cases, if you're receiving commercial support for the OS you 
might want to verify that the distro provider will still support an OS 
running on Xen - particularly if running in PV mode.


> Andris wrote:
> > Hello all!
> >
> > I read in xen user archive thread where some people asked whether is
> > possible to run not modified 32 bit pv DomU's just copying 64bit kernel
> > modules in DomU and starting it with 64bit kernel (prepared for pv
> > guests). So people in that thread got answer- "may try".
> > We have a newer kernels and newer xen since 2006.
> > So I have the same situation:
> > need to run 32bit Ubuntu distro (commercial product, based on ubuntu
> > dapper) on 64bit Dom0 (Gentoo 2007.0, xen 3.1.1).
> > I have succes to run in as HVM, but I/O performance is far from
> > desirable. I made disk image and cloned that 32bit system on it. Modified
> > fstab added swap disk and copied modules directory from 64bit Dom0 and
> > tried to boot with my 64bit kernel prepared for 64bit pv guests.
> > System booted and seems to be working. Is such approach is ok or I may
> > encounter certain problems in future?
> >
> > andris
> >
> > _______________________________________________
> > Xen-users mailing list
> > Xen-users@xxxxxxxxxxxxxxxxxxx
> > http://lists.xensource.com/xen-users
> _______________________________________________
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Dave: Just a question. What use is a unicyle with no seat?  And no pedals!
Mark: To answer a question with a question: What use is a skateboard?
Dave: Skateboards have wheels.
Mark: My wheel has a wheel!

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