RE: [Xen-users] dom0 32 bits and domU 64 bits

["Petersson, Mats" <Mats.Petersson@xxxxxxx>]

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From: xen-users-bounces@xxxxxxxxxxxxxxxxxxx [mailto:xen-users-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of lflesch.domicile@xxxxxxxxxxx
Sent: 24 October 2006 14:10
To: xen-users
Subject: [Xen-users] dom0 32 bits and domU 64 bits

Hi all,

 

I'm actually trying to make a new xen installation on a 64 bits AMD server.

My purpose is to install several HVM servers in 32 and 64 bits.

 

My question : if I use a 32 bits xen distribution for my xen installation, like eisxen, will it possible to install xen domUs in 64 bits ? 

 

 

No, definitely not. In HVM (full virtualization with hardware support like AMD-V and Intel VT) you can run "smaller on bigger", and there is work going on to allow that in Para-virtual (i.e. non-HVM) domains too - but don't expect that to turn up for another several weeks, I'd say.

 

The main reason for this is that the Xen hypervisor needs to be able to access all the CPU registers of the guest - since there's no guarantee that data needed by Xen is in the "lower 8" registers that are available in 32-bit mode. On top of that, you have the problem that 64-bit uses 4-level page-tables, which needs to be understood (and accessed) by the Xen hypervisor. Although this is technically possible in 32-bit mode, the code isn't written for this mode of operation (although from a programming standpoint, it would be easier to solve this than the register access, which technically a more challenging scenario).

 

--

Mats

 

 

Thanks a lot for you explanations,

Laurent

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