RE: [Xen-users] 64-Bit Xen?

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

> -----Original Message-----
> From: xen-users-bounces@xxxxxxxxxxxxxxxxxxx 
> [mailto:xen-users-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of 
> Donofrio, Lewis
> Sent: 15 May 2007 13:33
> To: xen-users@xxxxxxxxxxxxxxxxxxx
> Subject: [Xen-users] 64-Bit Xen?
> 
> Question,
> 
> If you have a 64-Bit host running Xen will the guest's be 64-bit as
> well?

Depends on several things. But first, lets just quickly recap the modes
that exist biggest to smallest:
64-bit - supports HUGE memory addresses (physical memory up to 40 bits
(1024 GB = 1TB), virtual memory to 48 bits (256 TB), with future
extension up to, I think, 53 bits if necessary).

32-bit PAE - supports 32-bit (4GB) virtual address, but up to 36 bits
(64GB) of physical memory [although Xen limits this to 34 bits/16GB]. 

32-bit non-PAE - 32-bit virtual, 32-bit physical address. 

So now to what's supported:
HVM (Fully virtualized guest) can be anything up to the same as the
hypervisor, so a 64-bit hypervisor supports all of the above modes,
whilst a 32-bit PAE can't use 64-bit, and a 32-bit non-PAE can't do
anything but 32-bit non-PAE guests. 

Para-virtual guest have traditionally had the requitement of being
identical to the hypervisor. With the upcomming release of 3.1.0 of Xen,
this will be reduced somewhat by allowing 32-bit PAE guests on top of
64-bit hypervisor. With this version, it's still not possible to run
32-bit non-PAE on a 64-bit hypervisor. 

--
Mats

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