RE: [Xen-users] 64-bit guest in 32-bit Xen/dom0 - ever?

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

> -----Original Message-----
> From: xen-users-bounces@xxxxxxxxxxxxxxxxxxx 
> [mailto:xen-users-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of Matt Ayres
> Sent: 02 March 2006 22:36
> To: xen-users@xxxxxxxxxxxxxxxxxxx
> Subject: [Xen-users] 64-bit guest in 32-bit Xen/dom0 - ever?
> 
> Hi,
> 
> Will it ever be possible to run a 64-bit guest kernel under a 32-bit
> (PAE) Xen and domain0 ?

We have had discussions about this (for fully virtualized solutions),
and the end question always comes to "Why would you want that?". We
couldn't come up with any reasonable reason why someone would want to
use a 32-bit Xen with 32-Bit dom0 and run a 64-Bit domain on top of it. 

For para-virtualized it's literally impossible to achieve this, as a
call to the hypervisor would need to be 64-bit, which a 32-bit Xen can't
handle, and thus it fails immediately. It's perhaps possible to do this
in a fully-virtualized scenario, but in our discussions, we've come to
the conclusion that it's MUCH MORE work than it's got valuable meaning.
[And that is assuming that it's actually POSSIBLE to achieve this -
which it MAY be, but it's also likely that there will be some
"interesting" problems arising. One obvious example would be what to do
with the 8 extra registers that x86-64 has, which you can't access in
32-bit mode - and thus, they can't be saved. How do you switch from Dom1
to Dom2 if you can't save all the registers for Dom1 before entering
Dom2?]

One important reason that we come to this conclusion is that the 64-bit
Linux (and 64-bit Windows) supports 99.9% of 32-bit apps directly in the
64-bit kernel, so there are VERY few apps that wouldn't run directly.
The remaining 0.1% of applications that aren't directly supported would
be ones that have direct interfaces with the kernel in such a way that
the difference between 64-bit kernel and 32-bit kernel makes a
difference to the application itself. I think "iptables" is one of those
in linux. 

Also, in fully virtualized world, you can run 32-bit kernels on top of
64-bit Xen, so only Domain0 needs to be 64-bit, all guests can be any
combination of 32-bit, PAE or 64-bit without any problem. 

If you actually think there is a GOOD, preferrably commercial, reason to
do this, please say so. But the reason that "It's an interesting idea"
or "Would be fun" aren't good ones. ;-)

--
Mats




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