Xen users - Mar 2006 - 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 ?

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> Will it ever be possible to run a 64-bit guest kernel under a 32-bit
> (PAE) Xen and domain0 ?
It seems pretty unlikely.  The guest kernel would be expecting 64-bit virtual 
addressing, 64-bit registers, etc, which just wouldn''t be available on
a CPU
which wasn''t in (or didn''t have) 64-bit mode.

Supporting non-PAE 32bit kernels on a PAE hypervisor, and maybe supporting 
32-bit kernels on 64-bit Xen are rather more likely.  i.e. being able to run 
a guest on a hypervisor that only uses *more* features of the hardware.

The first effort in this direction may be (something like) supporting non-PAE 
guests on a PAE hypervisor.

Cheers,
Mark

-- 
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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Mark Williamson wrote:
> .. and maybe supporting  32-bit kernels on 64-bit Xen are rather more
likely. ..
I haven''t been working/testing under 64-bit mode at all, but this is
not
yet possible? I thought it was a feature discussion brought about it 
months ago.

Thanks,
Matt

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> > .. and maybe supporting  32-bit kernels on 64-bit Xen are rather more
> > likely. ..
>
> I haven''t been working/testing under 64-bit mode at all, but this
is not
> yet possible? I thought it was a feature discussion brought about it
> months ago.
The interface for 64-bit Xen is rather different, because it''s tailored
to the
characteristics of the x86_64 architecture (for instance, the fact that
it''s
not possible to use segmentation for protection in the same way complicates 
the task of protecting Xen from kernels and apps...).

The most promising way of dealing with this is probably to implement a
"shim"
that will adapt hypercalls by the 32-bit kernel to work correctly with a 
64-bit Xen.  The domain builder would be able to insert this shim when 
building a legacy 32-bit domain to enable it to run correctly on Xen64.  
There''ll likely also need to be some Xen support.

It certainly seems doable, but there''s not yet been enough motivation
to
enable mixed platforms for paravirtualised guests.  If you really need 32-bit 
(or PAE) guests on 64-bit, it should be possible to run them unmodified in a 
VT-x / SVM virtual machine.

HTH,
Mark

-- 
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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> -----Original Message-----
> From: xen-users-bounces@lists.xensource.com 
> [mailto:xen-users-bounces@lists.xensource.com] On Behalf Of Matt Ayres
> Sent: 02 March 2006 22:36
> To: xen-users@lists.xensource.com
> 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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Petersson, Mats wrote:
>> Hi,
>>
>> Will it ever be possible to run a 64-bit guest kernel under a 32-bit
>> (PAE) Xen and domain0 ?
I mistyped this actually.  My brain is laughing at itself right now. :)

The question was supposed to be: Will it be possible to run 32-bit guest 
(incl PAE) under a 64-bit Xen and dom0 ?

This was already addressed by Mark though.

My reason for asking is this will allow for a transition to 64-bit for 
users.

Thanks,
Matt

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> -----Original Message-----
> From: xen-users-bounces@lists.xensource.com 
> [mailto:xen-users-bounces@lists.xensource.com] On Behalf Of Matt Ayres
> Sent: 03 March 2006 14:25
> To: Petersson, Mats
> Cc: mark.williamson@cl.cam.ac.uk; xen-users@lists.xensource.com
> Subject: Re: [Xen-users] 64-bit guest in 32-bit Xen/dom0 - ever?
> 
> 
> 
> Petersson, Mats wrote:
> 
> >> Hi,
> >>
> >> Will it ever be possible to run a 64-bit guest kernel 
> under a 32-bit
> >> (PAE) Xen and domain0 ?
> 
> I mistyped this actually.  My brain is laughing at itself 
> right now. :)
> 
> The question was supposed to be: Will it be possible to run 
> 32-bit guest (incl PAE) under a 64-bit Xen and dom0 ?
> 
> This was already addressed by Mark though.
> 
> My reason for asking is this will allow for a transition to 
> 64-bit for users.
Yes, but so does the native (non-Xen) 64-bit solutions - you can run
most 32-bit apps under 64-bit Linux (or Windows). The few that don''t
work are special cases, and almost always software run by root only [or
during system setup], so not really a big concern for users as such.

--
Mats
> 
> Thanks,
> Matt
> 
> _______________________________________________
> Xen-users mailing list
> Xen-users@lists.xensource.com
> http://lists.xensource.com/xen-users
> 
> 

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> The question was supposed to be: Will it be possible to run 32-bit guest
> (incl PAE) under a 64-bit Xen and dom0 ?
I did wonder ;-)
> This was already addressed by Mark though.
>
> My reason for asking is this will allow for a transition to 64-bit for
> users.
I guess if you really want *the same* environment for 32-bit users, i.e. using 
a XenLinux kernel, you could run a 32-bit Xen under VT under 64-bit Xen.  The 
performance implications of this are a bit painful though.  Better to either:

* migrate to a 64-bit kernel and userland if possible (best)
* use 32-on-64 support for Xen when & if it becomes available
* migrate to a 32-bit native i386 kernel, running in VT

You''ll have a migration path, but it''s likely to have some
fiddling involved.

Cheers,
Mark

-- 
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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