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RE: [Xen-devel] Xen 3.0 x86 + PAE with > 16 GB RAM?

To: "Derrik Pates" <demon@xxxxxxxxxxxxx>
Subject: RE: [Xen-devel] Xen 3.0 x86 + PAE with > 16 GB RAM?
From: "Petersson, Mats" <mats.petersson@xxxxxxx>
Date: Mon, 9 Jan 2006 14:40:39 +0100
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Thread-topic: [Xen-devel] Xen 3.0 x86 + PAE with > 16 GB RAM?

> -----Original Message-----
> From: Derrik Pates [mailto:demon@xxxxxxxxxxxxx] 
> Sent: 09 January 2006 13:24
> To: Petersson, Mats
> Cc: Keir Fraser; xen-devel@xxxxxxxxxxxxxxxxxxx
> Subject: Re: [Xen-devel] Xen 3.0 x86 + PAE with > 16 GB RAM?
> Petersson, Mats wrote:
> > If I understand things right, the V40z is an AMD Opteron 
> based system, 
> > which means that you could run x86-64.
> This is correct, yes.
> >                                        Is there any 
> particular reason 
> > you don't want to do this? This would give you 63TB (or is 
> that 63-7 =
> > 56TB?) or addressable memory, with full 32-bit compatibility for 
> > user-mode code.  You'd obviously have to run the Guest Kernels in 
> > 64-bit mode as well...
> I'm considering this as well, but it's not quite so simple 
> when running a 32-bit guest OS - 64-bit guests are obviously 
> a non-issue, but 32-bit OSes need several userspace tools 
> replaced with equivalents that understand certain internal 
> structures of the 64-bit kernel. I haven't decided yet if the 
> necessary hacking will be worthwhile to modify the userspace 
> tools for several OSes, or tweak the hypervisor (and 
> apparently the guest memory layouts as well, based on Keir's 
> statements) and use that instead.

Hmm, so these are some custom tools? Or something that normally is
distributed with the OS distribution you're planning to use? All
software distributed with the OS will, of course, be compiled to fit the
kernel - whether it's in 32-bit or 64-bit. Also, any tools not included
in the distribution would be perfectly fine to run in 32-bit mode [with
the relevant 32-bit restrictions, but that's the same if you're running
PAE], as all OS interfaces that are available to the 32-bit app within a
32-bit OS are also made available, through a thin translation layer, in
the 64-bit OS. As far as I'm aware, running 32-bit apps/tools will work
exactly the same in 64-bit OS as it does in 32-bit OS. I'm sure there
are some, rare, special cases where this is not true, but I'm not aware
of any at this time.


> > I'm not aware of any reason why this shouldn't be a 
> "better" solution?
> I'll agree it should be a higher-performance solution, at 
> least, but I have to make sure it all works right, as well... 
> :) The fact is the hardware in question hasn't even arrived 
> yet, so it's something I still have to look into.
> --
> Derrik Pates
> demon@xxxxxxxxxxxxx

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