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

To: "Keir Fraser" <Keir.Fraser@xxxxxxxxxxxx>, "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 12:34:11 +0100
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Thread-topic: [Xen-devel] Xen 3.0 x86 + PAE with > 16 GB RAM?
> -----Original Message-----
> From: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx 
> [mailto:xen-devel-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of 
> Keir Fraser
> Sent: 09 January 2006 10:30
> To: Derrik Pates
> Cc: xen-devel@xxxxxxxxxxxxxxxxxxx
> Subject: Re: [Xen-devel] Xen 3.0 x86 + PAE with > 16 GB RAM?
> On 6 Jan 2006, at 22:21, Derrik Pates wrote:
> > Can this safely be increased? From my reading of the code, 
> it appears 
> > that it is set as it is because each gigabyte of installed RAM 
> > requires a megabyte of RAM to be set aside for mapping tables and 
> > such. If I just increase the value of MACHPHYS_MBYTES to 32 (for 
> > example), should Xen boot and work happily, just consuming more RAM 
> > for its mapping tables?
> >
> > Please Cc: me with your reply, as I'm not subscribed to this list 
> > currently.
> The memory layout of Xen is hardcoded in asm-x86/config.h, 
> and also in public/arch-x86_32.h. If the addresses of certain 
> critical structures are modified appropriately then things 
> should work fine. Of course Linux will end up with less 
> kernel virtual address space and hence less lowmem.

If I understand things right, the V40z is an AMD Opteron based system,
which means that you could run x86-64. 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 not aware of any reason why this shouldn't be a "better" solution?

>   -- Keir
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