Re: [Xen-merge] Re: x86-64's __PAGE_OFFSET

[Andi Kleen <ak@suse.de>]

On Friday 06 January 2006 18:11, Jan Beulich wrote:
> >>> Keir Fraser <Keir.Fraser@cl.cam.ac.uk> 06.01.06 15:10:13 >>>
> >
> >On 6 Jan 2006, at 13:50, Jan Beulich wrote:
> >
> >> What, if any, is the reason for x86-64 xenlinux to have a different
> >> __PAGE_OFFSET value than native? While merging, it would seem 
> >> reasonable
> >> to drop this difference if not motivated technically.
> >
> >Jun might remember if there was a particular reason, but I can't think
> 
> >of any insurmountable technical reason why the native __PAGE_OFFSET 
> >value shouldn't work.
> 
> There must be a reason - the system dies immediately with that change.
> But I'd really like to understand why this is so, perhaps to check with
> Andi whether then we couldn't generally go with the Xen value...

It would lose 7TB of the kernel's theoretical 63TB main memory 
support. While that exceeds the address space of all current x86-64 CPUs
(1TB max) given that there are already multi TB machines around it might 
not be a good idea.

What does Xen need that big hole for?

-Andi