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>>> 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...

Jan

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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

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>>> Keir Fraser <Keir.Fraser@cl.cam.ac.uk> 07.01.06 10:11:28 >>>
>
>On 7 Jan 2006, at 05:22, Nakajima, Jun wrote:
>
>> __PAGE_OFFSET for the native 0xffff810000000000UL has a conflict
with
>> Xen's address map (xen/include/asm-x86/config.h):
>
>Well, that is a good reason for the modified __PAGE_OFFSET. :-)
>
>I guess we stick with a Xen-specific value then.

To make this dependency more clear, I'd like to change Xen's definition
to use HYPERVISOR_VIRT_END then. While at present no assembly file (that
xenolinux uses) actually needs this value, I'd still like to provide for
using this value in assembly files. For that reason, the definition in
xen/include/public/arch-x86_64.h would need to be split into an
__ASSEMBLY__ and a C variant. However, while looking at that I found
that there are multiple definitions of HYPERVISOR_VIRT_{START,END}, and
I wonder whether these shouldn't be consolidated into a single one, or
if that is absolutely impossible, whether at least some compile-time
asserts should be added that make sure these don't get out of sync.

Jan

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On 9 Jan 2006, at 09:39, Jan Beulich wrote:

> To make this dependency more clear, I'd like to change Xen's definition
> to use HYPERVISOR_VIRT_END then. While at present no assembly file
> (that
> xenolinux uses) actually needs this value, I'd still like to provide
> for
> using this value in assembly files. For that reason, the definition in
> xen/include/public/arch-x86_64.h would need to be split into an
> __ASSEMBLY__ and a C variant. However, while looking at that I found
> that there are multiple definitions of HYPERVISOR_VIRT_{START,END}, and
> I wonder whether these shouldn't be consolidated into a single one, or
> if that is absolutely impossible, whether at least some compile-time
> asserts should be added that make sure these don't get out of sync.

Okay, I'll make the necessary changes to allow the definitions' use in
assembly files.

-- Keir


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