RE: [Xen-devel] x86-64's __PAGE_OFFSET

["Nakajima, Jun" <jun.nakajima@xxxxxxxxx>]

Jan Beulich wrote:
>>>> Keir Fraser <Keir.Fraser@xxxxxxxxxxxx> 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

__PAGE_OFFSET for the native 0xffff810000000000UL has a conflict with
Xen's address map (xen/include/asm-x86/config.h):

 *  0xffff810000000000 - 0xffff817fffffffff [512GB, 2^39 bytes,
PML4:258]
 *    Guest linear page table.
 *  0xffff818000000000 - 0xffff81ffffffffff [512GB, 2^39 bytes,
PML4:259]
 *    Shadow linear page table.
 *  0xffff820000000000 - 0xffff827fffffffff [512GB, 2^39 bytes,
PML4:260]
 *    Per-domain mappings (e.g., GDT, LDT).
 *  0xffff828000000000 - 0xffff8283ffffffff [16GB,  2^34 bytes,
PML4:261]
 *    Machine-to-phys translation table.
 *  0xffff828400000000 - 0xffff8287ffffffff [16GB,  2^34 bytes,
PML4:261]
 *    Page-frame information array.
 *  0xffff828800000000 - 0xffff828bffffffff [16GB,  2^34 bytes,
PML4:261]
 *    ioremap()/fixmap area.
 *  0xffff828c00000000 - 0xffff82ffffffffff [464GB,
PML4:261]
 *    Reserved for future use.
 *  0xffff830000000000 - 0xffff83ffffffffff [1TB,   2^40 bytes,
PML4:262-263]
 *    1:1 direct mapping of all physical memory. Xen and its heap live
here.
 *  0xffff840000000000 - 0xffff87ffffffffff [4TB,   2^42 bytes,
PML4:264-271]
 *    Reserved for future use.
 *  0xffff880000000000 - 0xffffffffffffffff [120TB, PML4:272-511]
 *    Guest-defined use.

So 0xffff880000000000UL was chosen for __PAGE_OFFSET in XenLinux.

Jun
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Intel Open Source Technology Center

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