Re: Question regarding x86_64 __PHYSICAL_MASK_SHIFT

[Andi Kleen]

On Tuesday 04 October 2005 20:52, Tejun Heo wrote:
>  Hello, Andi.
>
> On Tue, Oct 04, 2005 at 07:24:56PM +0200, Andi Kleen wrote:
> > You're right - PHYSICAL_MASK shouldn't be applied to PFNs, only to full
> > addresses. Fixed with appended patch.
> >
> > The 46bits limit is because half of the 48bit virtual space
> > is used for user space and the other 47 bit half is divided into
> > direct mapping and other mappings (ioremap, vmalloc etc.). All
> > physical memory has to fit into the direct mapping, so you
> > end with a 46 bit limit.
>
>  __PHYSICAL_MASK is only used to mask out non-pfn bits from page table
> entries.  I don't really see how it's related to virtual space
> construction.

Any other bits are not needed and should be pte_bad()ed.

Ok there could be IO mappings beyond 46bits in theory, but I will worry about
these when they happen. For now it's better to error out to easier detect
memory corruptions in page tables (some x86-64 CPUs tend to machine
check when presented with unmapped physical addresses, which 
is nasty to track down) 

>
> > See also Documentation/x86-64/mm.txt
>
>  Thanks.  :-)
>
>  I think PHYSICAL_PAGE_MASK and PTE_FILE_MAX_BITS should also be
> updated.  How about the following patch?  Compile & boot tested.

No, I think the existing code with my patch is fine.

-Andi
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