amd64: change VM_KMEM_SIZE_SCALE to 1?

[Kostik Belousov]

On Thu, Sep 09, 2010 at 11:03:48AM +0300, Andriy Gapon wrote:
> on 26/07/2010 22:29 Peter Wemm said the following:
> > 
> > That hard limit of 512G of physical ram doesn't seem so distant anymore..
> 
> I was reviewing this thread and got curious about this statement.
> What is this limit?  Where does it come from?
I do not think that such limit is enforced by hardware. In the recent Intel
x86 sw architecture specification, there is a statement (vol 3a, 4.1.4):

CPUID.80000008H:EAX[7:0] reports the physical-address width supported by
the processor. (For processors that do not support CPUID function 80000008H,
the width is generally 36 if CPUID.01H:EDX.PAE [bit 6] = 1 and 32 otherwise.)
This width is referred to as MAXPHYADDR. MAXPHYADDR is at most 52.

I suspect that "512GB" might refer to the current direct map sizing in
the amd64 pmap module. The direct map is provided by single entry in
pml4 table, that allows to map 512GB.
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