Question regarding x86_64 __PHYSICAL_MASK_SHIFT

[Tejun Heo]

 Hello, Andi.

 In include/asm-x86_64/page.h, __VIRTUAL_MASK_SHIFT is defined as 48 
bits which is the size of virtual address space on current x86_64 
machines as used as such.  OTOH, __PHYSICAL_MASK_SHIFT is defined as 46 
and used as mask shift for physical page address (i.e. physaddr >> 12).

 In addition to being a bit confusing due to similar names but 
different meanings, this means that we assume processors can physically 
address 58 (46 + 12) bits, but both amd64 and IA-32e manuals say that 
current architectural limit is 52 bits and bits 52-62 are reserved in 
all page table entries.  This currently (and in foreseeable future) 
doesn't cause any problem but it's still a bit weird.

 Am I missing something?

 Thanks.

--
tejun
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