Actually you are using a 64Gb capable processor that is only capable of
sanely addressing 4Gb at a time, total across both user and kernel space
and takes a hefty hit whenever you switch which 4Gb you are peering at.
If you want to make sensible use of even 4Gb user/4Gb kernel you need to
take a page table reload at syscall time and deal with quite messy handling
for copy to/from user.
[If someone from Intel disagrees please do so publically - I'd love to have
someone prove the limit can be dealt with 8)]
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