What determines the size of kernel virtual space?
  Is it the start location of the kernel to end of memory, or
is it VM_MIN_KERNEL_ADDRESS in vmparam.h?
  It appears that I need to adjust VM_MAXUSER_ADDRESS and VM_MAX_ADDRESS
as well.... perhaps this parameter in comments, KPTDI is the key?

  Secondly, if I just want to mmap a 2^28 area into a user process, and
I don't care if the kernel can access it, then I suspect that I don't really
need to expand the size of kernel VM. 

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