illumos gate

every(?) other system puts the user limit below the amd64 va hole, effectively meaning that that the memory hole forms the boundary between user and kernel mappings. This is taken for granted by some things that used tagged pointers, notably firefox.
It would make life markedly easier if we were to do the same.

We did this already when running under Xen, and it can be trivially done in /etc/system via

set _userlimit=0x7fffc0000000

The possible downside of this is that because our userlimit is above the va hole, we get the beneficial effect that the stack and shared library mappings tend to be above the va hole, and the heap below it, meaning that they cannot under any circumstances meet. We likely wish to reproduce this with a guard page (if we don't already).