33.10.1.2. ARM SP0 vs SPx
See ARMv8 architecture reference manual db D1.6.2 "The stack pointer registers".
There is one SP per exception level.
This can also be seen clearly on the analysis at gem5 ExecContext::readIntRegOperand
register resolution.
TODO create a minimal runnable example.
TODO: how to select to use SP0 in an exception handler?