If we do not add Smip & Ssip extensions - we use standard mret/sret instructions in both interrupt and exception handlers.

However, Smnip & Ssnip extensions add new behavior to mret/sret instructions - they update xpistatus.pithreshold from xithreshold on mret/sret execution.

So, if we get exception trap during interrupt handler execution - we'll return with modified xithreshold and break interrupt nesting logic.