power failure interrupt on NIOS 2

By default, the software looks for the highest priority interrupt which is pending and handles that. All interrupts are disabled while it is searching for that interrupt.

A low priority interrupt can choose to enable higher priority interrupts while it is handled (using the function alt_irq_interruptible), but this is only of benefit to slow interrupt handlers (those which take more than 50 cycles or so).

If you call alt_irq_interruptable from a fast interrupt handler (as interrupt handlers should be) then this will result in a longer maximum interrupt latency.