kernel Stack vs User space stack

We are using ArmV8 64 in our project. would like to know the answers for the below question ?

Question 1 : 

I couldn't figure out, what the above code does ? 

Question 2 :  When a user space app is interrupted, how the el1h_irq_handler is invoked () ?

Could you give a step by step info, or documented anywhere ?

Question 3 : Does ARM ABI plays a role in transition from user space to kernel space ?