Exception Model
Exception Level: EL0 (Application), EL1(Rich OS), EL2(Hypervisor), EL3(Firmware/Secure Monitor)
不同Level的权限不同，主要是两类权限：
Memory Privilege（访问内存）: 通过MMU管控。EL0是非特权模式，EL1/2/3要做权限检查。
Register Access(访问处理器资源)
exception types
synchronous exceptions: caused by, or related to, the instruction that has just been executed
asynchronous exceptions: generated externally, not synchronous with the current instruction stream
Physical interrupts: SError(system error), IRQ, FIQ
Virtual interrupts: vSError, vIRQ, vFIQ
不同的level收到不同的type的exception，根据对应的访问控制策略处理