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Saturday, December 7, 2019

On ARM64 processors, what exception level does the kernel run in?

On ARM64 processors, what exception level does the kernel run in?

  • EL3
  • EL2
  • EL0
  • EL1


Privilege and Exception levels

Before we explain the details of the Armv8-A exception model, let's start by introducing the concept of privilege. Modern software expects to be split into different modules, each with a different level of access to system and processor resources. An example of this is the split between the operating system kernel, which has a high level of access to system resources, and user applications, which have a more limited ability to configure the system.

Armv8-A enables this split by implementing different levels of privilege. The current level of privilege can only change when the processor takes or returns from an exception. Therefore, these privilege levels are referred to as Exception levels in the Armv8-A architecture. Each Exception level is numbered, and the higher levels of privilege have higher numbers.
As shown in the following diagram, the Exception levels are referred to as EL<x>, with x as a number between 0 and 3. For example, the lowest level of privilege is referred to as EL0.

A common usage model has application code running at EL0, with an operating system running at EL1. EL2 is used by a hypervisor, with EL3 being reserved by low-level firmware and security code.


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