J-Link ARMv8-AR

This article describes the debugging specifics for ARMv8-A/R based chips & cores.

Debugging through execution state changes

The ARMv8-A/R architecture describes 2 execution states:

• AArch64
• AArch32

An execution state change can only happen when changing the exception level (EL0-EL3).
While J-Link supports debugging through code that changes between AArch32 and AArch64, most debuggers / IDEs do not. For example with GDB it is not possible to debug an application that contains AArch32 and AArch64 code because the register set as well as the instruction set changes with the execution state, which is problematic.

EL2 registers

If exception level 2 (EL2) is implemented (this is an optional exception level), there are some registers that are specific to EL2.
If EL2 runs in AArch32 execution state:

• ELR_hyp
• HSR
• SPSR_hyp

If EL2 runs in AArch64 execution state:

• ELR_EL2
• ESR_EL2
• SPSR_EL2

The above mentioned registers are mapped to each other (so they only exist once in hardware).

Note:

• The EL2 registers are only accessible if the core is in EL2 or higher. For example if the core is halted in EL1, they cannot be written / read by J-Link.
  If J-Link would temporarily change to EL2 to read them, the EL switch would cause the register values to become invalid / unknown (by ARMv8-A/R architecture definition).
  
• If a debugger requests these registers while the core is below EL2 mode, J-Link will return all 0s for them.

Vector catch support

For ARMv8-A/R, vector catch does not generate a halt request to the core (entry to Debug state)
but instead generates a debug exception that jumps to the debug exception handler.
Vector catch is designed for self-hosted debug (e.g. debugging an application under a Linux OS running on the chip and without an external debug probe).
Therefore, vector catch is not available when debugging with J-Link.

Device Support

J-Link has general support for the ARMv8-A/R architecture. This means Cortex-A53, A55, A72, ... cores are supported.

However, here is the pitfall: For most v8-A/R based devices you need special connect, reset, ... sequences so just because the core is supported does not necessarily mean that the device is working out-of-the box. You may need to create a J-Link script file as long as we do not list a particular device as being supported: https://www.segger.com/supported-devices/jlink/

This comes as no surprise. Most vendors design these big devices with the goal to have a Linux running on it. The documentation about the debug interface is next to no-existent. Last but not least, as most people are running a Linux on such devices, they don‘t need a debug probe at all.