CODEWITHSUNDEEP
arm64
user3200696
user3200696

Reputation: 31

ARMv8 exception vector significance of EL0_SP

I am new to ARMv8 architecture and while reading the v8 exception vectors I am not able to understand significance of adding SP_EL0 level vectors while SP_ELx vector set exists. What I am trying to find a use case where this is useful. I understand that when the exception is taken at same level by default the stack of same exception level is used for example if EL2 (Hyp) mode is defined then if exception occurs while being at EL2 level the stack defined at EL2 will be used but not sure if I configure to use EL0 level stack under what use cases this could be useful ? Can some one give a use case and explain ?

Also while going through the spec it seems these two different stack usage idea is taken from Cortex-M architecture but i am not very clear with thread and handler modes as well. Can any one explain it ?

Just wanted to add here that ARMv7 doesn't have thread and handler concept so i am not clear requirement in ARMv8.

Thanks

Upvotes: 2

Views: 3028

Answers (1)

Variable Length Coder
Variable Length Coder

Reputation: 8116

SP_EL0 is the "normal" stack pointer. Your code that runs in any EL should be running on SP_EL0 whenever it can. SP_EL1/2/3 is the "exception" stack pointer for that mode. When an exception, fault, or interrupt happens, the processor switches to this stack (and possibly switches EL). Your exception handler code should do what it needs to save the exception state and get itself onto SP_EL0 to complete handling the exception.

That is, your EL0 user code, your EL1 kernel code, your EL2 hypervisor, and your EL3 monitor should all be using SP_EL0 except in the early stages of their exception handlers. If you were to take a nested exception while on SP_ELx (non-zero), you could blow over your stack, ELR, FAR, etc. which is why you want to save these and get back to SP_EL0 as soon as you can.

Upvotes: 2

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