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FreeRTOS A53 port : ELx stack pointers in save and restore context

Posted by mathieu-42 on November 29, 2016

Hello,

I am working with A53 port of FreeRTOS and I do not understand something in portASM.S file.

My program is running in EL3.

Why when the context is saved we switch stack pointer to EL0 stack pointer?

Then at the end of "portSAVE_CONTEXT" macro, we switch back to EL3 stack pointer.

~~~ .macro portSAVE_CONTEXT /* Switch to use the EL0 stack pointer. */ MSR SPSEL, #0

...

/* Switch to use the ELx stack pointer. */
MSR 	SPSEL, #1

.endm

~~~ Same thing for .macro portRESTORE_CONTEXT.

If program is in EL3, why we do not use EL3 stack pointer? Are there some security issues when we switch to EL0 stack pointer from EL3?

Best regards,

Mathieu


FreeRTOS A53 port : ELx stack pointers in save and restore context

Posted by rtel on November 29, 2016

I will have to consult the hardware reference manual to recall....get back to you.


FreeRTOS A53 port : ELx stack pointers in save and restore context

Posted by mathieu-42 on November 30, 2016

Thanks.


FreeRTOS A53 port : ELx stack pointers in save and restore context

Posted by rtel on December 11, 2016

Looking at the code it looks like the EL0 stack pointer is used so stack relative pushes/pops can be performed without corrupting the stack pointer used by the interrupt handler itself.


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