is there CPS instruction alternatives to jump from EL1 to EL0 at boot time?

Hello,

I would like to switch from EL1 to EL0 and update my PC in one instruction because I would like to prevent code execution in EL0 mode in my supervisor memory zone.

In order to do that I would like to use ERET instruction or to pop CPSR from stack at boot time instead of using CPS instruction.

I didn't find in the documentation a specific reason to not use ERET at boot time but when I am using it (or when i amp poping cpsr from stack "pop pc^") the behavior is erroneous.

1. Can someone explain that?
2. Is there a way  to update PC and CPSR in one instruction at boot time (without erroneous behavior)?

Thanks by advance,

Kevin