ARMv8-M: internal low-level (TrustZone) API & implementation for configuring IRQ target

[ioannisg]

This issue aims to define an internal API and implement it, to address #6376 for ARMv8-M (with Security Extension).

In irq_manage.c we implement the following ARM Cortex-M-specific functions:

1. void irq_target_state_set(unsigned int irq, int secure_state);

2. int irq_target_state_is_nonsecure(unsigned int irq);

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Function irq_target_state_set(unsigned int irq, int secure_state) sets the security state (Secure or Non-Secure) targeted by the given irq.

irq_target_state_set(unsigned int irq, int nonsecure_state):

• requires ARMv8-M
• is only compiled if ARM_SECURE_FIRMWARE is defined
• shall only be called if we are in Secure state, otherwise, a write attempt to NVIC.ITNS[x] register is WRITE-IGNORED (WI), because the ITNS register is not banked between security states
• shall set the NVIC.ITNS[x].irq bit accordingly, depending on the secure_state
• shall _ASSERT if the operation is not performed successfully

Function irq_target_state_is_secure(unsigned int irq) returns the security state targeted by the given irq.

irq_target_state_is_nonsecure(unsigned int irq)

• requires ARMv8-M (mainline or baseline)
• is only compiled if ARM_SECURE_FIRMWARE is defined
• shall only be called if we are in Secure state, otherwise, a read attempt to NVIC.ITNS[x] register is READ-AS-ZERO, because the ITNS register is not banked between security states.
• shall get the NVIC.ITNS[x].irq bit and return the security state it corresponds to