Correct way to mask interrupts in secure world ARMv8M M33

Hello,

I'm wondering what the correct way to mask non secure interrupts is, on entering secure world on an ARMv8-M processor, with Main and Security extensions. The scenario I have is as follows:

The SOC has 1 M33 core. I have a non secure OS that has an RTOS and schedules threads etc.(RTX RTOS2). The core has 2 Systick timers implemented, 1 for each world. The RTOS configures the non secure timer with priority 0. The secure world sets up AIRCR.PRIS=1 during boot, which means, the NS view of the timer priority is 0, but the view from secure world(SHPR*_NS) is 0x80.

The secure side exposes a bunch of secure function calls, that are called from RTX/NS threads. Some of these functions are not thread safe, so I  would like to disable interrupts during execution of these functions, so that the NS timer does not fire and secure world execution is not preempted.

On ARMv8-A, on entry into secure world through SMC, the processor automatically masks interrupts. However, the SG instruction does no such masking. My questions are as follows:

1) Is there a way to make the core automatically boost the current execution priority to mask interrupts on entry into the secure world ? Or is there some banked register, whose state is retained and has the effect of masking interrupts only when executing in a particular world ?

2) I tried setting PRIMASK to 1 and it works, but it seems like I have to set PRIMASK back to 0 before leaving secure world. If not the execution priority does not drop back and the non secure interrupts never fire again. Is this expected behavior? I thought that the PRIMASK would only affect the secure world, since the PRIMASK register is banked between security states.

3) I tried writing the BASEPRI register but the write was ignored, even though the core was in privileged/handler mode. How would I use the BASEPRI register to address my scenario ?

It seems like the best approach to solve my issue would be to implement secure world mutex's that set PRIMASK to 1 on mutex acquire and PRIMASK to 0 on mutex release. Is this the correct approach ? it seems like using BASEPRI to mask non secure interrupts only would be a better approach, in case I have secure interrupts that need to be handled.

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