CLIC supports mintstatus.MIL (RO) and mcause.MPIL (RW) for the current
interrupt level and the previous interrut level before a trap. Each ISR
must execute MRET to set mcause.MPIL back to mintstatus.MIL.
This commit introduces CONFIG_CLIC_SUPPORT_INTERRUPT_LEVEL to handle
mcause.MPIL for interrupt preemption in nested ISR, and uses
CONFIG_RISCV_ALWAYS_SWITCH_THROUGH_ECALL to ensure ISR always switch out
with MRET.
e.g.
With CONFIG_RISCV_ALWAYS_SWITCH_THROUGH_ECALL=n, a context-switch in
ISR may skip MRET in this flow:
IRQ -> _isr_wrapper -> z_riscv_switch() -> retrun to arch_switch()
Signed-off-by: Jimmy Zheng <jimmyzhe@andestech.com>
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