arch: riscv: handle interrupt level for CLIC

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_PREEMPTION 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>

arch/riscv/Kconfig

@@ -168,9 +168,18 @@ config RISCV_HAS_PLIC
 config RISCV_HAS_CLIC
 	bool
 	depends on RISCV_PRIVILEGED
+	select RISCV_ALWAYS_SWITCH_THROUGH_ECALL if MULTITHREADING
+	select CLIC_SUPPORT_INTERRUPT_PREEMPTION if !NRFX_CLIC
 	help
 	  Does the SOC provide support for a Core-Local Interrupt Controller (CLIC).
 
+config CLIC_SUPPORT_INTERRUPT_PREEMPTION
+	bool
+	depends on RISCV_HAS_CLIC
+	help
+	  This option handles interrupt level in ISR to support interrupt
+	  preemption for nested interrupt.
+
 config RISCV_SOC_EXCEPTION_FROM_IRQ
 	bool
 	help

arch/riscv/core/isr.S

@@ -188,6 +188,12 @@ SECTION_FUNC(exception.entry, _isr_wrapper)
 	sr s0, __struct_arch_esf_s0_OFFSET(sp)
 	get_current_cpu s0
 
+#ifdef CONFIG_CLIC_SUPPORT_INTERRUPT_PREEMPTION
+	/* Save mcause register */
+	csrr t0, mcause
+	sr t0, __struct_arch_esf_mcause_OFFSET(sp)
+#endif /* CONFIG_CLIC_SUPPORT_INTERRUPT_PREEMPTION */
+
 	/* Save MEPC register */
 	csrr t0, mepc
 	sr t0, __struct_arch_esf_mepc_OFFSET(sp)
@@ -721,6 +727,13 @@ fp_trap_exit:
 	/* Restore MEPC and MSTATUS registers */
 	lr t0, __struct_arch_esf_mepc_OFFSET(sp)
 	lr t2, __struct_arch_esf_mstatus_OFFSET(sp)
+
+#ifdef CONFIG_CLIC_SUPPORT_INTERRUPT_PREEMPTION
+	/* Restore MCAUSE register for previous interrupt level. */
+	lr t1, __struct_arch_esf_mcause_OFFSET(sp)
+	csrw mcause, t1
+#endif /* CONFIG_CLIC_SUPPORT_INTERRUPT_PREEMPTION */
+
 	csrw mepc, t0
 	csrw mstatus, t2
 

arch/riscv/core/offsets/offsets.c

@@ -112,6 +112,10 @@ GEN_OFFSET_STRUCT(arch_esf, a7);
 GEN_OFFSET_STRUCT(arch_esf, mepc);
 GEN_OFFSET_STRUCT(arch_esf, mstatus);
 
+#ifdef CONFIG_CLIC_SUPPORT_INTERRUPT_PREEMPTION
+GEN_OFFSET_STRUCT(arch_esf, mcause);
+#endif /* CONFIG_CLIC_SUPPORT_INTERRUPT_PREEMPTION */
+
 GEN_OFFSET_STRUCT(arch_esf, s0);
 
 #ifdef CONFIG_USERSPACE

arch/riscv/core/thread.c

@@ -106,6 +106,11 @@ void arch_new_thread(struct k_thread *thread, k_thread_stack_t *stack,
 	stack_init->soc_context = soc_esf_init;
 #endif
 
+#ifdef CONFIG_CLIC_SUPPORT_INTERRUPT_PREEMPTION
+	/* Clear the previous interrupt level. */
+	stack_init->mcause = 0;
+#endif
+
 	thread->callee_saved.sp = (unsigned long)stack_init;
 
 	/* where to go when returning from z_riscv_switch() */

include/zephyr/arch/riscv/exception.h

@@ -78,6 +78,10 @@ struct arch_esf {
 	unsigned long a7;		/* function argument */
 #endif /* !CONFIG_RISCV_ISA_RV32E */
 
+#ifdef CONFIG_CLIC_SUPPORT_INTERRUPT_PREEMPTION
+	unsigned long mcause;		/* machine cause register */
+#endif /* CONFIG_CLIC_SUPPORT_INTERRUPT_PREEMPTION */
+
 	unsigned long mepc;		/* machine exception program counter */
 	unsigned long mstatus;	/* machine status register */