arch/risc-v: Set Supervisor User Memory (access) for idle process too

This has been a long issue for me as it results in random crashes when
asynchronous events occur when the idle process is active.

The problem is that the kernel cannot access user memory, because the CPU
status prevents it.

arch/risc-v/src/common/riscv_getnewintctx.c

@@ -70,3 +70,12 @@ uintptr_t riscv_get_newintctx(void)
 #endif
   );
 }
+
+void riscv_set_idleintctx(void)
+{
+  /* Set SUM for idle process if needed */
+
+#ifdef CONFIG_ARCH_USE_MMU
+  SET_CSR(CSR_STATUS, STATUS_SUM);
+#endif
+}

arch/risc-v/src/common/riscv_initialstate.c

@@ -78,6 +78,10 @@ void up_initial_state(struct tcb_s *tcb)
 
       riscv_stack_color(tcb->stack_alloc_ptr, 0);
 #endif /* CONFIG_STACK_COLORATION */
+
+      /* Set idle process' initial interrupt context */
+
+      riscv_set_idleintctx();
       return;
     }
 

arch/risc-v/src/common/riscv_internal.h

@@ -200,6 +200,7 @@ void riscv_copystate(uintptr_t *dest, uintptr_t *src);
 void riscv_sigdeliver(void);
 int riscv_swint(int irq, void *context, void *arg);
 uintptr_t riscv_get_newintctx(void);
+void riscv_set_idleintctx(void);
 void riscv_exception_attach(void);
 
 #ifdef CONFIG_ARCH_FPU