arch: arc: use a separate stack for exception handling

* use a separate stack for exception handling, this
will gurantee the exception handling always work, not
affected by some speical cases, e.g., stack check exception
or mem. protection exception at the exception entry.

* this commit can fix the second case of #8092

* note: the thread switch is still possible in exception
handling, e.g, caused by thread_abort. But the switched out
thread cannot be recovered, as the thread context is not
setup.

Signed-off-by: Wayne Ren <[email protected]>

Author: Wayne Ren

arch/arc/core/fault_s.S

@@ -36,6 +36,15 @@ GTEXT(__ev_maligned)
 
 GDATA(exc_nest_count)
 
+	.balign 4
+SECTION_VAR(BSS, saved_value)
+	.word 0
+
+#define EXCEPTION_STACK_SIZE 256
+
+SECTION_VAR(NOINIT, _exception_stack)
+	.space EXCEPTION_STACK_SIZE
+
 /*
  * @brief Fault handler installed in the fault and reserved vectors
  */
@@ -53,21 +62,18 @@ SECTION_SUBSEC_FUNC(TEXT,__fault,__ev_div_zero)
 SECTION_SUBSEC_FUNC(TEXT,__fault,__ev_dc_error)
 SECTION_SUBSEC_FUNC(TEXT,__fault,__ev_maligned)
 
-	/*
-	 * Before invoking exception handler, the kernel switches to an exception
-	 * stack, to save the faulting thread's registers.
-	 * The exception is fatal and all the kernel can do is just print
-	 * a diagnostic message and halt.
-	 */
 
 #ifdef CONFIG_ARC_STACK_CHECKING
-	push_s r2
+	st r0, [saved_value]
 	/* disable stack checking */
-	lr r2, [_ARC_V2_STATUS32]
-	bclr r2, r2, _ARC_V2_STATUS32_SC_BIT
-	kflag r2
-	pop_s r2
+	lr r0, [_ARC_V2_STATUS32]
+	bclr r0, r0, _ARC_V2_STATUS32_SC_BIT
+	kflag r0
+	ld r0, [saved_value]
 #endif
+	st sp, [saved_value]
+	mov_s sp, _exception_stack
+	add sp, sp, EXCEPTION_STACK_SIZE
 
 	/* save caller saved registers */
 	_create_irq_stack_frame
@@ -81,30 +87,11 @@ SECTION_SUBSEC_FUNC(TEXT,__fault,__ev_maligned)
 	lr r0,[_ARC_V2_ERET]
 	st_s r0, [sp, ___isf_t_pc_OFFSET] /* eret into pc */
 
-	ld r1, [exc_nest_count]
-	add r0, r1, 1
-	st r0, [exc_nest_count]
-	cmp r1, 0
-
-	bgt.d  exc_nest_handle
 	mov r0, sp
 
-	mov r1, _kernel
-	ld sp, [r1, _kernel_offset_to_irq_stack]
-exc_nest_handle:
-	push_s r0
-
 	jl _Fault
 
 _exc_return:
-	pop sp
-
-	mov	r1, exc_nest_count
-	ld	r0, [r1]
-	sub	r0, r0, 1
-	cmp	r0, 0
-	bne.d	_exc_return_from_exc
-	st 	r0, [r1]
 
 #ifdef CONFIG_PREEMPT_ENABLED
 	mov_s r1, _kernel
@@ -114,11 +101,6 @@ _exc_return:
 	ld_s r0, [r1, _kernel_offset_to_ready_q_cache]
 	breq r0, r2, _exc_return_from_exc
 
-	_save_callee_saved_regs
-
-	st _CAUSE_RIRQ, [r2, _thread_offset_to_relinquish_cause]
-	/* note: Ok to use _CAUSE_RIRQ since everything is saved */
-
 	ld_s r2, [r1, _kernel_offset_to_ready_q_cache]
 	st_s r2, [r1, _kernel_offset_to_current]
 
@@ -152,6 +134,7 @@ _exc_return:
 
 _exc_return_from_exc:
 	_pop_irq_stack_frame
+	ld sp, [saved_value]
 	rtie
 
 
@@ -204,13 +187,16 @@ valid_syscall_id:
 _do_other_trap:
 #endif /* CONFIG_USERSPACE */
 #ifdef CONFIG_ARC_STACK_CHECKING
-	push_s r2
+	st r0, [saved_value]
 	/* disable stack checking */
-	lr r2, [_ARC_V2_STATUS32]
-	bclr r2, r2, _ARC_V2_STATUS32_SC_BIT
-	kflag r2
-	pop_s r2
+	lr r0, [_ARC_V2_STATUS32]
+	bclr r0, r0, _ARC_V2_STATUS32_SC_BIT
+	kflag r0
+	ld r0, [saved_value]
 #endif
+	st sp, [saved_value]
+	mov_s sp, _exception_stack
+	add sp, sp, EXCEPTION_STACK_SIZE
 
 	/* save caller saved registers */
 	_create_irq_stack_frame
@@ -224,19 +210,8 @@ _do_other_trap:
 	lr r0,[_ARC_V2_ERET]
 	st_s r0, [sp, ___isf_t_pc_OFFSET] /* eret into pc */
 
-	ld r1, [exc_nest_count]
-	add r0, r1, 1
-	st r0, [exc_nest_count]
-	cmp r1, 0
-
-	bgt.d  trap_nest_handle
 	mov r0, sp
 
-	mov r1, _kernel
-	ld sp, [r1, _kernel_offset_to_irq_stack]
-trap_nest_handle:
-	push_s r0
-
 	mov blink,  _exc_return
 
 	cmp ilink, _TRAP_S_CALL_RUNTIME_EXCEPT