core: riscv: Remove thread_exit_user_mode()

Currently, the user mode abort and some system calls return to kernel
mode by thread_exit_user_mode(). It leads to some problems, such as
CFG_CORE_DEBUG_CHECK_STACKS=y, since thread_exit_user_mode() does not
update the core local flags which indicate current type of stack.

Fix it by removing thread_exit_user_mode(). So that the program flow to
return to kernel mode and user mode are both from common trap handler.

Signed-off-by: Alvin Chang <alvinga@andestech.com>

core/arch/riscv/include/kernel/thread_private_arch.h

@@ -126,10 +126,6 @@ static inline void thread_rpc(uint32_t rv[THREAD_RPC_NUM_ARGS])
 }
 
 void thread_scall_handler(struct thread_scall_regs *regs);
-void thread_exit_user_mode(unsigned long a0, unsigned long a1,
-			   unsigned long a2, unsigned long a3,
-			   unsigned long sp, unsigned long pc,
-			   unsigned long status);
 
 #endif /*__ASSEMBLER__*/
 #endif /*__KERNEL_THREAD_PRIVATE_ARCH_H*/

core/arch/riscv/kernel/abort.c

@@ -236,13 +236,10 @@ static void handle_user_mode_panic(struct abort_info *ai)
 	ai->regs->a0 = TEE_ERROR_TARGET_DEAD;
 	ai->regs->a1 = true;
 	ai->regs->a2 = 0xdeadbeef;
-	ai->regs->ra = (vaddr_t)thread_unwind_user_mode;
+	ai->regs->epc = (vaddr_t)thread_unwind_user_mode;
 	ai->regs->sp = thread_get_saved_thread_sp();
 	ai->regs->status = xstatus_for_xret(true, PRV_S);
-
-	thread_exit_user_mode(ai->regs->a0, ai->regs->a1, ai->regs->a2,
-			      ai->regs->a3, ai->regs->sp, ai->regs->ra,
-			      ai->regs->status);
+	ai->regs->ie = 0;
 }
 
 #ifdef CFG_WITH_VFP

core/arch/riscv/kernel/thread_arch.c

@@ -98,8 +98,9 @@ static void thread_lazy_restore_ns_vfp(void)
 
 static void setup_unwind_user_mode(struct thread_scall_regs *regs)
 {
-	regs->ra = (uintptr_t)thread_unwind_user_mode;
+	regs->epc = (uintptr_t)thread_unwind_user_mode;
 	regs->status = xstatus_for_xret(true, PRV_S);
+	regs->ie = 0;
 	/*
 	 * We are going to exit user mode. The stack pointer must be set as the
 	 * original value it had before allocating space of scall "regs" and
@@ -135,11 +136,12 @@ void thread_scall_handler(struct thread_scall_regs *regs)
 
 	assert(sess && sess->handle_scall);
 
-	if (!sess->handle_scall(regs)) {
+	if (sess->handle_scall(regs)) {
+		/* We're about to switch back to user mode at ecall + 4 */
+		regs->epc += 4;
+	} else {
+		/* We're returning from __thread_enter_user_mode() */
 		setup_unwind_user_mode(regs);
-		thread_exit_user_mode(regs->a0, regs->a1, regs->a2,
-				      regs->a3, regs->sp, regs->ra,
-				      regs->status);
 	}
 }
 

core/arch/riscv/kernel/thread_rv.S

@@ -385,27 +385,35 @@ ecall_from_user:
 	addi	t0, sp, THREAD_SCALL_REGS_SIZE
 	store_xregs a0, THREAD_CTX_KERN_SP, REG_T0
 
-	/*
-	 * We are returning to U-Mode, on return, the program counter
-	 * is set to xsepc (pc=xepc), we add 4 (size of an instruction)
-	 * to continue to next instruction.
-	 */
+	/* Restore XEPC */
 	load_xregs sp, THREAD_SCALL_REG_EPC, REG_T0
-	addi	t0, t0, 4
 	csrw	CSR_XEPC, t0
-
 	/* Restore XIE */
 	load_xregs sp, THREAD_SCALL_REG_IE, REG_T0
 	csrw	CSR_XIE, t0
 	/* Restore XSTATUS */
 	load_xregs sp, THREAD_SCALL_REG_STATUS, REG_T0
 	csrw	CSR_XSTATUS, t0
-	/* Set scratch as thread_core_local */
+	/* Check previous privilege mode by status.SPP */
+	b_if_prev_priv_is_u t0, 1f
+	/*
+	 * We are going to XRET to kernel mode.
+	 * XSCRATCH is already zero to indicate that we are in kernel mode.
+	 * We must keep kernel gp & tp, so skip restoring user gp & tp.
+	 */
+	j	2f
+1:
+	/*
+	 * We are going to XRET to user mode.
+	 * XSCRATCH must be tp to be used in next trap.
+	 * We also need to restore user gp & tp
+	 */
 	csrw	CSR_XSCRATCH, tp
-	/* Restore caller-saved registers */
-	load_xregs sp, THREAD_SCALL_REG_RA, REG_RA
 	load_xregs sp, THREAD_SCALL_REG_GP, REG_GP
 	load_xregs sp, THREAD_SCALL_REG_TP, REG_TP
+2:
+	/* Restore remaining caller-saved registers */
+	load_xregs sp, THREAD_SCALL_REG_RA, REG_RA
 	load_xregs sp, THREAD_SCALL_REG_T0, REG_T0, REG_T2
 	load_xregs sp, THREAD_SCALL_REG_A0, REG_A0, REG_A7
 	load_xregs sp, THREAD_SCALL_REG_T3, REG_T3, REG_T6
@@ -499,18 +507,32 @@ abort_from_user:
 	/* Restore XSTATUS */
 	load_xregs sp, THREAD_ABT_REG_STATUS, REG_T0
 	csrw	CSR_XSTATUS, t0
-	/* Set scratch as thread_core_local */
-	csrw	CSR_XSCRATCH, tp
 
 	/* Update core local flags */
 	lw	a0, THREAD_CORE_LOCAL_FLAGS(tp)
 	srli	a0, a0, THREAD_CLF_SAVED_SHIFT
 	sw	a0, THREAD_CORE_LOCAL_FLAGS(tp)
 
-	/* Restore all GPRs */
-	load_xregs sp, THREAD_ABT_REG_RA, REG_RA
+	/* Check previous privilege mode by status.SPP */
+	b_if_prev_priv_is_u t0, 1f
+	/*
+	 * We are going to XRET to kernel mode.
+	 * XSCRATCH is already zero to indicate that we are in kernel mode.
+	 * We must keep kernel gp & tp, so skip restoring user gp & tp.
+	 */
+	j	2f
+1:
+	/*
+	 * We are going to XRET to user mode.
+	 * XSCRATCH must be tp to be used in next trap.
+	 * We also need to restore user gp & tp
+	 */
+	csrw	CSR_XSCRATCH, tp
 	load_xregs sp, THREAD_ABT_REG_GP, REG_GP
 	load_xregs sp, THREAD_ABT_REG_TP, REG_TP
+2:
+	/* Restore remaining GPRs */
+	load_xregs sp, THREAD_ABT_REG_RA, REG_RA
 	load_xregs sp, THREAD_ABT_REG_T0, REG_T0, REG_T2
 	load_xregs sp, THREAD_ABT_REG_S0, REG_S0, REG_S1
 	load_xregs sp, THREAD_ABT_REG_A0, REG_A0, REG_A7
@@ -550,37 +572,6 @@ FUNC thread_unwind_user_mode , :
 	ret
 END_FUNC thread_unwind_user_mode
 
-/*
- * void thread_exit_user_mode(unsigned long a0, unsigned long a1,
- *			       unsigned long a2, unsigned long a3,
- *			       unsigned long sp, unsigned long pc,
- *			       unsigned long status);
- */
-FUNC thread_exit_user_mode , :
-	/* Set kernel stack pointer */
-	mv	sp, a4
-
-	/* Set xSTATUS */
-	csrw	CSR_XSTATUS, a6
-
-	/*
-	 * Zeroize xSCRATCH to indicate to thread_trap_vect()
-	 * that we are executing in kernel.
-	 */
-	csrw	CSR_XSCRATCH, zero
-
-	/*
-	 * Mask all interrupts first. Interrupts will be unmasked after
-	 * returning from __thread_enter_user_mode().
-	 */
-	csrw	CSR_XIE, zero
-
-	/* Set epc as thread_unwind_user_mode() */
-	csrw	CSR_XEPC, a5
-
-	XRET
-END_FUNC thread_exit_user_mode
-
 /*
  * uint32_t __thread_enter_user_mode(struct thread_ctx_regs *regs,
  *				     uint32_t *exit_status0,
@@ -608,7 +599,7 @@ FUNC __thread_enter_user_mode , :
 
 	/*
 	 * Save kernel stack pointer to ensure that
-	 * thread_exit_user_mode() uses correct stack pointer.
+	 * exception_from_user() uses correct stack pointer.
 	 */
 
 	store_xregs s0, THREAD_CTX_KERN_SP, REG_SP