core: riscv: Improve thread user mode record

The following steps descript the program flow of kernel context during
entering and exiting user mode:

1. thread_enter_user_mode(): prepare user mode context
2. __thread_enter_user_mode(): allocate thread_user_mode_rec from kernel
   stack, and save kernel RA and callee-saved registers into
   thread_user_mode_rec. Save kernel SP into thread context.
   Enter user mode context.

If there is a user ecall which wants to return to kernel mode:
  3. ecall_from_user(): Already set kernel SP, GP, and TP.
  4. setup_unwind_user_mode(): prepare to return to kernel mode.
If there is a user abort which should return to kernel mode:
  3. abort_from_user(): Already set kernel SP, GP, and TP.
  4. handle_user_mode_panic(): prepare to return to kernel mode.

5. XRET to kernel mode with kernel SP, GP, and TP.
6. thread_unwind_user_mode(): Restore kernel RA and callee-saved
   registers from thread_user_mode_rec.

Thus, it's unnecessary to save kernel SP and GP into
thread_user_mode_rec, since they have already set by trap handler and
it's unnecessary to set them again in thread_unwind_user_mode().

Also, rename THREAD_USER_MODE_REC_XX to their register names.

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

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

@@ -51,11 +51,11 @@ struct thread_user_mode_rec {
 	unsigned long pad;
 	/*
 	 * x[] is used to save registers for user/kernel context-switching
-	 * 0-3: ra-tp
-	 * 4-6: s0-s1
-	 * 6-15: s2-s11
+	 * 0: ra
+	 * 1-2: s0-s1
+	 * 3-12: s2-s11
 	 */
-	unsigned long x[16];
+	unsigned long x[13];
 };
 
 extern long thread_user_kcode_offset;

core/arch/riscv/kernel/asm-defines.c

@@ -55,14 +55,12 @@ DEFINES
 	/* struct thread_user_mode_rec */
 	DEFINE(THREAD_USER_MODE_REC_CTX_REGS_PTR,
 	       offsetof(struct thread_user_mode_rec, ctx_regs_ptr));
-	DEFINE(THREAD_USER_MODE_REC_X1,
+	DEFINE(THREAD_USER_MODE_REC_RA,
 	       offsetof(struct thread_user_mode_rec, x[0]));
-	DEFINE(THREAD_USER_MODE_REC_X4,
+	DEFINE(THREAD_USER_MODE_REC_S0,
+	       offsetof(struct thread_user_mode_rec, x[1]));
+	DEFINE(THREAD_USER_MODE_REC_S2,
 	       offsetof(struct thread_user_mode_rec, x[3]));
-	DEFINE(THREAD_USER_MODE_REC_X8,
-	       offsetof(struct thread_user_mode_rec, x[4]));
-	DEFINE(THREAD_USER_MODE_REC_X18,
-	       offsetof(struct thread_user_mode_rec, x[6]));
 	DEFINE(THREAD_USER_MODE_REC_SIZE, sizeof(struct thread_user_mode_rec));
 
 	/* struct thread_abort_regs */

core/arch/riscv/kernel/thread_rv.S

@@ -537,30 +537,22 @@ END_FUNC trap_from_user
 
 /*
  * void thread_unwind_user_mode(uint32_t ret, uint32_t exit_status0,
- * 		uint32_t exit_status1);
+ * 				uint32_t exit_status1);
  * See description in thread.h
  */
 FUNC thread_unwind_user_mode , :
-
 	/* Store the exit status */
 	load_xregs sp, THREAD_USER_MODE_REC_CTX_REGS_PTR, REG_A3, REG_A5
 	sw	a1, (a4)
 	sw	a2, (a5)
-
-	/* Save user callee regs */
+	/* Save user callee-saved regs */
 	store_xregs a3, THREAD_CTX_REG_S0, REG_S0, REG_S1
 	store_xregs a3, THREAD_CTX_REG_S2, REG_S2, REG_S11
-	store_xregs a3, THREAD_CTX_REG_SP, REG_SP, REG_TP
-
-	/* Restore kernel callee regs */
-	mv	a1, sp
-
-	load_xregs a1, THREAD_USER_MODE_REC_X1, REG_RA, REG_GP
-	load_xregs a1, THREAD_USER_MODE_REC_X8, REG_S0, REG_S1
-	load_xregs a1, THREAD_USER_MODE_REC_X18, REG_S2, REG_S11
-
+	/* Restore kernel ra(thread_enter_user_mode()) & callee-saved regs */
+	load_xregs sp, THREAD_USER_MODE_REC_RA, REG_RA
+	load_xregs sp, THREAD_USER_MODE_REC_S0, REG_S0, REG_S1
+	load_xregs sp, THREAD_USER_MODE_REC_S2, REG_S2, REG_S11
 	add	sp, sp, THREAD_USER_MODE_REC_SIZE
-
 	/* Return from the call of thread_enter_user_mode() */
 	ret
 END_FUNC thread_unwind_user_mode
@@ -579,9 +571,9 @@ FUNC __thread_enter_user_mode , :
 	 */
 	addi	sp, sp, -THREAD_USER_MODE_REC_SIZE
 	store_xregs sp, THREAD_USER_MODE_REC_CTX_REGS_PTR, REG_A0, REG_A2
-	store_xregs sp, THREAD_USER_MODE_REC_X1, REG_RA, REG_GP
-	store_xregs sp, THREAD_USER_MODE_REC_X8, REG_S0, REG_S1
-	store_xregs sp, THREAD_USER_MODE_REC_X18, REG_S2, REG_S11
+	store_xregs sp, THREAD_USER_MODE_REC_RA, REG_RA
+	store_xregs sp, THREAD_USER_MODE_REC_S0, REG_S0, REG_S1
+	store_xregs sp, THREAD_USER_MODE_REC_S2, REG_S2, REG_S11
 
 	/*
 	 * Save the kernel stack pointer in the thread context