[PATCH v2] firmware: fw_base: Improve exception stack setup in trap handler

[Anup Patel]

Currently, the low-level trap handler (i.e. _trap_handler()) uses
branch instructions to conditionally setup exception stack based
on which mode trap occured.

This patch implements exception stack setup using xor instructions
which is faster with same number of instructions due to lack of
branch instructions.

The new exception stack setup approach can be best described by the
following pseudocode:

 Came_From_M_Mode = ((MSTATUS.MPP < PRV_M) ? 1 : 0) - 1;
 Exception_Stack = TP ^ (Came_From_M_Mode & (SP ^ TP))

 Came_From_M_Mode = 0    ==>    Exception_Stack = TP
 Came_From_M_Mode = -1   ==>    Exception_Stack = SP

Signed-off-by: Anup Patel <anup.patel at wdc.com>
---
Changes since v1:
- Don't assume that 0b10 privilege mode will never be used
- Use AND operation in-place of MUL operation
---
 firmware/fw_base.S | 47 +++++++++++++++++++++-------------------------
 1 file changed, 21 insertions(+), 26 deletions(-)

diff --git a/firmware/fw_base.S b/firmware/fw_base.S
index b66ac41..0271d9a 100644
--- a/firmware/fw_base.S
+++ b/firmware/fw_base.S
@@ -490,35 +490,30 @@ _trap_handler:
 	/* Save T0 in scratch space */
 	REG_S	t0, SBI_SCRATCH_TMP0_OFFSET(tp)
 
-	/* Check which mode we came from */
+	/*
+	 * Set T0 to appropriate exception stack
+	 *
+	 * Came_From_M_Mode = ((MSTATUS.MPP < PRV_M) ? 1 : 0) - 1;
+	 * Exception_Stack = TP ^ (Came_From_M_Mode & (SP ^ TP))
+	 *
+	 * Came_From_M_Mode = 0    ==>    Exception_Stack = TP
+	 * Came_From_M_Mode = -1   ==>    Exception_Stack = SP
+	 */
 	csrr	t0, CSR_MSTATUS
 	srl	t0, t0, MSTATUS_MPP_SHIFT
 	and	t0, t0, PRV_M
-	xori	t0, t0, PRV_M
-	beq	t0, zero, _trap_handler_m_mode
-
-	/* We came from S-mode or U-mode */
-_trap_handler_s_mode:
-	/* Set T0 to original SP */
-	add	t0, sp, zero
-
-	/* Setup exception stack */
-	add	sp, tp, -(SBI_TRAP_REGS_SIZE)
-
-	/* Jump to code common for all modes */
-	j	_trap_handler_all_mode
-
-	/* We came from M-mode */
-_trap_handler_m_mode:
-	/* Set T0 to original SP */
-	add	t0, sp, zero
-
-	/* Re-use current SP as exception stack */
-	add	sp, sp, -(SBI_TRAP_REGS_SIZE)
-
-_trap_handler_all_mode:
-	/* Save original SP (from T0) on stack */
-	REG_S	t0, SBI_TRAP_REGS_OFFSET(sp)(sp)
+	slti	t0, t0, PRV_M
+	add	t0, t0, -1
+	xor	sp, sp, tp
+	and	t0, t0, sp
+	xor	sp, sp, tp
+	xor	t0, tp, t0
+
+	/* Save original SP on exception stack */
+	REG_S	sp, (SBI_TRAP_REGS_OFFSET(sp) - SBI_TRAP_REGS_SIZE)(t0)
+
+	/* Set SP to exception stack and make room for trap registers */
+	add	sp, t0, -(SBI_TRAP_REGS_SIZE)
 
 	/* Restore T0 from scratch space */
 	REG_L	t0, SBI_SCRATCH_TMP0_OFFSET(tp)
-- 
2.25.1