[PATCH] firmware: fw_base: Optimize trap handler for RV32 systems

[Anup Patel]

On RV32 systems, we have two CSRs for M-mode status (MSTATUS and
MSTATUSH) when H-extension is implemented. This means we have to
save/restore MSTATUSH for RV32 systems only when H-extension is
implemented. The current _trap_handler() has extra instructions
(roughly 10) to conditional save/restore of MSTATUSH CSR.

These extra instructions in RV32 _trap_handler() can be avoided
if we create separate low-level trap handler for RV32 systems
having H-extension. This patch optimizes low-level trap handler
for RV32 systems accordingly.

Signed-off-by: Anup Patel <anup.patel at wdc.com>
---
 firmware/fw_base.S | 87 ++++++++++++++++++++++++++++++++++++----------
 1 file changed, 68 insertions(+), 19 deletions(-)

diff --git a/firmware/fw_base.S b/firmware/fw_base.S
index 1d9b375..fb504e8 100644
--- a/firmware/fw_base.S
+++ b/firmware/fw_base.S
@@ -413,6 +413,14 @@ _start_warm:
 
 	/* Setup trap handler */
 	la	a4, _trap_handler
+#if __riscv_xlen == 32
+	csrr	a5, CSR_MISA
+	srli	a5, a5, ('H' - 'A')
+	andi	a5, a5, 0x1
+	beq	a5, zero, _skip_trap_handler_rv32_hyp
+	la	a4, _trap_handler_rv32_hyp
+_skip_trap_handler_rv32_hyp:
+#endif
 	csrw	CSR_MTVEC, a4
 
 	/* Initialize SBI runtime */
@@ -476,10 +484,7 @@ fw_platform_init:
 	add	a0, a1, zero
 	ret
 
-	.section .entry, "ax", %progbits
-	.align 3
-	.globl _trap_handler
-_trap_handler:
+.macro	TRAP_SAVE_AND_SETUP_SP_T0
 	/* Swap TP and MSCRATCH */
 	csrrw	tp, CSR_MSCRATCH, tp
 
@@ -519,23 +524,23 @@ _trap_handler:
 
 	/* Swap TP and MSCRATCH */
 	csrrw	tp, CSR_MSCRATCH, tp
+.endm
 
+.macro	TRAP_SAVE_MEPC_MSTATUS have_mstatush
 	/* Save MEPC and MSTATUS CSRs */
 	csrr	t0, CSR_MEPC
 	REG_S	t0, SBI_TRAP_REGS_OFFSET(mepc)(sp)
 	csrr	t0, CSR_MSTATUS
 	REG_S	t0, SBI_TRAP_REGS_OFFSET(mstatus)(sp)
-	REG_S	zero, SBI_TRAP_REGS_OFFSET(mstatusH)(sp)
-#if __riscv_xlen == 32
-	csrr	t0, CSR_MISA
-	srli	t0, t0, ('H' - 'A')
-	andi	t0, t0, 0x1
-	beq	t0, zero, _skip_mstatush_save
+	.if \have_mstatush
 	csrr	t0, CSR_MSTATUSH
 	REG_S	t0, SBI_TRAP_REGS_OFFSET(mstatusH)(sp)
-_skip_mstatush_save:
-#endif
+	.else
+	REG_S	zero, SBI_TRAP_REGS_OFFSET(mstatusH)(sp)
+	.endif
+.endm
 
+.macro	TRAP_SAVE_GENERAL_REGS_EXCEPT_SP_T0
 	/* Save all general regisers except SP and T0 */
 	REG_S	zero, SBI_TRAP_REGS_OFFSET(zero)(sp)
 	REG_S	ra, SBI_TRAP_REGS_OFFSET(ra)(sp)
@@ -567,11 +572,15 @@ _skip_mstatush_save:
 	REG_S	t4, SBI_TRAP_REGS_OFFSET(t4)(sp)
 	REG_S	t5, SBI_TRAP_REGS_OFFSET(t5)(sp)
 	REG_S	t6, SBI_TRAP_REGS_OFFSET(t6)(sp)
+.endm
 
+.macro	TRAP_CALL_C_ROUTINE
 	/* Call C routine */
 	add	a0, sp, zero
 	call	sbi_trap_handler
+.endm
 
+.macro	TRAP_RESTORE_GENERAL_REGS_EXCEPT_SP_T0
 	/* Restore all general regisers except SP and T0 */
 	REG_L	ra, SBI_TRAP_REGS_OFFSET(ra)(sp)
 	REG_L	gp, SBI_TRAP_REGS_OFFSET(gp)(sp)
@@ -602,30 +611,70 @@ _skip_mstatush_save:
 	REG_L	t4, SBI_TRAP_REGS_OFFSET(t4)(sp)
 	REG_L	t5, SBI_TRAP_REGS_OFFSET(t5)(sp)
 	REG_L	t6, SBI_TRAP_REGS_OFFSET(t6)(sp)
+.endm
 
+.macro	TRAP_RESTORE_MEPC_MSTATUS have_mstatush
 	/* Restore MEPC and MSTATUS CSRs */
 	REG_L	t0, SBI_TRAP_REGS_OFFSET(mepc)(sp)
 	csrw	CSR_MEPC, t0
 	REG_L	t0, SBI_TRAP_REGS_OFFSET(mstatus)(sp)
 	csrw	CSR_MSTATUS, t0
-#if __riscv_xlen == 32
-	csrr	t0, CSR_MISA
-	srli	t0, t0, ('H' - 'A')
-	andi	t0, t0, 0x1
-	beq	t0, zero, _skip_mstatush_restore
+	.if \have_mstatush
 	REG_L	t0, SBI_TRAP_REGS_OFFSET(mstatusH)(sp)
 	csrw	CSR_MSTATUSH, t0
-_skip_mstatush_restore:
-#endif
+	.endif
+.endm
 
+.macro TRAP_RESTORE_SP_T0
 	/* Restore T0 */
 	REG_L	t0, SBI_TRAP_REGS_OFFSET(t0)(sp)
 
 	/* Restore SP */
 	REG_L	sp, SBI_TRAP_REGS_OFFSET(sp)(sp)
+.endm
+
+	.section .entry, "ax", %progbits
+	.align 3
+	.globl _trap_handler
+_trap_handler:
+	TRAP_SAVE_AND_SETUP_SP_T0
+
+	TRAP_SAVE_MEPC_MSTATUS 0
+
+	TRAP_SAVE_GENERAL_REGS_EXCEPT_SP_T0
+
+	TRAP_CALL_C_ROUTINE
+
+	TRAP_RESTORE_GENERAL_REGS_EXCEPT_SP_T0
+
+	TRAP_RESTORE_MEPC_MSTATUS 0
+
+	TRAP_RESTORE_SP_T0
 
 	mret
 
+#if __riscv_xlen == 32
+	.section .entry, "ax", %progbits
+	.align 3
+	.globl _trap_handler_rv32_hyp
+_trap_handler_rv32_hyp:
+	TRAP_SAVE_AND_SETUP_SP_T0
+
+	TRAP_SAVE_MEPC_MSTATUS 1
+
+	TRAP_SAVE_GENERAL_REGS_EXCEPT_SP_T0
+
+	TRAP_CALL_C_ROUTINE
+
+	TRAP_RESTORE_GENERAL_REGS_EXCEPT_SP_T0
+
+	TRAP_RESTORE_MEPC_MSTATUS 1
+
+	TRAP_RESTORE_SP_T0
+
+	mret
+#endif
+
 	.section .entry, "ax", %progbits
 	.align 3
 	.globl _reset_regs
-- 
2.25.1