diff options
| author | Anup Patel <anup.patel@wdc.com> | 2020-08-10 18:37:46 +0530 |
|---|---|---|
| committer | Anup Patel <anup@brainfault.org> | 2020-08-14 14:35:47 +0530 |
| commit | 4b18a2acc2f4568746efc82ec25705f33c4ae31e (patch) | |
| tree | fdd9a8c5ffa207ab3492ea6adeccc31647adaf90 /firmware | |
| parent | 937caee0833115f69d697ca190001ba0aa5c7368 (diff) | |
firmware: fw_base: Improve exception stack setup in trap handler
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@wdc.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>
Diffstat (limited to 'firmware')
| -rw-r--r-- | firmware/fw_base.S | 47 |
1 files 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) |