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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>
This commit is contained in:
Anup Patel 2020-08-10 18:37:46 +05:30 committed by Anup Patel
parent 937caee083
commit 4b18a2acc2
1 changed files with 19 additions and 24 deletions
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43
firmware/fw_base.S
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@ -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
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
/* We came from S-mode or U-mode */
_trap_handler_s_mode:
/* Set T0 to original SP */
add t0, sp, zero
/* Save original SP on exception stack */
REG_S sp, (SBI_TRAP_REGS_OFFSET(sp) - SBI_TRAP_REGS_SIZE)(t0)
/* 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)
/* 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)