RISC-V异常处理相关内容

异常处理相关内容

- - 异常处理相关内容
  - - 异常处理准备工作
    - 异常处理函数
    - Opensbi系统调用的注册

异常处理相关内容

异常处理准备工作

这里需要特殊强调的是异常处理构建的相关内容：
这里会将a4寄存器中的值存储到CSR_MTVEC这个状态寄存器，也就是异常处理程序的的入口；如果遇到异常、中断时，硬件会自动找到_trap_handler

	/* Setup trap handler */la	a4, _trap_handler
#if __riscv_xlen == 32csrr	a5, CSR_MISAsrli	a5, a5, ('H' - 'A')andi	a5, a5, 0x1beq	a5, zero, _skip_trap_handler_rv32_hypla	a4, _trap_handler_rv32_hyp
#endifcsrw	CSR_MTVEC, a4.section .entry, "ax", %progbits.align 3.globl _trap_handler
_trap_handler:TRAP_SAVE_AND_SETUP_SP_T0TRAP_SAVE_MEPC_MSTATUS 0TRAP_SAVE_GENERAL_REGS_EXCEPT_SP_T0TRAP_CALL_C_ROUTINETRAP_RESTORE_GENERAL_REGS_EXCEPT_SP_T0TRAP_RESTORE_MEPC_MSTATUS 0TRAP_RESTORE_SP_T0mret

建立excption_stack空间，如所示M模式下的异常，则从SP指针开始构建；若不是M模式进入异常，则需要从TP指针开始构建，TP的值为MSCRARCH（这个寄存器会在非M模式下记录M模式下栈帧地址）

.macro	TRAP_SAVE_AND_SETUP_SP_T0/* Swap TP and MSCRATCH */csrrw	tp, CSR_MSCRATCH, tp/* Save T0 in scratch space */REG_S	t0, SBI_SCRATCH_TMP0_OFFSET(tp)/** 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_MSTATUSsrl	t0, t0, MSTATUS_MPP_SHIFTand	t0, t0, PRV_Mslti	t0, t0, PRV_Madd	t0, t0, -1xor	sp, sp, tpand	t0, t0, spxor	sp, sp, tpxor	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)/* Save T0 on stack */REG_S	t0, SBI_TRAP_REGS_OFFSET(t0)(sp)/* Swap TP and MSCRATCH */csrrw	tp, CSR_MSCRATCH, tp
.endm

TRAP_CALL_C_ROUTINE前面的宏流程用于状态的保存，TRAP_CALL_C_ROUTINE则会调用到C阶段，进入真正的异常处理程序：

.macro	TRAP_CALL_C_ROUTINE/* Call C routine */add	a0, sp, zerocall	sbi_trap_handler
.endm

下面将调用到sbi_trap_handler进行真正的异常处理函数。

异常处理函数

当Linux中发起ecall调用后，OpenSBI相关服务出发过程如下，主要分为以下几个阶段

上一节中讲到，在fw_base.S汇编阶段注册了M mode的trap handler，也就是sbi_trap_handler
在sbi_trap_handler中处理各种mcause，首先判断中断原因是否为外部设备中断（timer，ipi），若不是则会根据不同的异常类型比如illegal instructions，Misaligned load & store，S and M mode ecall等等

//lib/sbi_trap.c
/*** Handle trap/interrupt** This function is called by firmware linked to OpenSBI* library for handling trap/interrupt. It expects the* following:* 1. The 'mscratch' CSR is pointing to sbi_scratch of current HART* 2. The 'mcause' CSR is having exception/interrupt cause* 3. The 'mtval' CSR is having additional trap information* 4. The 'mtval2' CSR is having additional trap information* 5. The 'mtinst' CSR is having decoded trap instruction* 6. Stack pointer (SP) is setup for current HART* 7. Interrupts are disabled in MSTATUS CSR** @param regs pointer to register state*/
void sbi_trap_handler(struct sbi_trap_regs *regs)
{int rc = SBI_ENOTSUPP;const char *msg = "trap handler failed";ulong mcause = csr_read(CSR_MCAUSE);ulong mtval = csr_read(CSR_MTVAL), mtval2 = 0, mtinst = 0;struct sbi_trap_info trap;if (misa_extension('H')) {mtval2 = csr_read(CSR_MTVAL2);mtinst = csr_read(CSR_MTINST);}if (mcause & (1UL << (__riscv_xlen - 1))) {mcause &= ~(1UL << (__riscv_xlen - 1));switch (mcause) {case IRQ_M_TIMER:sbi_timer_process();break;case IRQ_M_SOFT:sbi_ipi_process();break;default:msg = "unhandled external interrupt";goto trap_error;};return;}switch (mcause) {case CAUSE_ILLEGAL_INSTRUCTION:rc  = sbi_illegal_insn_handler(mtval, regs);msg = "illegal instruction handler failed";break;case CAUSE_MISALIGNED_LOAD:rc = sbi_misaligned_load_handler(mtval, mtval2, mtinst, regs);msg = "misaligned load handler failed";break;case CAUSE_MISALIGNED_STORE:rc  = sbi_misaligned_store_handler(mtval, mtval2, mtinst, regs);msg = "misaligned store handler failed";break;case CAUS