4.1.2 中断跳转

本文章是对 Trap 的补充，如果没有看过的话最好先去看看。不看对后续影响不大，可以作为独立的文章阅读。

__alltrap

当操作系统产生中断时， cpu 会跳转到 stvec 控制寄存器中保存的地址。在 interrupt::init 中，我们设置这个地址为 __alltrap ：

.section .text
.globl __alltraps
__alltraps:
    SAVE_ALL
    mv a0, sp
    jal rust_trap
.globl __trapret
__trapret:
    RESTORE_ALL
    # return from supervisor call
    sret

.globl 表示该符号是一个全局符号，可以被外部访问到。 .section .text 将 __alltraps 和 __trapret 放到程序的同一个代码段中，使得 __alltrap 执行完后会立刻执行 __trapret 。在使用 SAVE_ALL 保存了所有寄存器状态后，将 栈帧地址（sp） 赋值给 a0 。 a0 是参数寄存器， jal 调用的函数（rusttrap）将把 a0 中的内容作为参数。在处理完中断后，由 **\_trapret** 将寄存器恢复到中断前的状态，继续执行中断前的指令。

SAVE_ALL

保存寄存器的工作只能在 内核态 进行。如果当前处于 用户态 ，则需要先返回内核态，再保存寄存器：

.macro SAVE_ALL
    # If coming from userspace, preserve the user stack pointer and load
    # the kernel stack pointer. If we came from the kernel, sscratch
    # will contain 0, and we should continue on the current stack.
    csrrw sp, sscratch, sp
    bnez sp, _save_context
_restore_kernel_sp:
    csrr sp, sscratch
    # sscratch = previous-sp, sp = kernel-sp
_save_context:
    # provide room for trap frame
    addi sp, sp, -36*XLENB
    # save x registers except x2 (sp)
    STORE x1, 1
    STORE x3, 3
    # tp(x4) = hartid. DON'T change.
    # STORE x4, 4
    STORE x5, 5
    STORE x6, 6
    STORE x7, 7
    STORE x8, 8
    STORE x9, 9
    STORE x10, 10
    STORE x11, 11
    STORE x12, 12
    STORE x13, 13
    STORE x14, 14
    STORE x15, 15
    STORE x16, 16
    STORE x17, 17
    STORE x18, 18
    STORE x19, 19
    STORE x20, 20
    STORE x21, 21
    STORE x22, 22
    STORE x23, 23
    STORE x24, 24
    STORE x25, 25
    STORE x26, 26
    STORE x27, 27
    STORE x28, 28
    STORE x29, 29
    STORE x30, 30
    STORE x31, 31

    # get sp, sstatus, sepc, stval, scause
    # set sscratch = 0
    csrrw s0, sscratch, x0
    csrr s1, sstatus
    csrr s2, sepc
    csrr s3, stval
    csrr s4, scause
    # store sp, sstatus, sepc, sbadvaddr, scause
    STORE s0, 2
    STORE s1, 32
    STORE s2, 33
    STORE s3, 34
    STORE s4, 35
.endm

这里我们通过 sscratch 寄存器来判断操作系统所处状态。在执行完 csrrw sp, sscratch, sp 后，如果 sp 的值为 0 ，则当前状态为内核态，直接跳转开始保存寄存器。否则需要先执行 csrr sp, sscratch ，跳转至内核态，再保存寄存器，并修改 sscratch ，表示已经处于内核态。

RESTORE_ALL

在恢复寄存器则与保存的过程相反。首先通过 sstatus 判断中断前操作系统所处状态。若为用户态，则需要恢复 sscratch 的值：

.macro RESTORE_ALL
    LOAD s1, 32             # s1 = sstatus
    LOAD s2, 33             # s2 = sepc
    andi s0, s1, 1 << 8     # sstatus.SPP = 1?
    bnez s0, _to_kernel     # s0 = back to kernel?
_to_user:
    addi s0, sp, 36*XLENB
    csrw sscratch, s0         # sscratch = kernel-sp
_to_kernel:
    # restore sstatus, sepc
    csrw sstatus, s1
    csrw sepc, s2

    # restore x registers except x2 (sp)
    LOAD x1, 1
    LOAD x3, 3
    # LOAD x4, 4
    LOAD x5, 5
    LOAD x6, 6
    LOAD x7, 7
    LOAD x8, 8
    LOAD x9, 9
    LOAD x10, 10
    LOAD x11, 11
    LOAD x12, 12
    LOAD x13, 13
    LOAD x14, 14
    LOAD x15, 15
    LOAD x16, 16
    LOAD x17, 17
    LOAD x18, 18
    LOAD x19, 19
    LOAD x20, 20
    LOAD x21, 21
    LOAD x22, 22
    LOAD x23, 23
    LOAD x24, 24
    LOAD x25, 25
    LOAD x26, 26
    LOAD x27, 27
    LOAD x28, 28
    LOAD x29, 29
    LOAD x30, 30
    LOAD x31, 31
    # restore sp last
    LOAD x2, 2
.endm

代码总览

将上述所有代码整理好后，我们的 trap.asm 应该长成这样：

.equ XLENB,     4
.equ XLENb,     32
.macro LOAD a1, a2
    lw \a1, \a2*XLENB(sp)
.endm
.macro STORE a1, a2
    sw \a1, \a2*XLENB(sp)
.endm

.macro SAVE_ALL
    # If coming from userspace, preserve the user stack pointer and load
    # the kernel stack pointer. If we came from the kernel, sscratch
    # will contain 0, and we should continue on the current stack.
    csrrw sp, sscratch, sp
    bnez sp, _save_context
_restore_kernel_sp:
    csrr sp, sscratch
    # sscratch = previous-sp, sp = kernel-sp
_save_context:
    # provide room for trap frame
    addi sp, sp, -36*XLENB
    # save x registers except x2 (sp)
    STORE x1, 1
    STORE x3, 3
    # tp(x4) = hartid. DON'T change.
    # STORE x4, 4
    STORE x5, 5
    STORE x6, 6
    STORE x7, 7
    STORE x8, 8
    STORE x9, 9
    STORE x10, 10
    STORE x11, 11
    STORE x12, 12
    STORE x13, 13
    STORE x14, 14
    STORE x15, 15
    STORE x16, 16
    STORE x17, 17
    STORE x18, 18
    STORE x19, 19
    STORE x20, 20
    STORE x21, 21
    STORE x22, 22
    STORE x23, 23
    STORE x24, 24
    STORE x25, 25
    STORE x26, 26
    STORE x27, 27
    STORE x28, 28
    STORE x29, 29
    STORE x30, 30
    STORE x31, 31

    # get sp, sstatus, sepc, stval, scause
    # set sscratch = 0
    csrrw s0, sscratch, x0
    csrr s1, sstatus
    csrr s2, sepc
    csrr s3, stval
    csrr s4, scause
    # store sp, sstatus, sepc, sbadvaddr, scause
    STORE s0, 2
    STORE s1, 32
    STORE s2, 33
    STORE s3, 34
    STORE s4, 35
.endm

.macro RESTORE_ALL
    LOAD s1, 32             # s1 = sstatus
    LOAD s2, 33             # s2 = sepc
    andi s0, s1, 1 << 8     # sstatus.SPP = 1?
    bnez s0, _to_kernel     # s0 = back to kernel?
_to_user:
    addi s0, sp, 36*XLENB
    csrw sscratch, s0         # sscratch = kernel-sp
_to_kernel:
    # restore sstatus, sepc
    csrw sstatus, s1
    csrw sepc, s2

    # restore x registers except x2 (sp)
    LOAD x1, 1
    LOAD x3, 3
    # LOAD x4, 4
    LOAD x5, 5
    LOAD x6, 6
    LOAD x7, 7
    LOAD x8, 8
    LOAD x9, 9
    LOAD x10, 10
    LOAD x11, 11
    LOAD x12, 12
    LOAD x13, 13
    LOAD x14, 14
    LOAD x15, 15
    LOAD x16, 16
    LOAD x17, 17
    LOAD x18, 18
    LOAD x19, 19
    LOAD x20, 20
    LOAD x21, 21
    LOAD x22, 22
    LOAD x23, 23
    LOAD x24, 24
    LOAD x25, 25
    LOAD x26, 26
    LOAD x27, 27
    LOAD x28, 28
    LOAD x29, 29
    LOAD x30, 30
    LOAD x31, 31
    # restore sp last
    LOAD x2, 2
.endm

.section .text
.globl __alltraps
__alltraps:
    SAVE_ALL
    mv a0, sp
    jal rust_trap
.globl __trapret
__trapret:
    RESTORE_ALL
    # return from supervisor call
    sret