CPU 上下文切换中断

10 中断里进行线程上下文切换如何做到的？

发布于 2023-05-23 09:47:50 浏览：146 订阅该版

```
rt_hw_context_switch_interrupt
    EXPORT rt_hw_context_switch_interrupt
rt_hw_context_switch    PROC
    EXPORT rt_hw_context_switch

; set rt_thread_switch_interrupt_flag to 1
    LDR     r2, =rt_thread_switch_interrupt_flag
    LDR     r3, [r2]
    CMP     r3, #1
    BEQ     _reswitch
    MOV     r3, #1
    STR     r3, [r2]

LDR     r2, =rt_interrupt_from_thread   ; set rt_interrupt_from_thread
    STR     r0, [r2]

_reswitch
    LDR     r2, =rt_interrupt_to_thread     ; set rt_interrupt_to_thread
    STR     r1, [r2]

LDR     r0, =NVIC_INT_CTRL              ; trigger the PendSV exception (causes context switch)
    LDR     r1, =NVIC_PENDSVSET
    STR     r1, [r0]
    BX      LR
    ENDP
```
------------

上面是context-m 这个中断时线程上下文切换也是通过触发PendSV进行的，但是一些其他的CPU架构在中断环境下不触发类似PendSV，那么是如何进行真正切换呢？例如下面
```
rt_hw_context_switch:
    /* compare rt_thread_switch_interrupt_flag and set it */
    lis     r5,rt_thread_switch_interrput_flag@h
    ori     r5,r5,rt_thread_switch_interrput_flag@l
    lwz     r6,0(r5)
    cmpi    cr0,0,r6,0x1                        /* whether is 1 */
    beq     _reswitch                           /* set already, goto _reswitch */
    li      r6,0x1                              /* set rt_thread_switch_interrput_flag to 1*/
    stw     r6,0(r5)

/* set rt_interrupt_from_thread to 'from' */
    lis     r5,rt_interrupt_from_thread@h
    ori     r5,r5,rt_interrupt_from_thread@l
    stw     r3,0(r5)

_reswitch:
    /* set rt_interrupt_to_thread to 'to' */
    lis     r6,rt_interrupt_to_thread@h
    ori     r6,r6,rt_interrupt_to_thread@l
    stw     r4,0(r6)

/* trigger a system call */
    sc

blr
rt_hw_context_switch_interrupt:
    /* compare rt_thread_switch_interrupt_flag and set it */
    lis     r5,rt_thread_switch_interrput_flag@h
    ori     r5,r5,rt_thread_switch_interrput_flag@l
    lwz     r6,0(r5)
    cmpi    cr0,0,r6,0x1                        /* whether is 1 */
    beq     _int_reswitch                       /* set already, goto _int_reswitch */
    li      r6,0x1                              /* set rt_thread_switch_interrput_flag to 1*/
    stw     r6,0(r5)

/* set rt_interrupt_from_thread to 'from' */
    lis     r5,rt_interrupt_from_thread@h
    ori     r5,r5,rt_interrupt_from_thread@l
    stw     r3,0(r5)

_int_reswitch:
    /* set rt_interrupt_to_thread to 'to' */
    lis     r6,rt_interrupt_to_thread@h
    ori     r6,r6,rt_interrupt_to_thread@l
    stw     r4,0(r6)

blr
```
目前的情况就是当我两个优先级相同的线程以时间片轮转时会在定时器中断tick增加时产生线程切换，并且调试模式打印是进了 `rt_hw_context_switch_interrupt`，现象就是没有进行切换。从这个汇编来看确实只是获取了from和to线程，真正的切换寄存器压栈进栈并没有进行。
所以我想问问不和context-m一样在里面触发，那么是怎么做到在中断下上下文切换呢？