Dovetail 对CPU状态寄存器中中断状态的虚拟化

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Dovetail通过对中断屏蔽的虚拟化，其实是放弃了对CPU物理中断开关的操纵，并且只是操纵内存中特殊结构的一个比特位INBAND_STALL_BIT来表示中断的开关。

在关中断的时候，设置了INBAND_STALL_BIT。

static __always_inline void stall_inband_nocheck(void)
{
        __set_bit(INBAND_STALL_BIT, &current->stall_bits);
        barrier();
}
/**
 *      inband_irq_disable - disable interrupts for the inband stage
 *
 *      Disable interrupts for the inband stage, disabling in-band
 *      interrupts. Out-of-band interrupts can still be taken and
 *      delivered to their respective handlers though.
 */
notrace void inband_irq_disable(void)
{
        check_inband_stage();
        stall_inband_nocheck();
}
static inline notrace void arch_local_irq_disable(void)
{
        inband_irq_disable();
        barrier();
}

在打开中断的时候清掉了对应的bit 位 INBAND_STALL_BIT

static __always_inline void unstall_inband_nocheck(void)
{
        barrier();
        __clear_bit(INBAND_STALL_BIT, &current->stall_bits);
}
/**
 *      inband_irq_disable - disable interrupts for the inband stage
 *
 *      Disable interrupts for the inband stage, disabling in-band
 *      interrupts. Out-of-band interrupts can still be taken and
 *      delivered to their respective handlers though.
 */
notrace void inband_irq_disable(void)
{
        check_inband_stage();
        stall_inband_nocheck();
}
static inline notrace void arch_local_irq_disable(void)
{
        inband_irq_disable();
        barrier();
}

对CPU状态寄存器的中断位的虚拟化，也是通过利用这个BIT来实现的。

static inline notrace
unsigned long arch_irqs_virtual_to_native_flags(int stalled)
{
        return (!stalled) << X86_EFLAGS_IF_BIT;
}
static __always_inline int test_and_stall_inband_nocheck(void)
{
        return __test_and_set_bit(INBAND_STALL_BIT, &current->stall_bits);
}

/**
 *      inband_irq_save - test and disable (virtual) interrupts
 *
 *      Save the virtual interrupt state then disables interrupts for
 *      the inband stage.
 *
 *      Returns the original interrupt state.
 */
trace_on_debug unsigned long inband_irq_save(void)
{
        check_inband_stage();
        return test_and_stall_inband_nocheck();
}
EXPORT_SYMBOL(inband_irq_save);

static inline notrace unsigned long arch_local_irq_save(void)
{
        int stalled = inband_irq_save();
        barrier();
        return arch_irqs_virtual_to_native_flags(stalled);
}

通过test_and_stall_inband_nocheck获取到这个BIT是否设置，然后把它的状态虚拟成CPU状态寄存器的中断状态位。