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.macro INTERRUPT_RETURN  ; 中断返回
    iret
.endm
.macro SAVE_ALL          ; 保护现场
    ...
.macro RESTORE_INT_REGS
    ...
.endm

ENTRY(system_call)
    SAVE_ALL
syscall_call:
    call *sys_call_table(,%eax,4)
    movl %eax, PT_EAX(%esp)  ; store the return value
syscall exit:
    testl $_TIF_ALLWORK_MASK, %ecx # current->work
    jne syscall_exit_work
restore_all:
    RESTORE_INT_REGS
irq_return:
    INTERRUPT_RETURN      ; 到这里就算执行完了
ENDPROC(system_call)

syscall_exit_work:
    testl  $_TIF_WORK_SYSCALL_EXIT, %ecx
    jz work_pending
END(syscall_exit_work)

work_pending:
    testb $_TIF_NEED_RESCHED, %cl
    jz work_notifysig
work_resched:
    call schedule
    jz restore_all
work_notifysig:
    ...                  ; deal with pending signals
END(work_pending)

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    SAVE_ALL 
    GET_THREAD_INFO(%ebp)  
                    # system call tracing in operation / emulation  
    testl $_TIF_WORK_SYSCALL_ENTRY,TI_flags(%ebp)  
    jnz syscall_trace_entry  
    cmpl $(NR_syscalls), %eax  
    jae syscall_badsys  
syscall_call:  
    call *sys_call_table(,%eax,4)  

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syscall_exit:  
    LOCKDEP_SYS_EXIT  
    DISABLE_INTERRUPTS(CLBR_ANY)
    TRACE_IRQS_OFF  
    movl TI_flags(%ebp), %ecx  
    testl $_TIF_ALLWORK_MASK, %ecx  # current->work  
    jne syscall_exit_work  
restore_all:  
    TRACE_IRQS_IRET  
restore_all_notrace:  
    movl PT_EFLAGS(%esp), %eax  # mix EFLAGS, SS and CS 
    movb PT_OLDSS(%esp), %ah  
    movb PT_CS(%esp), %al  
    andl $(X86_EFLAGS_VM | (SEGMENT_TI_MASK << 8) | SEGMENT_RPL_MASK), %eax  
    cmpl $((SEGMENT_LDT << 8) | USER_RPL), %eax  
    CFI_REMEMBER_STATE  
    je ldt_ss           # returning to user-space with LDT SS
restore_nocheck:  
    RESTORE_REGS 4          # skip orig_eax/error_code  
irq_return:  
    INTERRUPT_RETURN