Kernel散记——系统调用

2.3 在Kernel空间

首先处理SWI指令

$kernel_root/arch/arm/kernel/entry-common.S

/*=============================================================================
* SWI handler
*-----------------------------------------------------------------------------
*/

    .align  5
ENTRY(vector_swi)
    sub sp, sp, #S_FRAME_SIZE
    stmia   sp, {r0 - r12}          @ Calling r0 - r12 
ARM(   add r8, sp, #S_PC       )
ARM(   stmdb   r8, {sp, lr}^       )   @ Calling sp, lr
THUMB( mov r8, sp          )   
THUMB( store_user_sp_lr r8, r10, S_SP  )   @ calling sp, lr
    mrs r8, spsr            @ called from non-FIQ mode, so ok. 
    str lr, [sp, #S_PC]         @ Save calling PC
    str r8, [sp, #S_PSR]        @ Save CPSR
    str r0, [sp, #S_OLD_R0]     @ Save OLD_R0
    zero_fp

...

      addne   scno, r7, #__NR_SYSCALL_BASE    @ put OS number in //从R7把系统调用号读出来

...

    adr tbl, sys_call_table     @ load syscall table pointer //再根据系统调用号，查询sys_call_table这个表，找到相应的系统调用

#include "calls.S"

sys_call_table即calls.S

=>calls.S

...

/* 120 */   CALL(sys_clone)//120号对应的就是sys_clone

... 

=>entry-common.S

 ldrcc   pc, [tbl, scno, lsl #2]     @ call sys_* routine//这里执行系统调用。

那么在哪实现sys_clone的呢？不同的系统的调用，在不同的目录里，比较open就是在fs/下面，clone就就在kernel下面。使用宏SYSCALL_DEFINE来配合使用。

=>kernel/fork.c

#ifdef __ARCH_WANT_SYS_CLONE  
#ifdef CONFIG_CLONE_BACKWARDS
SYSCALL_DEFINE5(clone, unsigned long, clone_flags, unsigned long, newsp,
         int __user *, parent_tidptr,
         int, tls_val,
         int __user *, child_tidptr)
#elif defined(CONFIG_CLONE_BACKWARDS2)
SYSCALL_DEFINE5(clone, unsigned long, newsp, unsigned long, clone_flags,
         int __user *, parent_tidptr,
         int __user *, child_tidptr,
         int, tls_val)
#elif defined(CONFIG_CLONE_BACKWARDS3)
SYSCALL_DEFINE6(clone, unsigned long, clone_flags, unsigned long, newsp,
        int, stack_size,
        int __user *, parent_tidptr,
        int __user *, child_tidptr,
        int, tls_val)
#else
SYSCALL_DEFINE5(clone, unsigned long, clone_flags, unsigned long, newsp,
         int __user *, parent_tidptr,
         int __user *, child_tidptr,
         int, tls_val)
#endif
{
    return do_fork(clone_flags, newsp, 0, parent_tidptr, child_tidptr);                                                                                        
}
#endif

=>

最终是调用forkc.c/do_fork()的.

整个流程是这样的，通了流程，有需要有精力就可以深入细节啦。