linux-gcc下asmlinkage是什么意思

它是GCC对C程序的一种扩展, #define asmlinkage __attribute__((regparm(0)))
表示用0个寄存器传递函数参数，这样，所有的函数参数强迫从栈中提取。
这个asmlinkage大都用在系统调用中，系统调用需要在entry.s文件中用汇编语言调用，所以必须要保证它符合C语言的参数传递规则，才能用汇编语言正确调用它。
这也是为何使用asmlinkage的原因吧！这是我的理解。

仔细看一下有asmlinkage的地方通常是系统调用的函数，因为在系统调用中，寄存器从用户空间传过来后SAVE_ALL压入堆栈，接着调用相应的系统调用函数，这样系统调用函数一定要保证是通过堆栈传递参数的

转贴一段：
The asmlinkage tag is one other thing that we should observe about this simple function. This is a #define for some gcc magic that tells the compiler that the function should not expect to find any of its arguments in registers (a common optimization), but only on the CPU's stack. Recall our earlier assertion that system_call consumes its first argument, the system call number, and allows up to four more arguments that are passed along to the real system call. system_call achieves this feat simply by leaving its other arguments (which were passed to it in registers) on the stack. All system calls are marked with the asmlinkage tag, so they all look to the stack for arguments. Of course, in sys_ni_syscall's case, this doesn't make any difference, because sys_ni_syscall doesn't take any arguments, but it's an issue for most other system calls. And, because you'll be seeing asmlinkage in front of many other functions, I thought you should know what it was about.

具体可以研究一下arch/i386/kernel/entry.S

是不是为了保护寄存器内容不受更改 才用堆栈传递参数 以便以后能RESTORE--ALL?

系统调用把参数存放在寄存器中然后进入内核空间，其实就和正常的函数调用一样，在内核中通过SAVE_ALL构造一个函数调用的栈帧环境，然后调用相应的系统调用函数而已

这里使用的堆栈就是原来SAVE--ALL的寄存器的内容吧??

对的

ENTRY(system_call)
pushl %eax # save orig_eax
SAVE_ALL
GET_THREAD_INFO(%ebp)
# system call tracing in operation
/* Note, _TIF_SECCOMP is bit number 8, and so it needs testw and not testb */
testw $(_TIF_SYSCALL_TRACE|_TIF_SYSCALL_AUDIT|_TIF_SECCOMP),TI_flags(%ebp)
jnz syscall_trace_entry
cmpl $(nr_syscalls), %eax
jae syscall_badsys
syscall_call:
call *sys_call_table(,%eax,4)
movl %eax,EAX(%esp) # store the return value