linux - Linux中的系统调用是如何实现的？

Question

当我在用户模式下调用系统调用时，调用在 OS 中是如何处理的？

它是调用一些可执行的二进制文件还是一些标准库？

如果是，完成通话需要什么样的东西？

Answer 1

看看this .

Starting with version 2.5, linux kernel introduced a new system call entry mechanism on Pentium II+ processors. Due to performance issues on Pentium IV processors with existing software interrupt method, an alternative system call entry mechanism was implemented using SYSENTER/SYSEXIT instructions available on Pentium II+ processors. This article explores this new mechanism. Discussion is limited to x86 architecture and all source code listings are based on linux kernel 2.6.15.6.

1. 什么是系统调用？

   System calls provide userland processes a way to request services from the kernel. What kind of services? Services which are managed by operating system like storage, memory, network, process management etc. For example if a user process wants to read a file, it will have to make 'open' and 'read' system calls. Generally system calls are not called by processes directly. C library provides an interface to all system calls.

2. 系统调用会发生什么？

   A kernel code snippet is run on request of a user process. This code runs in ring 0 (with current privilege level -CPL- 0), which is the highest level of privilege in x86 architecture. All user processes run in ring 3 (CPL 3).

   So, to implement system call mechanism, what we need is

   1) a way to call ring 0 code from ring 3.

   2) some kernel code to service the request.

3. 很好的老方法

   Until some time back, linux used to implement system calls on all x86 platforms using software interrupts. To execute a system call, user process will copy desired system call number to %eax and will execute 'int 0x80'. This will generate interrupt 0x80 and an interrupt service routine will be called. For interrupt 0x80, this routine is an "all system calls handling" routine. This routine will execute in ring 0. This routine, as defined in the file /usr/src/linux/arch/i386/kernel/entry.S, will save the current state and call appropriate system call handler based on the value in %eax.

4. 新的 Shiny 方式

   It was found out that this software interrupt method was much slower on Pentium IV processors. To solve this issue, Linus implemented an alternative system call mechanism to take advantage of SYSENTER/SYSEXIT instructions provided by all Pentium II+ processors. Before going further with this new way of doing it, let's make ourselves more familiar with these instructions.