Upon return from the system call, syscall_exit is eventually reached, and a call to resume_userspace transitions back to user-space.

从系统调用中返回后，最终执行syscall_exit，并调用resume_userspace返回用户空间。

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Step two: data is copied from the kernel buffer into the user buffer, and the read system call returns. The return from the call caused a context switch from kernel back to user mode.

步骤二：数据从内核空间缓冲区复制到用户空间缓冲区，之后系统调用read返回，这导致了从内核空间向用户空间的上下文切换。

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