System Call Optimization with the SYSENTER Instruction
有些看不懂，/////////////////////////////////////////////////
If you have paid close attention so far, you might have noticed that there is no "SYSEXIT_EIP_MSR" or "SYSEXIT_ESP_MSR" register. So, how does the SYSEXIT instruction know where to return to in the user-mode code that initially called SYSENTER? When you think about it, such information could not be fixed in an MSR because each system call can potentially originate from completely different locations in user-mode. Therefore, it is the responsibility of the caller (the code that calls SYSENTER) to place the address the CPU is to return to after the system call has returned in the EDX register. The caller must also place the current stack pointer (the value of ESP) in the ECX register. The SYSEXIT instruction will then restore the original value in the EIP and ESP by copying the content from EDX and ECX respectively. This will cause the execution to continue at the instruction after the original SYSENTER instruction.
