Pentium fast system call learning

Fast system call
    System call is realized by soft interrupt instruction . stay x86 In protected mode , Handle INT Soft interrupt command ,CPU First, from the interrupt description table IDT Take out the corresponding door descriptor , Determine the type of gate descriptor , Then check the level of the door descriptor DPL and INT The level of the instruction caller CPL, When CPL Less than or equal to DPL, in other words INT When the caller level is higher than the level specified by the descriptor , To successfully call , Finally, according to the content of the descriptor , Stack 、 Jump and permission level promotion . After the kernel code is executed , call IRET Command return ,IRET Command recovery user stack , And jump back to low-level code . This series of permission checks will cost a lot CPU cycle .
    Intel stay Pentium II A new pair of instructions has been introduced into the processor SYSENTER/SYSEXIT, For fast system calls .SYSENTER Used to enter the kernel state ,SYSEXIT Used to return from kernel state . These two instructions avoid permission checking , Set the processor directly to the predefined run level . At the same time, the execution environment information required by the system call is saved in a set of model related registers （MSR） in , Avoid accessing memory , Further improve the speed of entering the kernel state .