Re: Doubts in System call mechanism in linux

"Daniel (Youngwhan) Song" <breadncup@xxxxxxxxx> · Mon, 10 May 2010 21:20:09 -0700

On 5/10/10 6:37 PM, Balachandar wrote:
Hello,
    We
transit from ring3 to ring0 using 'int' or the new 'syscall/sysenter'
instruction. Does that mean that the page tables and other stuffs that
needs to be modified for the kernel is automatically done by the 'int'
instruction or the interrupt handler for the 'int 0x80' will do the
required stuff and jump to the respective system call.

  Also
when returning from a system call, we again need to go to user space.
For this we need to know the instruction address in the user space to
continue the user application. Where is that address stored. Does the
'ret' instruction automatically changes the ring from ring3 to ring0 or
where/how this ring changing mechanism takes place?

Then, i read that changing from ring3 to ring0 is not as costly as
changing from ring0 to ring3. Why is this so??

  Thanks,
  Bala

Hi Bala,

Have you referred this?

http://tldp.org/HOWTO/html_single/Implement-Sys-Call-Linux-2.6-i386/

http://www.ibm.com/developerworks/linux/library/l-system-calls/index.html

Daniel