How to change the Core input voltage setting?

[김유석 책임연구원]

Dear Sir.

My HW enginner required to me. that Change the setting of "Core input 
voltage".

But, I don't know everything this one. Because x86 platform is first 
time of my develop life.

Anyway, I'm try to find the something on coreboot source code.

But, still unknow.

So, I need a start point or key point that Initial code for "Core input 
voltage".

Please advise to me.

Thank you.