Very Early boot time stamps support

[Abderrahmane Benbachir]

Hi Pavel,
I'm implementing a feature in ftrace to enable very early function  
tracing, I'm using tsc when x86_tsc feature is available, but it seems  
that you did similar work in your patch "[PATCH v9 0/6] Early boot  
time stamps for x86".

I need to record timestamps at the start of start_kernel(), the first  
function to be traced is set_task_stack_end_magic.

How early does the patch provide stable tsc ?

This is the patch that need to use very early time stamps :  
https://lkml.org/lkml/2017/11/8/1031