I am using Linux kernel 2.6.27 for building UML kernel.  It seems like "arch/x86/kernel/tsc.c" is not built in and the symbol "cpu_khz" is not available.  

My question is:  what's the reason that "tsc.c" not included in UML?  Is there any config or simple way(patch?)  to include it in UML? 

I have not tried the same on the newer kernels yet as my target system uses 2.6.27 only so I prefer to stick with 2.6.27 if possible.