Sometimes I encounter code that reads TSC with rdtsc instruction, but calls cpuid right before.
Why is calling cpuid necessary? I realize it may have something to do with different cores having TSC values, but what exactly happens when you call those two instructions in sequence?
It's to prevent out-of-order execution. See here for full details, relevant bit below:
The Pentium Pro and Pentium II processors support out-of-order execution instructions may be executed in another order as you programmed them. This can be a source of errors if not taken care of.
To prevent this the programmer must serialize the the instruction queue. This can be done by inserting a serializing instruction like CPUID instruction before the RDTSC instruction.