There's a talk here: https://www.youtube.com/watch?v=jMUMRNRkcMg but I'm not much clearer after scrubbing through some of the slides. It sounds like all GPIO pins on recent Intel chips support this Timed I/O functionality. I don't know if they could measure the timing of transactions to a NIC - I would think this is only for specialist hardware or testing. They mention using a logic analyser to compare the clocks of two systems after synchronisation.