CPUs issue instructions through a few ports, each of which services a set of instruction types (eg, arithmetic, memory load/store, vector instructions, etc.). For example here are Skylake’s ports: https://en.wikichip.org/wiki/intel/microarchitectures/skylak...

So getting a trace from the load ports is basically a trace of all memory accesses in the system. Something particularly cool about this work is that you can even see loads that are hidden from software, like a hardware page table walk.