Why isn't RDTSC a serializing instruction?

Question

The Intel manuals for the RDTSC instruction warn that out of order execution can change when RDTSC is actually executed, so they recommend inserting a CPUID instruction in f

Answer 1

As paxdiably explains, RDTSC predates the concept of "serializing" instructions because it was implemented on an in-order CPU. Adding that behavior later would change the memory access behavior of code using it, and thus be incompatible for some purposes.

Instead, more recent CPUs have a related RDTSCP instruction that is defined as serializing (actually stronger: it promises to wait until all instructions issued before it have completed, not just that memory accesses have been done), for exactly this reason. Use that if you are running on modern CPUs.