The article quotes the Intel docs: "Instruction ordering: Instructions following a SYSCALL may be fetched from memory before earlier instructions complete execution, but they will not execute (even speculatively) until all instructions prior to the SYSCALL have completed execution (the later instructions may execute before data stored by the earlier instructions have become globally visible)."

More detail here would be great, especially using the terms "issue" and "commit" rather than execute.

A barrier makes sense to me, but preventing instructions from issuing seems like too hard of a requirement, how could anyone tell?