The first HT processors had smaller caches and that was before operating systems had SMT-aware schedulers. The place where SMT really seems to benefit is in pa

The first HT processors had smaller caches and that was before operating systems had SMT-aware schedulers. The place where SMT really seems to benefit is in particular applications that can use arbitrarily many threads. This also points to a solution to the security issue, because you get most of the benefit even if you only schedule threads from the same process group together on the same core and it would be possible for programs to specify whether it's safe to do that (e.g. yes for a render, no for a web browser). That would also improve SMT performance in general since SMT makes the performance of some programs worse, and they could use the same mechanism to opt out of it.