* Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote: > This was marked for stable, and honestly, nowhere in the discussion > did I see any mention of just *how* bad the performance impact of this > was. Yeah. This was an oversight - we'll fix it! > When performance goes down by 50% on some loads, people need to start > asking themselves whether it was worth it. It's apparently better to > just disable SMT entirely, which is what security-conscious people do > anyway. > > So why do that STIBP slow-down by default when the people who *really* > care already disabled SMT? > > I think we should use the same logic as for L1TF: we default to > something that doesn't kill performance. Warn once about it, and let > the crazy people say "I'd rather take a 50% performance hit than > worry about a theoretical issue". Yeah, absolutely. We'll also require performance measurements in changelogs enabling any sort of mitigation feature from now on - this requirement was implicit but 53c613fe6349 flew in under the radar, so it's going to be explicit an explicit requirement. Thanks, Ingo
