Reply to post: Re: A Kludge

OpenBSD disables Intel’s hyper-threading over CPU data leak fears

Ken Hagan Gold badge

Re: A Kludge

"Hyperthreading always was a kludge to desperately keep overlong pipelines full."

I don't think that's fair. It keeps an OoO processor core busier than it would otherwise be if it could only take instructions from one thread of execution. You lose if the second stream of instructions blows your caches (so your workload is memory-bandwidth-limited) and you win if it doesn't (so your workload is computation-limited). It presumably takes fewer transistors than a second core but makes fault handling more gnarly.

End-users have always been able to switch it off in the BIOS, so OpenBSD are merely making it easy to do that after you've booted the OS. Obviously I don't know whether this is inspired by advance warning of another Spectre-like bug, but it seems more likely to me that they just thought it would be prudent in the current climate to have a switch already in place for their users. As the article points out, it is quite believable that being able to share a core with your victim makes a timing attack easier.

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