Messages in this thread | | | Date | Mon, 16 May 2005 10:00:20 -0700 (PDT) | From | Linus Torvalds <> | Subject | Re: Hyper-Threading Vulnerability |
| |
On Fri, 13 May 2005, Scott Robert Ladd wrote: > > Alan Cox wrote: > > HT for most users is pretty irrelevant, its a neat idea but the > > benchmarks don't suggest its too big a hit > > On real-world applications, I haven't seen HT boost performance by more > than 15% on a Pentium 4 -- and the usual gain is around 5%, if anything > at all. HT is a nice idea, but I don't enable it on my systems.
HT is _wonderful_ for latency reduction.
Why people think "performace" means "throughput" is something I'll never understand. Throughput is _always_ secondary to latency, and really only becomes interesting when it becomes a latency number (ie "I need higher throughput in order to process these jobs in 4 hours instead of 8" - notice how the real issue was again about _latency_).
Now, Linux tends to have pretty good CPU latency anyway, so it's not usually that big of a deal, but I definitely enjoyed having a HT machine over a regular UP one. I'm told the effect was even more pronounced on XP.
Of course, these days I enjoy having dual cores more, though, and with multiple cores, the latency advantages of HT become much less pronounced.
As to the HT "vulnerability", it really seems to be not a whole lot different than what people saw with early SMP and (small) direct-mapped caches. Thank God those days are gone.
I'd be really surprised if somebody is actually able to get a real-world attack on a real-world pgp key usage or similar out of it (and as to the covert channel, nobody cares). It's a fairly interesting approach, but it's certainly neither new nor HT-specific, or necessarily seem all that worrying in real life.
(HT and modern CPU speeds just means that the covert channel is _faster_ than it has been before, since you can test the L1 at core speeds. I doubt it helps the key attack much, though, since faster in that case cuts both ways: the speed of testing the cache eviction may have gone up, but so has the speed of the operation you're trying to follow, and you'd likely have a really hard time trying to catch things in real life).
It does show that if you want to hide key operations, you want to be careful. I don't think HT is at fault per se.
Linus - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
| |