Re: Hyper-Threading Vulnerability

[Linus Torvalds]

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
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