Re: Privilege Levels Below User

On 2024-06-11 2:22, John Savard wrote:

On Mon, 10 Jun 2024 07:16:48 GMT, anton@mips.complang.tuwien.ac.at
(Anton Ertl) wrote:
 

John Savard <quadibloc@servername.invalid> writes:

 

In the case of Spectre, fixing the hardware has a cost in performance.

>
How do you know?
>
Papers on so-called "invisible speculation" schemes have reported
slowdowns <10% for the more advanced schemes, with IIRC some even
reporting a speedup.

 I've heard claims - especially from Mitch Alsup - that, indeed, all
one has to do is avoid certain _mistakes_ when designing a pipeline,
and there's no room for Spectre any more.
 I'm no expert on these things at all, so I don't know that this can't
be true. But I also don't know that it _is_ true.
 What does Spectre exploit? it exploits the fact that speculative
execution keeps around data that was fetched into cache by the
speculative execution of some code that was never supposed to be
executed. Just in case it might be useful later.
 Obviously, keeping around any data that just happens to be
accidentally in cache, just in case it might be useful later, does
have a positive (but likely very slight) effect on performance.

Not always. If the mistakenly speculated cache-fetch /evicted/ some other data from the (finite-sized) cache, and the evicted data are needed later on the /true/ execution path, the mistakenly speculated fetch has a /negative/ effect on performance. (This kind of "timing anomaly" is very bothersome in static WCET analysis.)