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From: John Savard <quadibloc@servername.invalid>
Newsgroups: comp.arch
Subject: Re: Privilege Levels Below User
Date: Mon, 10 Jun 2024 17:22:05 -0600
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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. Being
strict about what speculative execution can do, on the other hand, so
nothing is allowed to leak information, will reduce performance... at
least a little bit.

It could well be that the losses aren't enough to be concerned about,
if this is done carefully. That is, not even the 3% quoted as the cost
of one of the earliest fixes. But since I've heard higher figures for
the fixes for later variants, without positive knowledge, I have to be
skeptical about claims that all possible variants of this kind of
attack can be prevented at little cost.

And Rowhammer is even worse. It's not at all clear to me what can be
done without adding an expensive layer of monitoring to memory
accesses. However, only DRAM is vulnerable to Rowhammer, and so it may
be possible to turn cache into a bulwark against it somehow.

John Savard