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From: mitchalsup@aol.com (MitchAlsup1)
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Subject: Re: Chipsandcheese article on the CDC6600
Date: Thu, 25 Jul 2024 17:22:44 +0000
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On Thu, 25 Jul 2024 10:59:16 +0000, Anton Ertl wrote:

> Lawrence D'Oliveiro <ldo@nz.invalid> writes:
>>On Wed, 24 Jul 2024 13:22:46 GMT, Scott Lurndal wrote:
>>
>>> Lawrence D'Oliveiro <ldo@nz.invalid> writes:
>>>>
>>>>I was wondering how those code patches would
>>>>impact on shared code.
>>>
>>> Global branch prediction, of course.
>>
>>But the characteristics of a program run in one thread/process might not
>>match those in another.
>
> They might not, or they might.  When the hardware branch predictor
> researchers looked into it, they found that there is more synergy than
> interference.  Consequently, they did not take measures to avoid
> sharing.  And for the approach of patching the hints in the code, the
> results of sharing will be beneficial on average, too, because the
> only difference from the 2-bit/branch predictor is that the latter is
> in microarchitectural state instead of in the code.
>
> Now somebody will point out that sharing makes it possible for an
> attacker to train branch predictors in one process to attack a
> different process through Spectre and friends.  While preventing
> sharing would close that, it does not close training the predictors in
> the same thread.

Not allowing a dependent AGEN to happen when the first AGEN takes
a fault ALSO prevents SPectré like attacks {Whether the crack is
opened up by the BP, IBP, or any other predictor.} Then not modifying
any cache prior to instruction retirement cements the door closed.
>
> Closing Spectre through invisible speculation (several papers exist
> about that) makes it irrelevant (for Spectre) whether the predictors
> are shared or not.  Of course, for invisible speculation the permanent
> branch predictors must not be updated speculatively, but that's
> probably better anyway.
>
> - anton