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From: mitchalsup@aol.com (MitchAlsup1)
Newsgroups: comp.arch
Subject: Re: Microarchitectural support for counting
Date: Sat, 5 Oct 2024 22:55:47 +0000
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On Sat, 5 Oct 2024 15:11:29 +0000, EricP wrote:

> MitchAlsup1 wrote:
>> On Fri, 4 Oct 2024 18:11:23 +0000, EricP wrote:
>>> On interrupt, if the core starts fetching instructions from the handler
>>> and
>>> stuffing them into the instruction queue (ROB) while there are still
>>> instructions in flight, and if those older instructions get a branch
>>> mispredict, then the purge of mispredicted older instructions will also
>>> purge the interrupt handler.
>>
>> Not necessary, you purge all of the younger instructions from the
>> thread at retirement, but none of the instructions associated with
>> the new <interrupt> thread at the front.
>
> That's difficult with a circular buffer for the instruction queue/rob
> as you can't edit the order. For a branch mispredict you might be able
> to mark a circular range of entries as voided, and leave the entries
> to be recovered serially at retire.

Every instruction needs a way to place itself before or after
any mispredictable branch. Once you know which branch mispredicted, you
know instructions will not retire, transitively. All you really need to
know is if the instruction will retire, or not. The rest of the
mechanics
play out naturally in the pipeline.
>
> But voiding doesn't look like it works for exceptions or conflicting
> interrupt priority adjustments. In those cases purging the interrupt
> handler and rejecting the hand-off looks like the only option.

Can you make this statement again and use different words?
>
> If one can live with the occasional replay of an interrupt hand-off and
> handler execute due to mispredict/exception/interrupt_priority_adjust
> then the interrupt pipelining looks much simplified.

You just have to cover the depth of the pipeline.