On Mon, 2 Jan 2012, Steve Reinhardt wrote:
Hi Nilay,
I think Gabe's point is that if you take the problem he is describing and
replace "misprediction" with "interrupt" then what you're seeing is just a
different manifestation of the same underlying problem. In a sense an
interrupt is just a control-flow misprediction, only much rarer and mostly
unavoidable... the pipeline predicts that control will always go to the
next sequential instruction for non-control-flow-instructions (and to
either the next instruction or the target for control-flow instructions)
and an interrupt is a situation where that prediction is wrong.
One difference is that in the branch misprediction case, the pipeline
immediately refetches along the correct path, while in the interrupt case,
the interrupt handler gets executed first, and the problem doesn't show up
until you return from the interrupt.
I think Gabe is also saying that if we did a larger restructuring of how
squashes are handled we could solve both problems at once (and perhaps
other related cases we haven't encountered yet). I agree that that sounds
good in theory, but in practice if there's a straightforward fix for the
current issue, I'm not opposed to putting that in.
Steve, from the way I have been able to fix the problem, it seems that the
issue is with the way interrupts are handled. As such I never saw
squashing being the problem, though it might be.
--
Nilay
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