AR: Clarify itrigger and trigger number translation

[timsifive]

itrigger numbers relate to the value written to *cause when the trap is taken.

Addresses #889.

[pdonahue-ventana]

I don't think that this sentence will make sense to people who aren't familiar with AIA. I would say something like:

Suggested change

	trap is taken. For interrupts whose numbers are translated, that means
	the number checked is the one in the mode that the trap handler executes
	in.
	trap is taken. For virtualized interrupts that have different interrupt numbers
	when viewed from different modes, that means
	the number checked is the one in the mode that the trap handler executes
	in.

[pdonahue-ventana]

Do we also need to say something about how we don't care why the interrupt is taken? It could be that the input wire asserted, it could be that M mode wrote mie.STIE=1 in the original priv arch, it could be that the hypervisor is playing with hvip, etc. This was one of John's concerns. We don't have to spell out the cases but we should say something about how the trigger is based on the interrupt trap regardless of the original cause of the interrupt.

[timsifive]

I kind of agree with you, but I'm not sure how to say that. Aren't interrupt traps by definition taken in response to interrupts?

[pdonahue-ventana]

I think that John's question was whether one category of reasons for taking an interrupt would be treated differently than another category of reasons for taking an interrupt.

Rereading what's currently there, it basically says that the trigger matches/fires only if we trap but it doesn't say that it fires every time that we trap. Maybe it's as simple as saying "The trigger fires if and only if the hart takes a trap because of the interrupt." That would presumably clarify that the reason for the trap doesn't matter.

[pdonahue-ventana]

If possible, I like to avoid "masked" because sometimes you have positive masks (A & mask) and sometimes you have negative masks (A & ~mask) so it can be ambiguous. What you have is probably not that ambiguous but how about using something closer to the original language? That also applies to global enable (mstatus.MIE) rather than just local enables (mie).

Suggested change

	interrupt. (E.g.\ it does not fire for an interrupt that is masked.)
	interrupt. (E.g.\ it does not fire for an interrupt that is not enabled.)

[timsifive]

disabled? Done.

[timsifive]

I've rewritten this, hopefully to be more clear.

[pdonahue-ventana]

I wasn't sure what the first "it" is. It seems like it refers to an interrupt but singular "it" doesn't match "interrupts" plural. Then I wondered if it would be clearer to just remove everything before the comma.

[timsifive]

How about:
This trigger fires when an interrupt trap is taken from a privilege mode
where the trigger is enabled and that interrupt is also enabled in the
trigger.

[pdonahue-ventana]

Now I understand. There are a lot of enables here: the mode bits in tdata1, the interrupt number in tdata2, and the interrupt enables (both mie and mstatus.MIE). I like something more like this tweak to the earlier language:

    For an interrupt where that interrupt number is enabled in tdata2, this trigger fires when an interrupt
    trap is taken from a privilege mode where the trigger is enabled.

That doesn't actually talk about the interrupt enables but that's implicit when you talk about taking a trap.

[timsifive]

I don't want to explicitly call out tdata2 here, because nmi is also an option. I agree there are a lot of enables. Maybe I just need to capitulate and make a numbered list.

[timsifive]

I made it even simpler. We don't need to talk about the mode at all in this sentence. It's already in the bits. The only real need is to indicate what the trigger is useful for. 2 paragraphs down (When the trigger matches...) already makes it clear that it only happens when a trap is actually taken. We don't need the parenthetical about interrupts being masked or whatever. The trigger fires when the trap is taken. Done.

[pdonahue-ventana]

That's simpler but still conveys all the necessary information. Looks good.