Still allow triggering on ASID as well. Also leave space for future H
mode support.

There was some e-mail discussion, subject:"ContextID trigger/trace"

* Mostly match "official" style for credits.

* Fix Megan's e-mail address.

Fixes #358.

* Abstract commands might work on a hung hart.

From a discussion on today's call.

* Rephrase.

Can't change harts during operations, and the current hart becoming unavailable may terminate the abstract command with error. (#322)

* Can't change harts during operations.

See #315.

* Only restrict hartsel changes during abstract cmds

reset/halt could take a very long time if a hart is asleep, and we don't
want to prevent debugging other harts in that case.

* Set cmderr for hartsel change during abstract cmd.

* Clarify reset and hasel during abstract command.

* Incorporate review feedback.

* Make debuggers handle harts becoming unavailable.

Instead of having the DM correctly handle a hart that becomes
unavailable, make debuggers responsible for that. This seems like a good
tradeoff because this should be rare.

* Clarifications requested in review.

* Clarify per review.

Fixes #301.

* Document breakpoint exception + enter debug mode

* Multiple triggers firing with different timing.

* Be a more lenient when trigger actions conflict

This came out of the July 11 meeting, where consensus was that requiring
both actions to be taken might significantly complicate hardware to
handle what is an uncommon corner case.

* Define MXLEN.

* Introduce DXLEN.

This lets us be more precise about register widths, to deal with the
fact that MXLEN can be changed by software.

* Mention debugger's responsibility when XLEN!=DXLEN

* DXLEN is the widest supported XLEN.

This simplifies implementations and validation by explicitly preventing
the debugger from doing things that aren't very useful anyway.

Fixes #320.

Note, the e.g. must be followed by a backslash to tell LaTeX that it
isn't the end of a sentence.

$0x400$ tells LaTeX to read "0 x 400" as math, not what you want

This is an alternate to #326.

Helps with #336.

* mprven may be tied high or low.

* Make mprven WARL.

* Solution to native triggers in M mode only systems

This came out of today's debug call.

* Incorporate feedback.

This is an alternate proposal to #329, which fixing #314 in a different
way.

* Clarify that resumereq is edge-sensitive.

* Make haltreq/resumereq more consistent.

Also clarify ackhavereset.

Clarifies #318.

This enables batch reading/writing of consecutively numbered registers.

Implements #250.

Fixes #319.

Also explain why there are separate bits in the first place, and make
them both W1.

Fixes #313.

* Allow extra harts to be reset.

Based on feedback by Allen that resetting just one hart in a multi-hart
core can be complex/poorly defined, depending on implementation.

* Remove halt-on-reset requirement for extra harts.

Fixes #317.