[dv] Implement testing and coverage for various mstatus states

[GregAC]

I recently discovered Ibex enabled/disabled interrupts in U mode based on the mstatus.mie bit which is a violation of the spec (mie applies only to M-mode in U mode it is ignored, only the per interrupt MIE CSR bits should be considered. This is fixed in #1824

I found this when adding interrupts to the debug single step test, which does execute code in U mode with mstatus.mie set to 0, but clearly other testing (in particular the interrupt tests) don't do this otherwise we would have found this bug earlier.

This indicates a gap in our testing. We should add some coverage over different mstatus states that checks we see various states across U-mode/M-mode and with interrupts/debugs/exceptions occurring.

Coverpoints:

• cross - priv_modes, interrupts, mstatus.MIE (excluding mstatus.MIE == 1 && priv_mode == MACHINE as interrupt will not be taken in that case)
• cross - priv_modes, exceptions

Existing tests will require expanding to meet the new coverage

estimate 4

[GregAC]

Consider waiving this work for V2, postponing to V3 to discuss in V2 meeting.

[johngt]

I took a quick look at both review meeting notes and couldn't see an obvious waiver discussion on this so assume that this is still live?

[GregAC]

@johngt This has been waived for V2, I've added a comment to the notes tagging you to highlight where

[GregAC]

estimate range: 2 - 4