StallM while FPU Div/Sqrt is in DONE state

[infinitymdm]

This discussion is focused on covering this line in fpu/fdivsqrt/fdivsqrtfsm.sv.

In order to cover this line, we need to set StallM high - aka stall the memory stage - while the FPU is in the DONE state of completing a square root or division operation. The timing for this is a bit tricky, and we haven't quite figured it out so far.

Ideas:

• Try to stall instruction cache by reading an unexpected location while the FPU is just reaching the done stage
  • We couldn't get the timing to work out here. No matter what we tried, the FPU would complete the operation before the icache stalled.
  • In the process helping us test this, Ross discovered Possible bug in Wally Branch Predictor jalr ra t0 used as long call incorrectly interpreted as return #250
• Try to stall data cache by reading an unexpected address while the FPU is just reaching the done stage