The genvar stuff was switched to readable names to make it easier
to understand for the first time. In the LRUUpdate logic for loop,
a special case was added for simpler logic in the case of the root
node, to hit coverage.
For coverage.
LRUWriteEn is gated by FlushStage in cache.sv,
so removing the signal completely avoids future confusion.
Update cache.sv to reflect cacheLRU edit.
Spent a long time trying to find a way to see if this condition was
possible, only to become relativly convinced that it isn't.
Basically, since RO cache writes only happen after a long period of
stall for the bus access, there's no way a flushD can be active
at the same time as a RO cache write. TrapM causes a FlushD, but
interrupts are gated by the "commited" logic and the exception
pipeline stalls.
I feel like its worth keeping the logic to be safe
so I've chosen to exclude it rather than explicitely remove it.
The cachefsm hardwired ClearValid logic to zero.
This signal might've been added to potentially add extra functionality
later. Unless that functionality is added, however, it negatively
impacts coverage. If the goal is to maximize coverage, this signal
should be removed and only added when it becomes necessary.
I found the problem. We use a Committed(F/M) signal to indicate the IFU or LSU has an ongoing cache or bus transaction and should not be interrupted. At the time of the mret, the IFU is fetching uncacheable invalid instructions asserting CommittedF. As the IFU finishes the request it unstalls the pipeline but continues to assert CommittedF. (This is not necessary for the IFU). In the same cycle the LSU d cache misses. Because CommittedF is blocking the interrupt the d cache submits a cache line fetch to the EBU.
I am thinking out loud here. At it's core the Committed(F/M) ensure memory operations are atomic and caches don't get into inconsistent states. Once the memory operation is completed the LSU/IFU removes the stall but continues to hold Committed(F/M) because the memory operation has completed and it would be wrong to allow an interrupt to occur with a completed load/store. However this is not true of the IFU. If we lower CommittedF once the operation is complete then this problem is solved. The interrupt won't be masked and the LSU will flush the d cache miss.
This requires a minor change in the cachebusfsm and cachefsm. I will report back after I've confirmed this works.
