Great read! Some boiled-down takeaways:
- Predictor updates may be deferred until sometime after a branch retires. Makes sense, otherwise I guess you'd expect that branches would take longer to retire!
- Dispatch-serializing instructions don't stall the pipeline for pending updates to predictor state. Also makes sense, considering you've already made a distinction between "committing the result of the branch instruction" and "committing the result of the prediction".
- Privilege-changing instructions don't stall the pipeline for pending updates either. Also makes sense, but only if you can guarantee that the privilege level is consistent between making/committing a prediction. Otherwise, you might be creating a situation where predictions generated by code in one privilege level may be committed to state used in a different one?
Maybe this is hard because "current privilege level" is not a single unambiguous thing in the pipeline?
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