After a bit of investigation, it seems the following are mitigations that could be worthwhile to briefly discuss in the book:

• (SW/HW) Flushing branch state when switching between sandboxes/distrusting contexts.
• (HW) partitioning branch predictor "real estate", so it doesn't get shared.
• (SW): remove branches, e.g. if-conversion - especially the branches that are dependent
  on secret data. algorithmically or through if-conversion.
• (HW): randomize branch predictor indexing, i.e. how branches alias.
• (HW/SW): Prevent predicting sensitive branches.
• (SW): obfuscating a set of conditional branches by converting them into a single indirect branch.
  Zigzagger, see [@lee2017].