Intermittence-Aware Speculative Page Coloring for Secure NVM

This paper revisits volatile scratchpad memory (SPM) for data confidentiality in an energy harvesting system (EHS) that suffers frequent power outages and thus features nonvolatile memory (NVM). Our insight is that low-cost data confidentiality can be achieved by using on-chip SPM as main memory and off-chip NVM as secondary storage. This strategic design naturally provides EHS with secure main memory (as SPM data vanish on an outage), encrypting them only when persistent in NVM. To realize this in a purely software manner, we introduce Manatee, a compiler-directed memory management scheme that logically structures NVM pages and preallocates them in SPM. A key innovation of Manatee is its intermittenceaware speculative page coloring that is explicitly designed taking into account frequent power outages. This design limits the requirement for page conflict resolution to the typically brief poweron period of EHS, providing greater flexibility in page coloring. If the period is longer than usual, two pages that would have been separated by an outage may instead be accessed within the same cycle, competing for the same SPM frame. To handle such a page miss (misspeculation), Manatee devises page manager runtime that evicts the old SPM frame, encrypting and persisting it, and fetches the new NVM page decrypting it upon retrieval. Our evaluation shows that the page miss rate is not significant $(\approx 1 \%)$, and Manatee delivers a $2-3 \times$ speedup over the state-of-the-art work. These results demonstrate that Manatee enables practical and secure memory for EHS, providing data confidentiality with minimal performance overhead.