NoHammer: Preventing Row Hammer With Last-Level Cache Management

Abstract

Row Hammer (RH) is a circuit-level phenomenon where repetitive activation of a DRAM row causes bit-flips in adjacent rows. Prior studies that rely on extra refreshes to mitigate RH vulnerability demonstrate that bit-flips can be prevented effectively. However, its implementation is challenging due to the significant performance degradation and energy overhead caused by the additional extra refresh for the RH mitigation. To overcome challenges, some studies propose techniques to mitigate the RH attack without relying on extra refresh. These techniques include delaying the activation of an aggressor row for a certain amount of time or swapping an aggressor row with another row to isolate it from victim rows. Although such techniques do not require extra refreshes to mitigate RH, the activation delaying technique may result in high-performance degradation in false-positive cases, and the swapping technique requires high storage overheads to track swap information. We propose NoHammer , an efficient RH mitigation technique to prevent the bit-flips caused by the RH attack by utilizing Last-Level Cache (LLC) management. NoHammer temporarily extends the associativity of the cache set that is being targeted by utilizing another cache set as the extended set and keeps the cache lines of aggressor rows on the extended set under the eviction-based RH attack. Along with the modification of the LLC replacement policy, NoHammer ensures that the aggressor row's cache lines are not evicted from the LLC under the RH attack. In our evaluation, we demonstrate that NoHammer gives 6% higher performance than a baseline without any RH mitigation technique by replacing excessive cache misses caused by the RH attack with LLC hits through sophisticated LLC management, while requiring 45% less storage than prior proposals.