Abstract

ColumnDisturb is a new data-disturbance error in which activations to an aggressor row cause bitflips in a victim row located hundreds of rows away (intra-subarray bitflips) and in victim rows in adjacent subarrays (inter-subarray bitflips). Existing row-level Rowhammer defenses are unable to tolerate ColumnDisturb, as they perform mitigations in a small localized region around the aggressor row. Intra-subarray ColumnDisturb can be tolerated by solutions (such as SALT and REGA) that operate at subarray granularity. However, to tolerate inter-subarray ColumnDisturb, such solutions must be extended with _ColumnDisturb Protection (CDP)_ , which performs additional _Adjacent-Counter Increment (ACI)_ for the neighboring subarrays. The ACIs ensure that adjacent subarrays also undergo mitigation, even if they receive no demand activations. Unfortunately, because ACIs occur at a 200% rate relative to demand activations, they effectively increase the activations perceived by the bank to 3x, which causes significant slowdowns (17% at a TRHD of 500) and refresh overheads. The goal of our paper is to tolerate ColumnDisturb while incurring negligible overheads. We propose _CODA_ , a ColumnDisturb mitigation that significantly reduces the rate of ACI required to securely tolerate ColumnDisturb. We present three variants of CODA. First, _CODA-E (Evade)_ , which leverages the insight that ACI can be skipped if the neighboring subarray receives a demand activation, and reduces ACI by 2x. Second, _CODA-F (Fraction)_ , which uses the timing duration of ColumnDisturb to do only a fractional increment for ACI, thereby reducing the rate of ACI by 2x-16x. Finally, _CODA-G (Gangskip)_ , which is designed for solutions that operate at multi-subarray granularity and skips ACI for neighboring subarrays within the same gang, further reduces overall ACI by 2x-8x. Overall, CODA reduces ACI by 12x-1300x, thereby making it possible to tolerate ColumnDisturb while incurring zero performance and power overhead.