Partitioned Tags, Shared Data: Reconciling Strict Cache Isolation with Write-Shared Coherence

SCP solves the problem that write-shared coherence fails under strict cache partitioning, a decade-old barrier to deploying eviction-based side-channel defenses in secure shared-OS settings. The method partitions only the tags while sharing a single data pool, sizes the data pool to prevent capacity-driven cross-partition eviction, and routes writes to the LLC after a leakage threshold to mitigate coherence-based leakage. Experimental evidence from gem5 shows SCP mitigates Prime+Probe, Flush+Reload, and shared-writeable-line attacks to no better than random guessing, with a +2.8% LLC SRAM hardware cost and IPC within 0.3% of DAWG on SPEC CPU2017. This matters because SCP reconciles strict cache isolation with write-shared coherence, enabling secure partitioning without sacrificing performance or coherence correctness.