Persistent Evictions Combined With Interconnect Backpressure Might Stall Write-Back No-Allocate Stores

Versal AI Core Series Production Errata (EN313)

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1.6 English

AMD Answer 73151

In a coherent ACE system, a write-back no-allocate (WBNA) store might be stalled if WriteUnique/WriteLineUnique (WU/WLU) transactions are enabled and the store is attempted when one or more cache evictions are pending. ACE requires that WU/WLU transactions do not bypass any outstanding evict type transactions (WriteBack/WriteEvict/WriteClean). To satisfy this requirement, a microarchitectural hazard is used to force a replay if a WU/WLU transaction is attempted when an eviction is pending. In rare scenarios with a persistent stream of L2 cache linefills and associated evictions, combined with significant backpressure in the interconnect, and with specific timing, it is possible for a WBNA store to be stalled indefinitely.

This is a third-party errata (Arm, Inc. 851022); this issue will not be fixed.