NVIDIA provides cache eviction priority hints such as evict_first and evict_last on recent GPUs.These hints allow users to specify the eviction priority that should be used for individual cache lines to improve cache utilization.However, NVIDIA does not disclose the microarchitectural details of these hints or cache eviction behaviors when using them, which makes their security and performance implications unclear.In this paper, we first reverse engineer the detailed comprehensive behaviors of these eviction priority hints.Then based on our findings, we analyze their impact on system security and performance.First, we found that these priority hints introduce new security problems.Specifically, we develop a new covert channel using the evict_first priority hint, which is more efficient than existing GPU covert channels.We also demonstrate a performance degradation attack using the evict_last priority hint, which is more stealthy compared to the known methods.Second, from the performance perspective, we show that marking a cache line as evict_last does not always keep it in the cache.In fact, if more than 12/16 (or 3/16, depending on the driver version) of the L2 cache size worth of data are marked as evict_last, cache thrashing can occur, which leads to performance degradation for real GPU workloads.