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Sounds tricky. For example if your CPU has hyperthreading, cache misses in speculative execution could leak to the other thread running on the same core, becaus
by CodesInChaos 9y ago
Sounds tricky. For example if your CPU has hyperthreading, cache misses in speculative execution could leak to the other thread running on the same core, because they share computational resources and the utilization of those is affected by cache misses in speculative execution.
- _chris_ 9y agoYou tag things and prevent bypassing uncommitted data from one thread to another. For example, it's a mistake (that's been made by other ISAs!) to allow two hardware threads to bypass store data out of the uncommitted StoreQueue from one thread to another. Your memory consistency model violates single-copy atomicity and now the programmer's life is much harder.
- CodesInChaos 9y agoI'm talking about the shared computation resources (e.g. ALU), not about shared caches. Cache hits and cache misses affect how much and when one thread utilizes the ALU, which means the other thread sharing it won't be able to use the same resources for computation. Since speculative execution also uses these shared resources, it leaks information about its cache misses to the other thread, even if a rolled-back speculative execution doesn't modify any caches.