Hyper-Threading Technology delivers significantly improved architectural performance at a lower-than-traditional power consumption and die size cost. However, increased logic complexity is one of the trade-offs of this technology. Hyper-Threading Technology exponentially increases the micro-architectural state space, decreases validation controllability, and creates a number of new and interesting micro-architectural boundary conditions. On the Intel® Xeon^TM processor family, which implements two logical processors per physical processor, there are multiple, independent logical processor selection points that use several algorithms to determine logical processor selection. Four types of resources: Duplicated, Fully Shared, Entry Tagged, and Partitioned, are used to support the technology. This complexity adds to the pre-silicon validation challenge.

Not only is the architectural state space much larger (see "Hyper-Threading Technology Architecture and Microarchitecture" in this issue of the Intel Technology Journal), but also a temporal factor is involved. Testing an architectural state may not be effective if one logical processor is halted before the other logical processor is halted. The multiple, independent, logical processor selection points and interference from simultaneously executing instructions reduce controllability. This in turn increases the difficulty of setting up precise boundary conditions to test. Supporting four resource types creates new validation conditions such as cross-logical processor corruption of the architectural state. Moreover, Hyper-Threading Technology provides support for inter- and intra-logical processor store to load forwarding, greatly increasing the challenge of memory ordering and memory coherency validation.

This paper describes how Hyper-Threading Technology impacts pre-silicon validation, the new validation challenges created by this technology, and our strategy for pre-silicon validation. Bug data are then presented and used to demonstrate the effectiveness of our pre-silicon Hyper-Threading Technology validation.

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