Security And Future Of Hyper-Threading

Hyper-threading officially termed Hyper-Threading Technology or HTT is an Intel-proprietary technology used to improve parallelization of computations performed on PC microprocessors via simultaneous multithreading. It is an improvement on super-threading. It debuted in U.S. Patent 4,847,755 (Gordon Morrison, et. al) and can be seen in use on the Intel Xeon processors and Pentium 4 processors. The technology improves processor performance under certain workloads by providing useful work for execution units that would otherwise be idle, for example during a cache miss. A Pentium 4 with Hyper-Threading enabled is treated by the operating system as two processors instead of one.

Hyper-threading relies on support in the operating system as well as the CPU. Conventional multiprocessor support is not enough to take advantage of hyper-threading. For example, even though Windows 2000 supports multiple CPUs, Intel does not recommend that hyper-threading be enabled under that operating system.

Security
In May 2005 Colin Percival presented a paper, Cache Missing for Fun and, demonstrating that a malicious thread operating with limited privileges can monitor the execution of another thread through their influence on a shared data cache, allowing for the theft of cryptographic keys. Note that while the attack described in the paper was demonstrated on an Intel Pentium 4 with Hyper Threading processor, the same techniques could theoretically apply to any system where caches are shared between two or more non-mutually-trusted execution threads; see also side channel attack.

Future
Older Pentium 4 based CPUs use Hyper-Threading, but the current-generation Pentium M based cores, Merom, Conroe, and Woodcrest, do not. Hyper-Threading is a specialized form of simultaneous multithreading (SMT), which has been said to be on Intel’s plans for the generation after Merom, Conroe and Woodcrest.

More recently Hyper-Threading has been criticized as being energy inefficient. For example, specialist low power CPU design company ARM has stated SMT can use up to 46% more power than dual CPU designs. Furthermore, they claim SMT increases cache thrashing by 42%, whereas dual core results in a 37% decrease. These considerations are claimed to be the reason Intel dropped SMT from the following micro architecture.

However, for 2008 Intel has claimed that the upcoming Nehalem will see the return of Hyper-Threading. Nehalem is projected to contain up to 8 cores and will be able to effectively scale 16+ threads.