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caches are dangerous" to "a shared L1 data cache is dangerous". This is a compromise between paranoia and performance: Unlike the L1 cache, nobody has publicly demonstrated a cryptographic side channel which exploits the L2 cache -- this is harder due to the larger size, lower bandwidth, and greater associativity -- and prohibiting shared L2 caches turns Intel Core Duo processors into Intel Core Solo processors. As before, the 'machdep.hyperthreading_allowed' sysctl will allow even the L1 data cache to be shared. Discussed with: jhb, scottl Security: See FreeBSD-SA-05:09.htt for background material.