Hyper-V RC1: Hyper-V launch failed; Processor 0x4 does not provide the features necessary to run Hyper-V (leaf 0x80000008, register 0x0: features needed 0x26, features supported 0x24).

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There's been many emails and posts about this since I replied to a Technet Forum post a few weeks back. This is a known issue we've investigated and identified as a bug in Hyper-V RC1. Limiting the number of processors to 1 physical sockets worth of cores using msconfig (eg 4 if dual quad procs, 2 if dual dual-procs) is one option. We have fixed this for the next public release. Although I can't say what and when that will be, for now, short of rebuilding and sticking with RC0, the numproc workaround is the only fix. With one exception to date, this has only been seen with dual-socket quad-core Intel processors (the one exception was on a box with Intel Xeon 5160 dual core processors). Additionally, if you see similar launch failures in the event log with RC1, I would strongly recommend you upgrade to the latest BIOS for your machine. In several instances, we have seen alternate "features needed" codes which a BIOS fixed.

Cheers,
John.