HyperV_Mon is a free tool for monitoring and diagnosing performance issues with Hyper-V.
After prompting from a user with a lot more processors then the test systems here, I updated the HyperV_Mon tool to handle resizing in general, and to adjust the root partition processor display. This new version is a minor release (1.8.0).
In the works will be a version 2.0 that will look at 2008 R2 with SP1 changes. This should come out in Q1 of next year.