Comment 3 for bug 1647260

I can also confirm that the Virtualbox 5.1.10 run on a Harpertown machine which reports the following in SystemProfiler

Hardware Overview:

  Model Name: Mac Pro
  Model Identifier: MacPro3,1
  Processor Name: Quad-Core Intel Xeon
  Processor Speed: 2.8 GHz
  Number of Processors: 2
  Total Number of Cores: 8
  L2 Cache (per Processor): 12 MB
  Memory: 10 GB
  Bus Speed: 1.6 GHz
  Boot ROM Version: MP31.006C.B05
  SMC Version (system): 1.25f4
  Serial Number (system): G884810ZXYL
  Hardware UUID: 07931828-CA7B-5776-9F2B-8685D5270A85

So you can see that Virtualbox is currently misrepresenting the available hardware as having 16 cores by blindly assuming that every cpu has hyper threading available in it.