Hyper-V: The Mystery of Limit Processor Functionality - Part 1

Natasha Mocke, Recently I became rather intrigued with Hyper-V's Limit Processor Functionality (LPF) function. One little checkbox became such an obsession that I start wasting hours of my time trying to find out exactly what it does. The dialogue says, "Limit processor functionality to run an older operating system such as Windows NT on this […]

Natasha Mocke, Recently I became rather intrigued with Hyper-V's Limit Processor Functionality (LPF) function. One little checkbox became such an obsession that I start wasting hours of my time trying to find out exactly what it does. The dialogue says, "Limit processor functionality to run an older operating system such as Windows NT on this virtual machine".

That sounds pretty plausible because the newer multiple core processors with hardware accelerated virtualization were not around in the old days these operating systems were around.

Simple I thought. I'd find a selection of system information tools and run those with the check box flagged and without. I'd then compare the processor details and get the answer. So I chose three popular tools I thought would help me out. They were Everest, SiSoft Sandra and CPU-Z. They actually ended up adding to my confusion!

To understand the results, it is important to understand what platforms I was testing on.

In order to test a base OS without a hypervisor, I ran Vista 32-bit and tested the CPU results on that.

I then ran tests on the parent partition for Windows Server 2008 with the Hyper-V role enabled, and then two child partitions running Windows XP with the Limit Processor Functionality checkbox on and off. I used XP SP2 because it is not enlightened and is not aware of VMBus. I also tried Windows XP SP3 with the Integration components for Hyper-V, but that made no difference to the result sets, so the results I'm providing apply to any version of Windows XP. The child partitions were only provided with a single CPU, even though I have a Core 2 Duo processor because Hyper-V only provides multi processor support for Windows Server 2003 and 2008 child partitions. The tools gave the same results for the Windows Server 2008 parent partition and the Windows XP child partition without LPF set, so the former is not shown.

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Hyper-V, Hypervisior, Windows Server 2008, WS2008, Processor, CPUID, Limit Processor Functionality, Troubleshooting, Knowledgebase, LPF