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I installed a Windows 7 professional 32 bit guest on a windows 8.1 hyper-v image. When I go to the settings and change the virtual processor count from 1 to 2 it no longer boots. Unless, I first use msconfig to limit it to using one processor. However, as soon as I remove that contstraint it again fails to boot.

My question to the gurus out there in serverfault land is how can I get my windows 7 guest to boot with 2 virtual processors. This is a build machine with complex software that takes days to put together so I am really looking to not have to reinstall just to add another virtual processor.

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migrated from Apr 12 '14 at 3:58

This question came from our site for system and network administrators.

Sounds like you're in dire need of a configuration management system. – EEAA Apr 11 '14 at 21:51
I don't know what that is, but it sounds horrible if you need a 'configuration management system' to add a vCPU. – David Houde Apr 12 '14 at 3:48
I assume your actually processor has HT support and has more then a single core? – Ramhound Apr 12 '14 at 4:02
My processor is a Core i7-2860QM CPU @ 2.5 GHz. – Joseph Southwell Apr 24 '14 at 15:03
I have verified that a new installation from the exact same installation media works with 4 virtual CPUs. – Joseph Southwell Apr 24 '14 at 16:22

This may be very obvious, but I had a similar problem with a VM after I replaced the motherboard on my computer. The new BIOS setting for Hyper-V was disabled. When I enabled it, my problems went away.

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I just checked and intel vt and intel vt direct i/o are enabled. – Joseph Southwell Apr 24 '14 at 16:18

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