I just wanted to make a post about this because it took me forever to figure out. And just in case someone else overlooked it and is searching endlessly for an answer as I was for hours. If you're not making it past grub on your virtual machine, whether that be getting stuck on uevents or rcu related CPU stall, when using more than one CPU then you should try checking for "Virtual Machine Platform" in your windows features. I looked for hours, and nobody mentioned this feature, and because of the name I assumed it was used by virtual box. Especially considering I could enable nesting paging and multiple CPUs in VirtualBox. Not only that, but it worked before. But disabling it solved the issue on all my machines. It also fixed a weird issue that prevented the guest from displaying in 720p or higher.
TL; DR: Disable "Virtual Machine Platform" in Windows Features to use multiple CPUs for VirtualBox and similar software.
PS: if you know how to tag more phrases to help this show in a google search result, please lmk.
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