Merge pull request #10065 from briancain/VIRT-INSTALL-NOTE

Update installation docs with multi-hypervisor warning

website/source/docs/installation/index.html.md

@@ -36,3 +36,61 @@ necessary sometimes for Windows).
   install via your system's package manager, it is very likely that you will
   experience issues. Please use the official installers on the downloads page.
 </div>
+
+## Running Multiple Hypervisors
+
+Sometimes, certain hypervisors do not allow you to bring up virtual machines
+if more than one hypervisor is in use. If you are lucky, you might see the following
+error message come up when trying to bring up a virtual machine with Vagrant and
+VirtualBox:
+
+    There was an error while executing `VBoxManage`, a CLI used by Vagrant for controlling VirtualBox. The command and stderr is shown below.
+
+    Command: ["startvm", <ID of the VM>, "--type", "headless"]
+
+    Stderr: VBoxManage: error: VT-x is being used by another hypervisor (VERR_VMX_IN_VMX_ROOT_MODE).
+    VBoxManage: error: VirtualBox can't operate in VMX root mode. Please disable the KVM kernel extension, recompile your kernel and reboot
+    (VERR_VMX_IN_VMX_ROOT_MODE)
+    VBoxManage: error: Details: code NS_ERROR_FAILURE (0x80004005), component ConsoleWrap, interface IConsole
+
+Other operating systems like Windows will blue screen if you attempt to bring up
+a VirtualBox VM with Hyper-V enabled. Below are a couple of ways to ensure you
+can use Vagrant and VirtualBox if another hypervisor is present.
+
+### Linux, VirtualBox, and KVM
+
+The above error message is because another hypervisor (like KVM) is in use.
+We must blacklist these in order for VirtualBox to run correctly.
+
+First find out the name of the hypervisor:
+
+    $ lsmod | grep kvm
+    kvm_intel             204800  6
+    kvm                   593920  1 kvm_intel
+    irqbypass              16384  1 kvm
+
+The one we're interested in is `kvm_intel`. You might have another.
+
+Blacklist the hypervisor (run the following as root):
+
+    # echo 'blacklist kvm-intel' >> /etc/modprobe.d/blacklist.conf
+
+Restart your machine and try running vagrant again.
+
+### Windows, VirtualBox, and Hyper-V
+
+If you wish to use VirtualBox on Windows, you must ensure that Hyper-V is not enabled
+on Windows. You can turn off the feature by running this Powershell command:
+
+```powershell
+Disable-WindowsOptionalFeature -Online -FeatureName Microsoft-Hyper-V-All
+```
+
+You can also disable it by going through the Windows system settings:
+
+- Right click on the Windows button and select ‘Apps and Features’.
+- Select Turn Windows Features on or off.
+- Unselect Hyper-V and click OK.
+
+You might have to reboot your machine for the changes to take effect. More information
+about Hyper-V can be read [here](https://docs.microsoft.com/en-us/virtualization/hyper-v-on-windows/quick-start/enable-hyper-v).