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I'm running Virtual Box 4.3.18 r96516 - the latest version at this time.
Host OS is Windows 8.1 x64.
Guest OS is Windows 10 Technical Preview x64

I successfully installed Windows 10 Technical Preview x64 in VBox. Usually, the virtual machine runs fine. But every now and then it 'forgets' that VT-x is enabled in my BIOS. The result is the well-known error: VT-x is not available. (VERR_VMX_NO_VMX).

If I restart my computer, VBox then 'remembers' that VT-x is enabled, and the virtual machine starts and runs well. After I have started and exited the virtual machine a few times, VBox then 'forgets' again than VT-x is enabled and I have to restart my computer to make it 'remember' again.

Hyper-V is not installed, as far as I know. (No check mark in Hyper-V in "Turn Windows features on or off.")

Possible clue: Without first considering the wisdom of doing this, I attempted to add guest additions to the virtual machine. Nothing happened when I tried this and no error messages.

How can I get Virtual Box to remember that VT-x is enabled?

EDIT: I re-started my computer just now and, of course, Virtual Box remembers that VT-x is enabled. However, the virtual machine is now all messed up. It thinks the guest operating system is Windows XP 32-bit. I may just scrap the whole thing and start from scratch.

Is VirtualBox crashing when you run Win10? If so then I imagine that could leave VT-x in an unstable state. That's what you get when you run an alpha release guest OS!

If you want to continue this discussion then please provide a VM log file of a failed session: with VM shut down you right click the VM in the GUI, select "Show Log", save "VBox.log" to a file (not interested in the older logs), add it to a zip and attach the zip here.

Thanks for your help. I decided to completely delete the existing virtual machine and create a new one. Now older (by a day) and wiser I think I did a better job of handling the virtual machine, e.g., ejecting the install .iso the 'right' way, didn't try to add guest additions. So far, so good. Next time I post with a problem I'll follow your advice about submitting the log.

EDIT: I spoke too soon. It was only a few hours before my virtual machine once again 'forgot' that VT-x was enabled.

In the attached log, see the line that says "WARNING! 64-bit guest type selected but the host CPU does NOT support HW virtualization." Oh yeah? So why did the virtual machine work perfectly earlier in the day?

That error suggests that something else has grabbed VT-x, and has not let go. I can't tell from here what that is. The log you provided isn't terribly useful since the error condition existed before that log started.

You didn't answer my question: does VirtualBox ever crash out when you are using Win10?

Ultimately - I assume you're aware that the Win10 Preview is an unsupported alpha, so it either works for you or it doesn't.

Virtual Box has not crashed while running the Windows 10 Technical Preview.

I do not see the connection between my problem and Windows 10. When the problem manifests, Virtual Box does not even allow the virtual machine to start because it detects no support for VT-x - not because the guest is Windows 10 Technical Preview.

As a test, I also created a virtual machine for the 32-bit version of the Windows 10 Technical Preview. This virtual machine also failed to start (until I restarted my computer) and for the same reason that the 64-bit version failed to start: no support for VT-x detected. The same thing happened even after I completely removed the 64-bit virtual machine.

Finally, I have read reports of the same thing happening to earlier versions of Virtual Box running earlier versions of Windows. So I have yet to be persuaded that the Windows 10 Technical Preview could be a root cause of this problem. It seems rather to originate in Virtual Box.

If you're running other applications that take up VT-x that this can happen. Is Hyper-V enabled on your host? Or any other hypervisor like VMWare installed?

I mentioned in my original post that Hyper-V is not enabled on my host computer. No other hypervisors are installed.

Is there some way to detect whether any of my installed software is somehow using VT-x?

Does this by any chance happen after you suspend/resume or hibernate your host system?

Hibernation is disabled on my system.

Here's how I use Virtual Box with Windows 10: I'll start the virtual machine, play with W10 for half an hour or so, then shut down the virtual machine (from the Windows Start menu) and close Virtual Box. I do this several times a day. My system is working throughout this time, i.e., it's not sleeping (S3), although the monitor and hard drives may occasionally power off due to my power options settings.

After a few hours, Virtual Box 'forgets' that VT-x is enabled.

In case there's any doubt that my system supports VT-x, see the Intel Processor Identification Utility report.

No one doubts that you have the ability to use hardware virtualization. It is apparent since you can start the 64-bit guest after a reboot. You have said that you do not have Hyper-V so the only thing left is something else is using it or I have a new idea for you to research. The model you specified was a late 2009 model and it is possible that the bios needs to be updated.

I wish a BIOS update was available, but the last one was issued in 2011 and I have it.

I've been going through my installed software to see if anything might incorporate a hypervisor. I only have one guess: My security software suite (Kaspersky Internet Security) includes a sandboxed browser intended for use in financial transactions. This browser might or might not include a hypervisor. In any case, I disabled this feature shortly after I installed the software, so this is not likely the cause of my problem.

All of the Windows Hyper-V services are stopped. (They are on manual startup.)

It's a good thing I only use Virtual Box for Windows 10 at this time, so no big deal if Virtual Box has an occasional problem.

There were some users who described that stopping Hyper-V wasn't sufficient, they had to completely disable it by removing the Hyper-V role.

I think my problem may be caused by my security software: Kaspersky Internet Security 2015. The software installs a driver (klhk.sys) that uses hypervisor technology - the Windows VMM. Another writer in a different user forum discovered that the same driver prevented him from adding Hyper-V to Windows 8.1.

I'll have to do more poking around to be sure about this, but I believe I'm headed in the right direction.

Bulldog wrote:I think my problem may be caused by my security software: Kaspersky Internet Security 2015. The software installs a driver (klhk.sys) that uses hypervisor technology - the Windows VMM. Another writer in a different user forum discovered that the same driver prevented him from adding Hyper-V to Windows 8.1.

Who needs viruses -- AV software does all the damage