I have yet to see a system whose default configuration enables MMU and directed I/O virtualization. Often this necessitates rebooting and going into the BIOS to enable it if you want, e.g., 64-bit support on your VMs.

Is there some kind of substantial processor overhead that occurs if this is switched on and you're not using virtualization? If not, then what's the reason for it being off by default?


5 Answers 5


There were some proof-of-concept rootkits like Blue Pill a while back that could own a system with VT on. After this discovery, most vendors began shipping their units with VT disabled as a general security precaution.

  • 8
    +1 People using VT on a desktop/laptop are the exception. That machine being susceptible to virii is the rule.
    – Chris S
    May 17, 2012 at 14:03
  • I'm suddenly having second thoughts about this shiny new powerful dev machine I've built that runs a bunch of VMs with hardware virtualization enabled. I don't need virtualization, but I figure it'd save me time in the long run not having to repave it every couple of months. How big an issue is this?
    – Paperjam
    May 17, 2012 at 21:16
  • 2
    Virtually non-existant (as of this comment). Blue Pill was a proof of concept. I am unaware of anything like this in the wild.
    – MDMarra
    May 17, 2012 at 21:20
  • 2
    Here is the Microsoft evaluation of the security problem, including the recommendation to switch VT off by default on client role systems: msdn.microsoft.com/en-us/windows/hardware/gg463498.aspx
    – Jpsy
    Jul 20, 2013 at 15:02
  • 5
    Microsoft's paper is no longer available on their web site, but you can read it on the Internet Archive if you want to. Sep 9, 2014 at 16:48

All the server blades we buy have virtualization enabled by default. It may be because our vendor has made a commercial decision to do so to save on so many support calls.

It is not disabled because of a processor hit, but depending on what is it being used for. It may be off by default, because enabling these features may affect peripheral access.

  • 6
    What peripheral access can this impact? I've never heard that before.
    – MDMarra
    May 17, 2012 at 13:25
  • Are you talking about VT-d when you mentioned peripheral access?
    – Zoredache
    May 17, 2012 at 19:03
  • No, it was MMU... I remember someone droning on about it recently. Something to do with Rasberry Pi.
    – Sirch
    May 18, 2012 at 9:46

Note that according to this other thread, it also seems to have some impact on performances https://superuser.com/questions/545101/why-does-hp-recommend-that-i-keep-hardware-virtualization-off

It seems relevant to me, since reduced set of instructions are more efficient. I just wouldn't have thought that changing a BIOS option could directly affect this.

I got no idea whether the impact is significant or not, but considering this and the security potential flaw, having such a rarely used feature disabled by default looks like a good choice to me.

  • 3
    The efficiency angle from the linked answer makes no sense. The instruction cache holds instructions of a software program. If the program doesn't use virtualization instructions then there is no effect on that cache. Furthermore there is no report that there is a separate execution unit that handles virtualization nor a part of the instruction decoder that can be bypassed, so there should be no efficiency difference either way.
    – Nimrod
    Mar 25, 2018 at 18:48

another reason is most user kernel function (like gettimeofday) are moved in VDSO.

sometimes under virtualization this fast path cannot be enabled.

so the system cannot:

gain the fast execution of these functions

avoid expensive switch from userland to kernel and return


In addition to the security concerns raised in @MDMarra answer, you should consider two additional points:

  1. any enabled, optional features has a small probability to have some nasty and unexpected interactions with other functionalities, requiring additional validation. PC vendors hate to spend resources on rarely used features, and client-side virtualization is one of them.

  2. I strongly suspect that due to #1, Intel basic client firmware/BIOS blob has VT disabled. So any vendor shipping a VT-enabled client need to customize that option when preparing its firmware.

That said, we are now in 2019; I see more clients with VT enabled by default. Servers are an entirely different matter: I not only always found VT enabled, but VT-d is also often active.

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