I recently got my hands on a new laptop with an i3-2310M which supports the VT-x extension. I want to guest install Windows 7.

  • How do I use VT-x on KVM?
  • How will the guest OS benefit/what more can the guest OS access with KVM using VT-x? Does it need additional driver installation in guest OS?
  • This is my first time properly owning and using a 64-bit machine--do I install 32-bit or 64-bit guest OS?
  • Finally, does any other virtualization software does a better job of using VT-x right now?

1 Answer 1

  1. VT-x (or "VirTualization eXtensions" for Intel, same as AMD-V for AMD) will be used by default by KVM if available.
  2. The guest OS benefits by running faster -- put simply, VT-x eliminates the overhead (or translation) that was previously involved in letting the guest OS access memory and use the CPU. See this Intel Technical Journal page for technical details.

    • Although no additional drivers are required to use VT-x per se, it's a good idea to install the "tools" or drivers which allow the guest to access disk/network as much faster "paravirtualized" devices rather than emulated ones. If your processor also supports VT-d, your guest OS may benefit from that too.
    • For KVM, this can be enabled with the virtio devices, for example:
    qemu-system-x86_64 -boot c -drive file=/images/xpbase.qcow2,if=virtio -m 384 -netdev type=tap,script=/etc/kvm/qemu-ifup,id=net0 -device virtio-net-pci,netdev=net0

    see this KVM wiki page for more info.

  3. 64-bit, as long as you have more than 1GB or so of RAM; especially if you're planning to do virtualization.
  4. VT-x/AMD-V are quite mature technologies at this point and I doubt that KVM lags behind any competing hypervisor (VMware, Virtualbox, etc.).
  • Thank you for your clear and concise answer. For the second point, I wasn't even sure if such driver existed for KVM in the first place--can you point me to where I can get them? Do you mean assigning more than 1 GB of RAM? The system in question enjoys 4 GB of RAM, I was thinking of assigning a maximum 1.5 GB to the guest, since it's Windows 7. And lastly, I heard that KVM does not do much to provide GPU power. VirtualBox is out of question with their supposedly messy drivers. As far as I'm aware, only VMware provides anything like Aero desktop or something.
    – Oxwivi
    Jul 18, 2012 at 18:44
  • @Oxwivi: sorry I missed this one. I edited the answer showing how to use paravirtualization in KVM. What I meant was that if you are assigning more than 1 GB or RAM, go 64-bit. Please let me know if you have any other questions -- if not, please consider accepting the answer. Thanks!
    – ish
    Aug 18, 2012 at 10:46
  • I do not seem to have VT-d support. And I've realized I don't understand the difference between the two. googling through I gather VT-d allows hardware paravirtualization and VT-x simply allows guest processes working at their intended privilege levels. Can I enable things like 3D for guest in that case?
    – Oxwivi
    Aug 20, 2012 at 5:16
  • @Oxwivi: VT-d allows the virtual machine to directly access hardware resources, bypassing the host. e.g. if you had two graphics cards, you could give your VM direct access to one of them for doing computing (CUDA), etc. VT-d is NOT necessary for anything else; 3D support in KVM, etc. will work just fine.
    – ish
    Aug 20, 2012 at 5:18
  • As far as I'm aware there's no driver for 3D support in KVM?
    – Oxwivi
    Aug 21, 2012 at 10:31

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