I am going to buy a new CPU. I have Intel Core 2 duo 4300. I've already chosen Intel i5-2500/i5-2500k (Sandy Bridge) as my new target CPU. I've read a lot of details about Sandy Bridge family, P67/H67 chipsets. My question is about TXT/VT-d.

As Intel COMPARISON pages shows there is a (major/minor?) difference between these two CPUs: lack of TXT/VT-d in i5-2500K model.

My question is.. when exactly and in what software do these options matter? Please, share some (or many) examples of real application of these two technologies.


There are two different aspects:

  • vt-d is a useful technology allowing VMs direct access to the hardware. Beyond the obvious performance gain the other advantage is that the VM does not have to rely on the hypervisor or host OS support of the HW.

  • TXT however is a potentially nastier kind of beast. Although it is presented by Intel as a security improvement for the user it is above all a device allowing vendor lock-in and DRM enforcement. And you and I know that you can't trust vendors to use these device in the sole customer's interest. I'm not saying we should boycott these CPUs, they will eventually be ubiquitous, however if you are in the process of selecting a CPU, that's probably one thing you might want to be aware of.

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  • Tell me more about those digital restrictions management stories. Are there any good articles about that? – AndrejaKo May 2 '11 at 11:25
  • Hmm.. I am aware of DRM "issue" but articles I had read didn't connect TXT with DRM. Thanks for pointing this out. – Krzysztof Szynter May 2 '11 at 11:34
  • @AndreajaKo, have a look at what the Linux geeks think about it or try the wikipedia TXT or TC entries. – Alain Pannetier May 2 '11 at 11:35
  • TXT seems like a worrying development indeed. Still, it's mentioned that a TPM chip is required for the system to work, so it probably won't be as dangerous until they become commonplace among home use computers. – AndrejaKo May 2 '11 at 11:46
  • @dygi and @AndreajaKo, why would a chip foundry company be interested in McAfee ? – Alain Pannetier May 2 '11 at 11:47

In theory VT-d could provide much better support for emulation and handling of hardware devices within a VM and, for example, take some of the grunt work out of moving data from the VM network interface to the host network interface.

In all honesty for home users you're probably not going to see a massive boost to your VM performance with VT-d, it looks to me more geared around multiple heavily used concurrent VMs, but there may be a bit more performance if your VM host software supports it.


Understanding VT-d: Intel Virtualization Technology for Directed I/O

Intel® Virtualization Technology for Directed I/O

Intel® Virtualization Technology (Intel® VT) for Directed I/O (Intel® VT-d) Architecture Specification

Looking at that last document (section 2.5 if you're interested) VT-d's main features appear to be allowing more separation of I/O to devices from VMs (so that each VM can effectively have it's own buffer to a device) and allowing VMs with appropriate drivers to have nearly direct access to the host hardware while still remaining protected and secure.

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  • Hmm.. I do work with virtual machines (few instances simultaneously max) but they tend not to be heavily used all the time. I suppose It would not make a major difference not having VT-d now but.. I change my desktop configuration no more than every two - three years. Trying to make best (near future) decision possible. – Krzysztof Szynter May 2 '11 at 8:48
  • I honestly wouldn't base my decision on what processor to buy based on whether or not it has VT-d, I really don't think it will provide enough of a speed boost for you to really notice or care about. If I were you I'd spend whatever money you save on a bigger hard drive, more memory or better graphics card, VT-d is not a "Killer Feature" for home users. – Mokubai May 2 '11 at 9:05
  • I generally love to experiment with different possibilities, as OSes, distros, and so on. The reason I ask about these features is that I would like to know, if there are some interesting scenarios where having VT-d/TXT is a must (and not having is like "u cannot do / run / try "sth"). As an example that i've recently read / found is Qubes where it seems that having VT-d makes more than a minor difference (though not strictly required). – Krzysztof Szynter May 2 '11 at 10:08
  • I was tempted to mention that VT-d looks to be most useful where a large number of VMs are in use simultaneously, and that looks to be the main feature of Qubes. Qubes looks pretty neat and I can see why it would work better with VT-d, as it can use VT-d to remap all the networking I/O from the other VMs into its network domain VM. I'll have to look into Qubes, it looks pretty neat for a secure machine... – Mokubai May 2 '11 at 17:36

I don't know much about TXT, but VT-d is great! Like mentioned above it allows hardware devices to be assigned directly to a virtual machine. From the VM's point of view, it is like the hardware is directly plugged in.

My use case for wanting this technology was to setup a virtualized multi-seat computer. I wanted to setup multiple VM's each as their own separate workstation, and I wanted them to have the speed and power of non-emulated 3D graphics. I.e. two or more separate gaming VM's running from the same physical box! With each VM having it's own monitor, graphics card, keyboard and mouse.

I've got two seats currently setup on my machine. Please be aware that it is non-trivial to setup and maintain. You also have to be careful with the make and model of your CPU, Motherboard and the BIOS you choose for the setup, as they all have to support VT-d for it to work. I've got it to work with the following hardware and software.


  • Intel i5-2500 (non-k)
  • AsRock Extreme 4 Gen 3 Z68 Motherboard
  • ATI HD 6850
  • ATI HD 6450
  • 16gb ram
  • 1tb storage
  • 3 x Monitor/Keyboard/Mouse (the extra monitor plugs in to dom0)


  • Dom0 - Debian 6, custom 3.1 kernel and Xen 4.1.2
  • 1st DomU - Win 7 Home Premium
  • 2nd DomU - Win 8

My setup isn't exactly the same as this guys, but check out the clip for a demonstration of what I'm talking about: http://www.youtube.com/watch?v=Gtmwnx-k2qg

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