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Well, don't I feel like a sucker.

I built a new Haswell box since my old computer was positively ancient. I built it with the explicit intention of using both virtualization and maybe even trying out TSX. Imagine my surprise when, just after the CPU return period expired, I realized that the top of the line processor doesn't have all of the features enabled!

Paying the extra for a K-series product also means giving up support for one of Haswell's key features, the TSX extensions that enable transactional memory. Intel has stripped out the VT-d device virtualization and vPro management features in the K series, as well.

In the end, enthusiasts face a rather unfortunate set of choices in Intel's Haswell-based product offerings. We can't help but think this situation wouldn't exist if AMD were putting more competitive pressure on Intel.

Is this some sort of a "soft" limitation that could be overridden, whether by microcode or BIOS etc, or is there no way to enable them on these very expensive CPUs?

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If you want the new instructions I highly suggest you return the 4770K unless its ability to be overclocked is worth missing out on those instructions. Unless you are at the cutting edge your likely safe because very few applications will use those instructions even 3 years from now. The hardware virtualization is likely the only thing you will really miss which is enabled by VT-d support. –  Ramhound Jul 22 '13 at 12:30

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It is impossible to enable as it is stripped out at hardware level. This was the case with the 3770k also.

As for their reasoning for doing so, there is no official statement. However, there is a lot of speculation on forums that it was an engineering decision; apparently VT-d becomes unstable when overclocking.

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Its likely a combination of several factors including Intel not putting the time in to make that portion of the design stable at higher frequencies. –  Ramhound Jul 22 '13 at 12:28

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