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Intel VT-x is an instruction set that facilitates virtualization. From Wikipedia

The CPU flag for VT-x capability is "vmx" [...] "vmx" stands for Virtual Machine Extensions, which adds ten new instructions: VMPTRLD, VMPTRST, VMCLEAR, VMREAD, VMWRITE, VMCALL, VMLAUNCH, VMRESUME, VMXOFF, and VMXON.

But

With some motherboards, users must enable Intel's VT-x feature in the BIOS setup before applications can make use of it

With some motherboards, VT-x technology is disabled by default and you have to go into the BIOS settings to enable it, while with other instruction sets, like SSE, that is not the case.

What would be the disadvantages of enabling this instruction set by default?

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    This depends entirely on the manufacturer. Many will not disable it by default, many will have it enabled by default. I've had many computers and they've all come enabled by default so this is either a specific question about your manufacturer or about reasons for disabling it rather than why it is "disabled by default" which it isn't in many cases. – Mokubai Sep 30 '17 at 10:29
  • I have slightly modified the question since it already says "With SOME motherboards" – raffaem Sep 30 '17 at 12:03
  • The reason that it is disable is impossible for us to know. – Ramhound Sep 30 '17 at 13:46
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    There are potential security and performance issues with VT-x. How serious they are is difficult to say. Some manufacturers consider it safer to have the option disabled by default, and easily enabled if needed. It is a subjective decision. – LMiller7 Sep 30 '17 at 14:05
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This is how I would think as a manufacturer and what I would do:

Considerations:

  • Effect: VT-x is a gateway to the entirely "new world" in the computer, because it gives a way to functionality of cardinality of original PC. This opens equally big space for potential issues.

  • How many users will actually use that? 2-10% of people who purchase the motherboard? Others are typically happy if their computer is working for basic use.

  • What advantages will default enablement bring to ordinary users? None. Until they discover it, learn it and start using it. Unlike other board features, it won't boost system performance, improve connectivity or sound or anything else immediately visible.

  • Are there risks of default enablement for ordinary users? Yes. Security - virtual machines potentially installed as malware and running without users knowing. Or at least performance and stability. E.g. CPU usage is reported differently. We do not need basic users to blame us for anything (and call our support) while actually it's them who do not understand what they are doing.

Enable it by default? Based on the above, no. Make the enablement optional for users who are interested in it.

I think main reasons are rather non-technical than technical.

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