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Why would vendors by default set hardware assisted virtualization off? I have a Lenovo Thinkpad X201 (64bit) but it seems this is the case with other vendors too. I want to run some virtual machines so I'm enabling it but I'm wondering if there are negative repercussions to this that I need to watch out for in the future.

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I too am curious about this. I believe it has to do with workload on hardware, and unneeded overhead for users that don't use the features, but I am not sure enough of that to add it as an answer. – DaBaer Feb 8 '11 at 21:35
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4 Answers

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I believe it is for security reasons. A rogue hypervisor can install itself and then run the main OS, the main OS can't tell that it's running under a hypervisor (sometimes considered ring -1). It could potentially be the ultimate virus. So you have to enable explicitly if you know you want to run a hypervisor.

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I believe another reason is power-efficiency, as shutting down any parts of the processor that do not need to be used will use less power, which is especially desirable on an laptop.

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From Wikipedia: "With hardware-assisted virtualization, the VMM can efficiently virtualize the entire x86 instruction set by handling these sensitive instructions using a classic trap-and-emulate model in hardware, as opposed to software."

VMM = Virtual Machine Monitor.

My guess: It's off by default because hardware-assisted virtualization incurs very high CPU loads, which in turn requires a lot more power than normal operation. You may also see performance degradation if it's always running on extremely high load. Remember, your Thinkpad isn't a server-grade system.

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Software virtualization incurs just as high (indeed, higher) CPU loads. That doesn't make any sense. – Billy ONeal Jun 1 '11 at 6:44
Considering this wasn't a comparison of software virtualization vs hardware virtualization I don't see the point of your comment. I simply meant that utilizing more hardware features requires more cycles -> more power. – aqua Jun 17 '11 at 17:27
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I don't buy the reasoning that "we're going to turn this off because you might actually use the performance offered by your CPU if it was on". Obviously the only people who are going to care whether the switch is on are going to be running that high a load anyway. Flipping a switch to off in the BIOS by default just makes that problem worse because it forces people to use the (much higher load) of software only virtualization. – Billy ONeal Jun 17 '11 at 17:53
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if you're using virtual machines alot (especially 64bit ones - they won't even start without virtualization), leave virtualization on

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