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<?xml version='1.0' encoding='UTF-8'?>
<!DOCTYPE topic PUBLIC "-//OASIS//DTD DITA Topic//EN" "topic.dtd">
<topic xml:lang="en-us" id="hwvirt">
<title>Hardware Virtualization</title>
<body>
<p>
<ph conkeyref="vbox-conkeyref-phrases/product-name"/> enables software in the virtual machine to run
directly on the processor of the host, but an array of complex
techniques is employed to intercept operations that would
interfere with your host. Whenever the guest attempts to do
something that could be harmful to your computer and its data,
<ph conkeyref="vbox-conkeyref-phrases/product-name"/> steps in and takes action. In particular, for lots
of hardware that the guest believes to be accessing,
<ph conkeyref="vbox-conkeyref-phrases/product-name"/> simulates a certain <i>virtual</i>
environment according to how you have configured a virtual
machine. For example, when the guest attempts to access a hard
disk, <ph conkeyref="vbox-conkeyref-phrases/product-name"/> redirects these requests to whatever you have
configured to be the virtual machine's virtual hard disk. This is
normally an image file on your host.
</p>
<p>
Unfortunately, the x86 platform was never designed to be
virtualized. Detecting situations in which <ph conkeyref="vbox-conkeyref-phrases/product-name"/> needs to
take control over the guest code that is executing, as described
above, is difficult. To achieve this, <ph conkeyref="vbox-conkeyref-phrases/product-name"/> uses
<i>hardware virtualization</i>.
</p>
<p>
Intel and AMD processors have support for hardware virtualization.
This means that these processors can help <ph conkeyref="vbox-conkeyref-phrases/product-name"/> to
intercept potentially dangerous operations that a guest operating
system may be attempting and also makes it easier to present
virtual hardware to a virtual machine.
</p>
<p>
These hardware features differ between Intel and AMD processors.
Intel named its technology VT-x, AMD calls theirs AMD-V. The Intel
and AMD support for virtualization is very different in detail,
but not very different in principle.
</p>
<note>
<p>
On many systems, the hardware virtualization features first need
to be enabled in the BIOS before <ph conkeyref="vbox-conkeyref-phrases/product-name"/> can use them.
</p>
</note>
<p>
Enabling hardware virtualization is <i>required</i>
in the following scenarios:
</p>
<ul>
<li>
<p>
Certain rare guest operating systems like OS/2 make use of
very esoteric processor instructions. For virtual machines
that are configured to use such an operating system, hardware
virtualization is enabled automatically.
</p>
</li>
<li>
<p>
<ph conkeyref="vbox-conkeyref-phrases/product-name"/>'s 64-bit guest and multiprocessing (SMP)
support both require hardware virtualization to be enabled.
This is not much of a limitation since the vast majority of
64-bit and multicore CPUs ship with hardware virtualization.
The exceptions to this rule are some legacy Intel and AMD
CPUs.
</p>
</li>
</ul>
<note type="caution">
<p>
Do not run other hypervisors, either open source or commercial
virtualization products, together with <ph conkeyref="vbox-conkeyref-phrases/product-name"/>. While
several hypervisors can normally be
<i>installed</i> in parallel, do not attempt to
<i>run</i> several virtual machines from competing
hypervisors at the same time. <ph conkeyref="vbox-conkeyref-phrases/product-name"/> cannot track what
another hypervisor is currently attempting to do on the same
host, and especially if several products attempt to use hardware
virtualization features such as VT-x, this can crash the entire
host.
</p>
</note>
<p>
See <xref href="hwvirt-details.dita#hwvirt-details"/> for a technical discussion of
hardware virtualization.
</p>
</body>
</topic>
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