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<chapter id="TechnicalBackground">
<title>Technical background</title>
<para>The contents of this chapter are not required to use VirtualBox
successfully. The following is provided as additional information for
readers who are more familiar with computer architecture and technology and
wish to find out more about how VirtualBox works "under the hood".</para>
<sect1 id="vboxconfigdata">
<title>Where VirtualBox stores its files</title>
<para>In VirtualBox, a virtual machine and its settings are described in a
virtual machine settings file in XML format. In addition, most virtual
machine have one or more virtual hard disks, which are typically
represented by disk images (e.g. in VDI format). Where all these files are
stored depends on which version of VirtualBox created the machine.</para>
<sect2>
<title>Machines created by VirtualBox version 4.0 or later</title>
<para>Starting with version 4.0, by default, each virtual machine has
one directory on your host computer where all the files of that machine
are stored -- the XML settings file (with a
<computeroutput>.vbox</computeroutput> file extension) and its disk
images.</para>
<para>By default, this "machine folder" is placed in a common folder
called "VirtualBox VMs", which VirtualBox creates in the current system
user's home directory. The location of this home directory depends on
the conventions of the host operating system:</para>
<itemizedlist>
<listitem>
<para>On Windows, this is
<computeroutput>%HOMEDRIVE%%HOMEPATH%</computeroutput>; typically
something like <computeroutput>C:\Documents and
Settings\Username\</computeroutput>.</para>
</listitem>
<listitem>
<para>On Mac OS X, this is
</listitem>
<listitem>
<para>On Linux and Solaris, this is
</listitem>
</itemizedlist>
<para>For simplicity, we will abbreviate this as
<computeroutput>$HOME</computeroutput> below. Using that convention, the
common folder for all virtual machines is
<para>As an example, when you create a virtual machine called "Example
VM", you will find that VirtualBox creates<orderedlist>
<listitem>
VM/</computeroutput> and, in that folder,</para>
</listitem>
<listitem>
<para>the settings file <computeroutput>Example
</listitem>
<listitem>
<para>the virtual disk image <computeroutput>Example
</listitem>
</orderedlist></para>
<para>This is the default layout if you use the "Create new virtual
machine" wizard as described in <xref linkend="gui-createvm" />. Once
you start working with the VM, additional files will show up: you will
find log files in a subfolder called
<computeroutput>Logs</computeroutput>, and once you have taken
snapshots, they will appear in a
<computeroutput>Snapshots</computeroutput> subfolder. For each VM, you
can change the location of its snapsnots folder in the VM
settings.</para>
<para>You can change the default machine folder by selecting
"Preferences" from the "File" menu in the VirtualBox main window. Then,
in the window that pops up, click on the "General" tab. Alternatively,
use <computeroutput>VBoxManage setproperty
machinefolder</computeroutput>; see <xref
linkend="vboxmanage-setproperty" />.</para>
</sect2>
<sect2>
<title>Machines created by VirtualBox versions before 4.0</title>
<para>If you have upgraded to VirtualBox 4.0 from an earlier version of
VirtualBox, you probably have settings files and disks in the earlier
file system layout.</para>
<para>Before version 4.0, VirtualBox separated the machine settings
files from virtual disk images. The machine settings files had an
<computeroutput>.xml</computeroutput> file extension and resided in a
folder called "Machines" under the global VirtualBox configuration
directory (see the next section). So, for example, on Linux, this was
directory. The default hard disks folder was called "HardDisks" and
resided in the <computeroutput>.VirtualBox</computeroutput> folder as
well. Both locations could be changed by the user in the global
preferences. (The concept of a "default hard disk folder" has been
abandoned with VirtualBox 4.0, since disk images now reside in each
machine's folder by default.)</para>
<para>The old layout had several severe disadvantages.<orderedlist>
<listitem>
<para>It was very difficult to move a virtual machine from one
host to another because the files involved did not reside in the
same folder. In addition, the virtual media of all machines were
registered with a global registry in the central VirtualBox
settings file
<para>To move a machine to another host, it was therefore not
enough to move the XML settings file and the disk images (which
were in different locations), but the hard disk entries from the
global media registry XML had to be meticulously copied as well,
which was close to impossible if the machine had snapshots and
therefore differencing images.</para>
</listitem>
<listitem>
<para>Storing virtual disk images, which can grow very large,
under the hidden <computeroutput>.VirtualBox</computeroutput>
directory (at least on Linux and Solaris hosts) made many users
wonder where their disk space had gone.</para>
</listitem>
</orderedlist></para>
<para>Whereas new VMs created with VirtualBox 4.0 or later will conform
to the new layout, for maximum compatibility, old VMs are
<emphasis>not</emphasis> converted to the new layout. Otherwise machine
settings would be irrevocably broken if a user downgraded from 4.0 back
to an older version of VirtualBox.</para>
</sect2>
<sect2>
<title>Global configuration data</title>
<para>In addition to the files of the virtual machines, VirtualBox
maintains global configuration data. On Linux and Solaris as of VirtualBox 4.3, this
is in the hidden directory <computeroutput>$HOME/.config/VirtualBox</computeroutput>, although <computeroutput>$HOME/.VirtualBox</computeroutput> will be used if it exists for compatibility with earlier versions; on Windows (and on Linux and Solaris with VirtualBox 4.2 and earlier) this is in <computeroutput>$HOME/.VirtualBox</computeroutput>; on a Mac it resides in
<para>VirtualBox creates this configuration directory automatically if
necessary. Optionally, you can supply an alternate configuration
directory by setting the
<computeroutput><literal>VBOX_USER_HOME</literal></computeroutput>
environment variable, or additionally on Linux or Solaris by using the standard <computeroutput><literal>XDG_CONFIG_HOME</literal></computeroutput> variable. (Since the global
all other configuration files, this allows for switching between several
VirtualBox configurations entirely.)</para>
<para>Most importantly, in this directory, VirtualBox stores its global
settings file, another XML file called
configuration options and the list of registered virtual machines with
pointers to their XML settings files. (Neither the location of this file
nor its directory has changed with VirtualBox 4.0.)</para>
<para>Before VirtualBox 4.0, all virtual media (disk image files) were
also contained in a global registry in this settings file. For
compatibility, this media registry still exists if you upgrade
VirtualBox and there are media from machines which were created with a
version before 4.0. If you have no such machines, then there will be no
global media registry; with VirtualBox 4.0, each machine XML file has
its own media registry.</para>
<para>Also before VirtualBox 4.0, the default "Machines" folder and the
default "HardDisks" folder resided under the VirtualBox configuration
directory (e.g.
If you are upgrading from a VirtualBox version before 4.0, files in
these directories are not automatically moved in order not to break
backwards compatibility.</para>
</sect2>
<sect2>
<title>Summary of 4.0 configuration changes</title>
<para>The following table gives a brief overview of the configuration
changes between older versions and version 4.0 or above:</para>
<table>
<title>Configuration changes in version 4.0 or above</title>
<tgroup cols="3">
<thead>
<row>
<entry><emphasis role="bold">Setting</emphasis></entry>
<entry><emphasis role="bold">Before 4.0</emphasis></entry>
<entry><emphasis role="bold">4.0 or above</emphasis></entry>
</row>
</thead>
<tbody>
<row>
<entry>Default machines folder</entry>
VMs</computeroutput></entry>
</row>
<row>
<entry>Default disk image location</entry>
<entry>In each machine's folder</entry>
</row>
<row>
<entry>Machine settings file extension</entry>
<entry><computeroutput>.xml</computeroutput></entry>
<entry><computeroutput>.vbox</computeroutput></entry>
</row>
<row>
<entry>Media registry</entry>
file</entry>
<entry>Each machine settings file</entry>
</row>
<row>
<entry>Media registration</entry>
<entry>Automatic on attach</entry>
</row>
</tbody>
</tgroup>
</table>
</sect2>
<sect2>
<title>VirtualBox XML files</title>
<para>VirtualBox uses XML for both the machine settings files and the
global configuration file,
<para>All VirtualBox XML files are versioned. When a new settings file
is created (e.g. because a new virtual machine is created), VirtualBox
automatically uses the settings format of the current VirtualBox
version. These files may not be readable if you downgrade to an earlier
version of VirtualBox. However, when VirtualBox encounters a settings
file from an earlier version (e.g. after upgrading VirtualBox), it
attempts to preserve the settings format as much as possible. It will
only silently upgrade the settings format if the current settings cannot
be expressed in the old format, for example because you enabled a
feature that was not present in an earlier version of
VirtualBox.<footnote>
<para>As an example, before VirtualBox 3.1, it was only possible to
enable or disable a single DVD drive in a virtual machine. If it was
enabled, then it would always be visible as the secondary master of
the IDE controller. With VirtualBox 3.1, DVD drives can be attached
to arbitrary slots of arbitrary controllers, so they could be the
secondary slave of an IDE controller or in a SATA slot. If you have
a machine settings file from an earlier version and upgrade
VirtualBox to 3.1 and then move the DVD drive from its default
position, this cannot be expressed in the old settings format; the
XML machine file would get written in the new format, and a backup
file of the old format would be kept.</para>
</footnote> In such cases, VirtualBox backs up the old settings file
in the virtual machine's configuration directory. If you need to go back
to the earlier version of VirtualBox, then you will need to manually
copy these backup files back.</para>
<para>We intentionally do not document the specifications of the
VirtualBox XML files, as we must reserve the right to modify them in the
future. We therefore strongly suggest that you do not edit these files
manually. VirtualBox provides complete access to its configuration data
through its the <computeroutput>VBoxManage</computeroutput> command line
tool (see <xref linkend="vboxmanage" />) and its API (see <xref
linkend="VirtualBoxAPI" />).</para>
</sect2>
</sect1>
<sect1 id="technical-components">
<title>VirtualBox executables and components</title>
<para>VirtualBox was designed to be modular and flexible. When the
VirtualBox graphical user interface (GUI) is opened and a VM is started,
at least three processes are running:<orderedlist>
<listitem>
<para><computeroutput>VBoxSVC</computeroutput>, the VirtualBox
service process which always runs in the background. This process is
started automatically by the first VirtualBox client process (the
GUI, <computeroutput>VBoxManage</computeroutput>,
<computeroutput>VBoxHeadless</computeroutput>, the web service or
others) and exits a short time after the last client exits. The
service is responsible for bookkeeping, maintaining the state of all
VMs, and for providing communication between VirtualBox components.
<para>When we refer to "clients" here, we mean the local clients
of a particular <computeroutput>VBoxSVC</computeroutput> server
process, not clients in a network. VirtualBox employs its own
all these processes run under the same user account on the host
operating system, and this is totally transparent to the
user.</para>
</note></para>
</listitem>
<listitem>
<para>The GUI process, <computeroutput>VirtualBox</computeroutput>,
a client application based on the cross-platform Qt library. When
started without the <computeroutput>--startvm</computeroutput>
option, this application acts as the VirtualBox manager, displaying
the VMs and their settings. It then communicates settings and state
changes to <computeroutput>VBoxSVC</computeroutput> and also
reflects changes effected through other means, e.g.,
<computeroutput>VBoxManage</computeroutput>.</para>
</listitem>
<listitem>
<para>If the <computeroutput>VirtualBox</computeroutput> client
application is started with the
<computeroutput>--startvm</computeroutput> argument, it loads the
VMM library which includes the actual hypervisor and then runs a
virtual machine and provides the input and output for the
guest.</para>
</listitem>
</orderedlist></para>
<para>Any VirtualBox front-end (client) will communicate with the service
process and can both control and reflect the current state. For example,
either the VM selector or the VM window or VBoxManage can be used to pause
the running VM, and other components will always reflect the changed
state.</para>
<para>The VirtualBox GUI application is only one of several available
front ends (clients). The complete list shipped with VirtualBox
is:<orderedlist>
<listitem>
<para><computeroutput>VirtualBox</computeroutput>, the Qt front end
implementing the manager and running VMs;</para>
</listitem>
<listitem>
<para><computeroutput>VBoxManage</computeroutput>, a less
user-friendly but more powerful alternative, described in <xref
linkend="vboxmanage" />.</para>
</listitem>
<listitem>
<para><computeroutput>VBoxSDL</computeroutput>, a simple graphical
front end based on the SDL library; see <xref
linkend="vboxsdl" />.</para>
</listitem>
<listitem>
<para><computeroutput>VBoxHeadless</computeroutput>, a VM front end
input, but allows redirection via VirtualBox Remote Desktop Extension;
see <xref linkend="vboxheadless" />.</para>
</listitem>
<listitem>
<para><computeroutput>vboxwebsrv</computeroutput>, the VirtualBox
web service process which allows for controlling a VirtualBox host
remotely. This is described in detail in the VirtualBox Software
Development Kit (SDK) reference; please see <xref
linkend="VirtualBoxAPI" /> for details.</para>
</listitem>
<listitem>
<para>The VirtualBox Python shell, a Python alternative to
VBoxManage. This is also described in the SDK reference.</para>
</listitem>
</orderedlist></para>
<para>Internally, VirtualBox consists of many more or less separate
components. You may encounter these when analyzing VirtualBox internal
error messages or log files. These include:</para>
<itemizedlist>
<listitem>
<para>IPRT, a portable runtime library which abstracts file access,
threading, string manipulation, etc. Whenever VirtualBox accesses host
operating features, it does so through this library for cross-platform
portability.</para>
</listitem>
<listitem>
<para>VMM (Virtual Machine Monitor), the heart of the
hypervisor.</para>
</listitem>
<listitem>
<para>EM (Execution Manager), controls execution of guest code.</para>
</listitem>
<listitem>
<para>REM (Recompiled Execution Monitor), provides software emulation
of CPU instructions.</para>
</listitem>
<listitem>
<para>TRPM (Trap Manager), intercepts and processes guest traps and
exceptions.</para>
</listitem>
<listitem>
<para>HWACCM (Hardware Acceleration Manager), provides support for
VT-x and AMD-V.</para>
</listitem>
<listitem>
<para>PDM (Pluggable Device Manager), an abstract interface between
the VMM and emulated devices which separates device implementations
from VMM internals and makes it easy to add new emulated devices.
Through PDM, third-party developers can add new virtual devices to
VirtualBox without having to change VirtualBox itself.</para>
</listitem>
<listitem>
<para>PGM (Page Manager), a component controlling guest paging.</para>
</listitem>
<listitem>
<para>PATM (Patch Manager), patches guest code to improve and speed up
software virtualization.</para>
</listitem>
<listitem>
<para>TM (Time Manager), handles timers and all aspects of time inside
guests.</para>
</listitem>
<listitem>
<para>CFGM (Configuration Manager), provides a tree structure which
holds configuration settings for the VM and all emulated
devices.</para>
</listitem>
<listitem>
<para>SSM (Saved State Manager), saves and loads VM state.</para>
</listitem>
<listitem>
<para>VUSB (Virtual USB), a USB layer which separates emulated USB
controllers from the controllers on the host and from USB devices;
this also enables remote USB.</para>
</listitem>
<listitem>
<para>DBGF (Debug Facility), a built-in VM debugger.</para>
</listitem>
<listitem>
<para>VirtualBox emulates a number of devices to provide the hardware
environment that various guests need. Most of these are standard
devices found in many PC compatible machines and widely supported by
guest operating systems. For network and storage devices in
particular, there are several options for the emulated devices to
access the underlying hardware. These devices are managed by
PDM.</para>
</listitem>
<listitem>
<para>Guest Additions for various guest operating systems. This is
code that is installed from within a virtual machine; see <xref
linkend="guestadditions" />.</para>
</listitem>
<listitem>
<para>The "Main" component is special: it ties all the above bits
together and is the only public API that VirtualBox provides. All the
client processes listed above use only this API and never access the
hypervisor components directly. As a result, third-party applications
that use the VirtualBox Main API can rely on the fact that it is
always well-tested and that all capabilities of VirtualBox are fully
exposed. It is this API that is described in the VirtualBox SDK
mentioned above (again, see <xref linkend="VirtualBoxAPI" />).</para>
</listitem>
</itemizedlist>
</sect1>
<sect1 id="hwvirt">
<title>Hardware vs. software virtualization</title>
<para>VirtualBox allows 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, VirtualBox steps in and takes action. In particular, for lots of
hardware that the guest believes to be accessing, VirtualBox simulates a
certain "virtual" environment according to how you have configured a
virtual machine. For example, when the guest attempts to access a hard
disk, VirtualBox redirects these requests to whatever you have configured
to be the virtual machine's virtual hard disk -- normally, an image file
on your host.</para>
<para>Unfortunately, the x86 platform was never designed to be
virtualized. Detecting situations in which VirtualBox needs to take
control over the guest code that is executing, as described above, is
difficult. There are two ways in which to achieve this:<itemizedlist>
<listitem>
<para>Since 2006, Intel and AMD processors have had support for
so-called <emphasis role="bold">"hardware
virtualization"</emphasis>. This means that these processors can
help VirtualBox 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.</para>
<para>These hardware features differ between Intel and AMD
processors. Intel named its technology <emphasis
role="bold">VT-x</emphasis>; AMD calls theirs <emphasis
role="bold">AMD-V</emphasis>. The Intel and AMD support for
virtualization is very different in detail, but not very different
in principle.<note>
<para>On many systems, the hardware virtualization features
first need to be enabled in the BIOS before VirtualBox can use
them.</para>
</note></para>
</listitem>
<listitem>
<para>As opposed to other virtualization software, for many usage
scenarios, VirtualBox does not <emphasis>require</emphasis> hardware
virtualization features to be present. Through sophisticated
techniques, VirtualBox virtualizes many guest operating systems
entirely in <emphasis role="bold">software</emphasis>. This means
that you can run virtual machines even on older processors which do
not support hardware virtualization.</para>
</listitem>
</itemizedlist></para>
<para>Even though VirtualBox does not always require hardware
virtualization, enabling it is <emphasis>required</emphasis> in the
following scenarios:<itemizedlist>
<listitem>
<para>Certain rare guest operating systems like OS/2 make use of
very esoteric processor instructions that are not supported with our
software virtualization. For virtual machines that are configured to
contain such an operating system, hardware virtualization is enabled
automatically.</para>
</listitem>
<listitem>
<para>VirtualBox's 64-bit guest support (added with version 2.0) and
multiprocessing (SMP, added with version 3.0) both require hardware
virtualization to be enabled. (This is not much of a limitation
since the vast majority of today's 64-bit and multicore CPUs ship
with hardware virtualization anyway; the exceptions to this rule are
e.g. older Intel Celeron and AMD Opteron CPUs.)</para>
</listitem>
</itemizedlist></para>
<warning>
<para>Do not run other hypervisors (open-source or commercial
virtualization products) together with VirtualBox! While several
hypervisors can normally be <emphasis>installed</emphasis> in parallel,
do not attempt to <emphasis>run</emphasis> several virtual machines from
competing hypervisors at the same time. VirtualBox 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. Also, within
VirtualBox, you can mix software and hardware virtualization when
running multiple VMs. In certain cases a small performance penalty will
be unavoidable when mixing VT-x and software virtualization VMs. We
recommend not mixing virtualization modes if maximum performance and low
overhead are essential. This does <emphasis>not</emphasis> apply to
AMD-V.</para>
</warning>
</sect1>
<sect1 id="gimproviders">
<title>Paravirtualization providers</title>
<para>VirtualBox allows exposing a paravirtualization interface to
facilitate accurate and efficient execution of software within a
virtual machine. These interfaces require the guest operating system
to recognize their presence and make use of them in order to leverage
the benefits of communicating with the VirtualBox hypervisor.</para>
<para>Most mainstream, modern operating systems, including Windows and
Linux, ship with support for one or more paravirtualization interfaces.
Hence, there is typically no need to install additional software in the
guest (including VirtualBox Guest Additions) to make use of this
feature.</para>
<para>VirtualBox provides the following interfaces:</para>
<itemizedlist>
<listitem>
<para><emphasis role="bold">Minimal</emphasis>: Announces the
presence of a virtualized environment, additionally reports the
TSC and APIC frequency to the guest operating system. This provider
is mandatory for running any Mac OS X guests.</para>
</listitem>
<listitem>
<para><emphasis role="bold">KVM</emphasis>: Presents a Linux KVM
hypervisor interface which is recognized by Linux kernels starting
with version 2.6.25. VirtualBox's implementation currently supports
paravirtualized clocks and SMP spinlocks. This provider is
recommended for Linux guests.</para>
</listitem>
<listitem>
<para><emphasis role="bold">Hyper-V</emphasis>: Presents a Microsoft
Hyper-V hypervisor interface which is recognized by Windows 7 and newer
operating systems. VirtualBox's implementation currently supports
paravirtualized clocks, APIC frequency reporting and relaxed timer
checks. This provider is recommended for Windows and FreeBSD guests.
</para>
</listitem>
</itemizedlist>
</sect1>
<sect1>
<title>Details about software virtualization</title>
<para>Implementing virtualization on x86 CPUs with no hardware
virtualization support is an extraordinarily complex task because the CPU
architecture was not designed to be virtualized. The problems can usually
be solved, but at the cost of reduced performance. Thus, there is a
constant clash between virtualization performance and accuracy.</para>
<para>The x86 instruction set was originally designed in the 1970s and
underwent significant changes with the addition of protected mode in the
1980s with the 286 CPU architecture and then again with the Intel 386 and
its 32-bit architecture. Whereas the 386 did have limited virtualization
support for real mode operation (V86 mode, as used by the "DOS Box" of
entire architecture.</para>
<para>In theory, software virtualization is not overly complex. In
addition to the four privilege levels ("rings") provided by the hardware
(of which typically only two are used: ring 0 for kernel mode and ring 3
for user mode), one needs to differentiate between "host context" and
"guest context".</para>
<para>In "host context", everything is as if no hypervisor was active.
This might be the active mode if another application on your host has been
scheduled CPU time; in that case, there is a host ring 3 mode and a host
ring 0 mode. The hypervisor is not involved.</para>
<para>In "guest context", however, a virtual machine is active. So long as
the guest code is running in ring 3, this is not much of a problem since a
hypervisor can set up the page tables properly and run that code natively
on the processor. The problems mostly lie in how to intercept what the
guest's kernel does.</para>
<para>There are several possible solutions to these problems. One approach
is full software emulation, usually involving recompilation. That is, all
code to be run by the guest is analyzed, transformed into a form which
will not allow the guest to either modify or see the true state of the
CPU, and only then executed. This process is obviously highly complex and
costly in terms of performance. (VirtualBox contains a recompiler based on
QEMU which can be used for pure software emulation, but the recompiler is
only activated in special situations, described below.)</para>
<para>Another possible solution is paravirtualization, in which only
specially modified guest OSes are allowed to run. This way, most of the
hardware access is abstracted and any functions which would normally
access the hardware or privileged CPU state are passed on to the
hypervisor instead. Paravirtualization can achieve good functionality and
performance on standard x86 CPUs, but it can only work if the guest OS can
actually be modified, which is obviously not always the case.</para>
<para>VirtualBox chooses a different approach. When starting a virtual
machine, through its ring-0 support kernel driver, VirtualBox has set up
the host system so that it can run most of the guest code natively, but it
has inserted itself at the "bottom" of the picture. It can then assume
control when needed -- if a privileged instruction is executed, the guest
traps (in particular because an I/O register was accessed and a device
needs to be virtualized) or external interrupts occur. VirtualBox may then
handle this and either route a request to a virtual device or possibly
delegate handling such things to the guest or host OS. In guest context,
VirtualBox can therefore be in one of three states:</para>
<para><itemizedlist>
<listitem>
<para>Guest ring 3 code is run unmodified, at full speed, as much as
possible. The number of faults will generally be low (unless the
guest allows port I/O from ring 3, something we cannot do as we
don't want the guest to be able to access real ports). This is also
referred to as "raw mode", as the guest ring-3 code runs
unmodified.</para>
</listitem>
<listitem>
<para>For guest code in ring 0, VirtualBox employs a nasty trick: it
actually reconfigures the guest so that its ring-0 code is run in
ring 1 instead (which is normally not used in x86 operating
systems). As a result, when guest ring-0 code (actually running in
ring 1) such as a guest device driver attempts to write to an I/O
register or execute a privileged instruction, the VirtualBox
hypervisor in "real" ring 0 can take over.</para>
</listitem>
<listitem>
<para>The hypervisor (VMM) can be active. Every time a fault occurs,
VirtualBox looks at the offending instruction and can relegate it to
a virtual device or the host OS or the guest OS or run it in the
recompiler.</para>
<para>In particular, the recompiler is used when guest code disables
interrupts and VirtualBox cannot figure out when they will be
switched back on (in these situations, VirtualBox actually analyzes
the guest code using its own disassembler). Also, certain privileged
instructions such as LIDT need to be handled specially. Finally, any
real-mode or protected-mode code (e.g. BIOS code, a DOS guest, or
any operating system startup) is run in the recompiler
entirely.</para>
</listitem>
</itemizedlist></para>
<para>Unfortunately this only works to a degree. Among others, the
following situations require special handling:</para>
<para><orderedlist>
<listitem>
<para>Running ring 0 code in ring 1 causes a lot of additional
instruction faults, as ring 1 is not allowed to execute any
privileged instructions (of which guest's ring-0 contains plenty).
With each of these faults, the VMM must step in and emulate the code
to achieve the desired behavior. While this works, emulating
thousands of these faults is very expensive and severely hurts the
performance of the virtualized guest.</para>
</listitem>
<listitem>
<para>There are certain flaws in the implementation of ring 1 in the
x86 architecture that were never fixed. Certain instructions that
<emphasis>should</emphasis> trap in ring 1 don't. This affects, for
Whereas the "load" operation is privileged and can therefore be
trapped, the "store" instruction always succeed. If the guest is
allowed to execute these, it will see the true state of the CPU, not
the virtualized state. The CPUID instruction also has the same
problem.</para>
</listitem>
<listitem>
<para>A hypervisor typically needs to reserve some portion of the
guest's address space (both linear address space and selectors) for
its own use. This is not entirely transparent to the guest OS and
may cause clashes.</para>
</listitem>
<listitem>
<para>The SYSENTER instruction (used for system calls) executed by
an application running in a guest OS always transitions to ring 0.
But that is where the hypervisor runs, not the guest OS. In this
case, the hypervisor must trap and emulate the instruction even when
it is not desirable.</para>
</listitem>
<listitem>
<para>The CPU segment registers contain a "hidden" descriptor cache
which is not software-accessible. The hypervisor cannot read, save,
or restore this state, but the guest OS may use it.</para>
</listitem>
<listitem>
<para>Some resources must (and can) be trapped by the hypervisor,
but the access is so frequent that this creates a significant
performance overhead. An example is the TPR (Task Priority) register
in 32-bit mode. Accesses to this register must be trapped by the
hypervisor, but certain guest operating systems (notably Windows and
Solaris) write this register very often, which adversely affects
virtualization performance.</para>
</listitem>
</orderedlist></para>
<para>To fix these performance and security issues, VirtualBox contains a
Code Scanning and Analysis Manager (CSAM), which disassembles guest code,
and the Patch Manager (PATM), which can replace it at runtime.</para>
<para>Before executing ring 0 code, CSAM scans it recursively to discover
problematic instructions. PATM then performs <emphasis>in-situ
</emphasis>patching, i.e. it replaces the instruction with a jump to
hypervisor memory where an integrated code generator has placed a more
suitable implementation. In reality, this is a very complex task as there
are lots of odd situations to be discovered and handled correctly. So,
with its current complexity, one could argue that PATM is an advanced
<emphasis>in-situ</emphasis> recompiler.</para>
<para>In addition, every time a fault occurs, VirtualBox analyzes the
offending code to determine if it is possible to patch it in order to
prevent it from causing more faults in the future. This approach works
well in practice and dramatically improves software virtualization
performance.</para>
</sect1>
<sect1>
<title>Details about hardware virtualization</title>
<para>With Intel VT-x, there are two distinct modes of CPU operation: VMX
root mode and non-root mode.<itemizedlist>
<listitem>
<para>In root mode, the CPU operates much like older generations of
processors without VT-x support. There are four privilege levels
("rings"), and the same instruction set is supported, with the
addition of several virtualization specific instruction. Root mode
is what a host operating system without virtualization uses, and it
is also used by a hypervisor when virtualization is active.</para>
</listitem>
<listitem>
<para>In non-root mode, CPU operation is significantly different.
There are still four privilege rings and the same instruction set,
but a new structure called VMCS (Virtual Machine Control Structure)
now controls the CPU operation and determines how certain
instructions behave. Non-root mode is where guest systems
run.</para>
</listitem>
</itemizedlist></para>
<para>Switching from root mode to non-root mode is called "VM entry", the
switch back is "VM exit". The VMCS includes a guest and host state area
controls which guest operations will cause VM exits.</para>
<para>The VMCS provides fairly fine-grained control over what the guests
can and can't do. For example, a hypervisor can allow a guest to write
certain bits in shadowed control registers, but not others. This enables
efficient virtualization in cases where guests can be allowed to write
control bits without disrupting the hypervisor, while preventing them from
altering control bits over which the hypervisor needs to retain full
control. The VMCS also provides control over interrupt delivery and
exceptions.</para>
<para>Whenever an instruction or event causes a VM exit, the VMCS contains
information about the exit reason, often with accompanying detail. For
example, if a write to the CR0 register causes an exit, the offending
instruction is recorded, along with the fact that a write access to a
control register caused the exit, and information about source and
destination register. Thus the hypervisor can efficiently handle the
condition without needing advanced techniques such as CSAM and PATM
described above.</para>
<para>VT-x inherently avoids several of the problems which software
virtualization faces. The guest has its own completely separate address
space not shared with the hypervisor, which eliminates potential clashes.
Additionally, guest OS kernel code runs at privilege ring 0 in VMX
non-root mode, obviating the problems by running ring 0 code at less
privileged levels. For example the SYSENTER instruction can transition to
ring 0 without causing problems. Naturally, even at ring 0 in VMX non-root
mode, any I/O access by guest code still causes a VM exit, allowing for
device emulation.</para>
<para>The biggest difference between VT-x and AMD-V is that AMD-V provides
a more complete virtualization environment. VT-x requires the VMX non-root
code to run with paging enabled, which precludes hardware virtualization
of real-mode code and non-paged protected-mode software. This typically
only includes firmware and OS loaders, but nevertheless complicates VT-x
hypervisor implementation. AMD-V does not have this restriction.</para>
<para>Of course hardware virtualization is not perfect. Compared to
software virtualization, the overhead of VM exits is relatively high. This
causes problems for devices whose emulation requires high number of traps.
One example is the VGA device in 16-color modes, where not only every I/O
port access but also every access to the framebuffer memory must be
trapped.</para>
</sect1>
<sect1 id="nestedpaging">
<title>Nested paging and VPIDs</title>
<para>In addition to "plain" hardware virtualization, your processor may
also support additional sophisticated techniques:<footnote>
<para>VirtualBox 2.0 added support for AMD's nested paging; support
for Intel's EPT and VPIDs was added with version 2.1.</para>
</footnote><itemizedlist>
<listitem>
<para>A newer feature called <emphasis role="bold">"nested
paging"</emphasis> implements some memory management in hardware,
which can greatly accelerate hardware virtualization since these
tasks no longer need to be performed by the virtualization
software.</para>
<para>With nested paging, the hardware provides another level of
indirection when translating linear to physical addresses. Page
tables function as before, but linear addresses are now translated
to "guest physical" addresses first and not physical addresses
directly. A new set of paging registers now exists under the
traditional paging mechanism and translates from guest physical
addresses to host physical addresses, which are used to access
memory.</para>
<para>Nested paging eliminates the overhead caused by VM exits and
page table accesses. In essence, with nested page tables the guest
can handle paging without intervention from the hypervisor. Nested
paging thus significantly improves virtualization
performance.</para>
<para>On AMD processors, nested paging has been available starting
with the Barcelona (K10) architecture -- they call it now "rapid
virtualization indexing" (RVI). Intel added support for nested
paging, which they call "extended page tables" (EPT), with their
Core i7 (Nehalem) processors.</para>
<para>If nested paging is enabled, the VirtualBox hypervisor can
also use <emphasis role="bold">large pages</emphasis> to reduce TLB
usage and overhead. This can yield a performance improvement of up
to 5%. To enable this feature for a VM, you need to use the
<computeroutput>VBoxManage modifyvm
</computeroutput><computeroutput>--largepages</computeroutput>
command; see <xref linkend="vboxmanage-modifyvm" />.</para>
</listitem>
<listitem>
<para>On Intel CPUs, another hardware feature called <emphasis
role="bold">"Virtual Processor Identifiers" (VPIDs)</emphasis> can
greatly accelerate context switching by reducing the need for
expensive flushing of the processor's Translation Lookaside Buffers
(TLBs).</para>
<para>To enable these features for a VM, you need to use the
<computeroutput>VBoxManage modifyvm --vtxvpid</computeroutput> and
<computeroutput>--largepages</computeroutput> commands; see <xref
linkend="vboxmanage-modifyvm" />.</para>
</listitem>
</itemizedlist></para>
</sect1>
</chapter>