<s>
In	O
computing	O
,	O
hardware-assisted	B-General_Concept
virtualization	I-General_Concept
is	O
a	O
platform	B-General_Concept
virtualization	I-General_Concept
approach	O
that	O
enables	O
efficient	O
full	B-Architecture
virtualization	I-Architecture
using	O
help	O
from	O
hardware	O
capabilities	O
,	O
primarily	O
from	O
the	O
host	O
processors	O
.	O
</s>
<s>
A	O
full	B-Architecture
virtualization	I-Architecture
is	O
used	O
to	O
emulate	O
a	O
complete	O
hardware	O
environment	O
,	O
or	O
virtual	B-Architecture
machine	I-Architecture
,	O
in	O
which	O
an	O
unmodified	O
guest	B-General_Concept
operating	I-General_Concept
system	I-General_Concept
(	O
using	O
the	O
same	O
instruction	B-General_Concept
set	I-General_Concept
as	O
the	O
host	B-Operating_System
machine	I-Operating_System
)	O
effectively	O
executes	O
in	O
complete	O
isolation	O
.	O
</s>
<s>
Hardware-assisted	B-General_Concept
virtualization	I-General_Concept
was	O
added	O
to	O
x86	B-Operating_System
processors	O
(	O
Intel	O
VT-x	O
,	O
AMD-V	O
or	O
VIA	O
VT	O
)	O
in	O
2005	O
,	O
2006	O
and	O
2010	O
(	O
respectively	O
)	O
.	O
</s>
<s>
Hardware-assisted	B-General_Concept
virtualization	I-General_Concept
is	O
also	O
known	O
as	O
accelerated	O
virtualization	O
;	O
Xen	B-Operating_System
calls	O
it	O
hardware	B-General_Concept
virtual	I-General_Concept
machine	I-General_Concept
(	O
HVM	O
)	O
,	O
and	O
Virtual	O
Iron	O
calls	O
it	O
native	B-General_Concept
virtualization	I-General_Concept
.	O
</s>
<s>
Hardware-assisted	B-General_Concept
virtualization	I-General_Concept
first	O
appeared	O
on	O
the	O
IBM	B-Device
System/370	I-Device
in	O
1972	O
,	O
for	O
use	O
with	O
VM/370	B-Application
,	O
the	O
first	O
virtual	B-Architecture
machine	I-Architecture
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
CAD	B-Application
)	O
,	O
virtualization	O
of	O
mainframes	B-Architecture
lost	O
some	O
attention	O
in	O
the	O
late	O
1970s	O
,	O
when	O
the	O
upcoming	O
minicomputers	B-Architecture
fostered	O
resource	O
allocation	O
through	O
distributed	B-Architecture
computing	I-Architecture
,	O
encompassing	O
the	O
commoditization	O
of	O
microcomputers	B-Architecture
.	O
</s>
<s>
IBM	O
offers	O
hardware	B-General_Concept
virtualization	I-General_Concept
for	O
its	O
IBM	B-Device
Power	I-Device
Systems	I-Device
hardware	O
for	O
AIX	B-Application
,	O
Linux	B-Application
and	O
IBM	B-Application
i	I-Application
,	O
and	O
for	O
its	O
IBM	B-Device
Z	I-Device
mainframes	B-Architecture
.	O
</s>
<s>
IBM	O
refers	O
to	O
its	O
specific	O
form	O
of	O
hardware	B-General_Concept
virtualization	I-General_Concept
as	O
"	O
logical	B-Device
partition	I-Device
"	O
,	O
or	O
more	O
commonly	O
as	O
LPAR	B-Device
.	O
</s>
<s>
The	O
increase	O
in	O
compute	O
capacity	O
per	O
x86	B-Operating_System
server	O
(	O
and	O
in	O
particular	O
the	O
substantial	O
increase	O
in	O
modern	O
networks	O
 '	O
bandwidths	O
)	O
rekindled	O
interest	O
in	O
data-center	O
based	O
computing	O
which	O
is	O
based	O
on	O
virtualization	O
techniques	O
.	O
</s>
<s>
The	O
most	O
visible	O
hallmark	O
of	O
a	O
return	O
to	O
the	O
roots	O
of	O
computing	O
is	O
cloud	B-Architecture
computing	I-Architecture
,	O
which	O
is	O
a	O
synonym	O
for	O
data	O
center	O
based	O
computing	O
(	O
or	O
mainframe-like	O
computing	O
)	O
through	O
high	O
bandwidth	O
networks	O
.	O
</s>
<s>
The	O
initial	O
implementation	O
x86	B-Operating_System
architecture	I-Operating_System
did	O
not	O
meet	O
the	O
Popek	B-Architecture
and	I-Architecture
Goldberg	I-Architecture
virtualization	I-Architecture
requirements	I-Architecture
to	O
achieve	O
"	O
classical	O
virtualization	O
"	O
:	O
</s>
<s>
This	O
made	O
it	O
difficult	O
to	O
implement	O
a	O
virtual	B-Operating_System
machine	I-Operating_System
monitor	I-Operating_System
for	O
this	O
type	O
of	O
processor	O
.	O
</s>
<s>
Specific	O
limitations	O
included	O
the	O
inability	O
to	O
trap	B-Application
on	O
some	O
privileged	O
instructions	O
.	O
</s>
<s>
To	O
compensate	O
for	O
these	O
architectural	O
limitations	O
,	O
designers	O
have	O
accomplished	O
virtualization	O
of	O
the	O
x86	B-Operating_System
architecture	I-Operating_System
through	O
two	O
methods	O
:	O
full	B-Architecture
virtualization	I-Architecture
or	O
paravirtualization	O
.	O
</s>
<s>
Both	O
create	O
the	O
illusion	O
of	O
physical	O
hardware	O
to	O
achieve	O
the	O
goal	O
of	O
operating	B-General_Concept
system	I-General_Concept
independence	O
from	O
the	O
hardware	O
but	O
present	O
some	O
trade-offs	O
in	O
performance	O
and	O
complexity	O
.	O
</s>
<s>
Full	B-Architecture
virtualization	I-Architecture
was	O
implemented	O
in	O
first-generation	O
x86	B-Operating_System
VMMs	O
.	O
</s>
<s>
It	O
relies	O
on	O
binary	O
translation	O
to	O
trap	B-Application
and	O
virtualize	O
the	O
execution	O
of	O
certain	O
sensitive	O
,	O
non-virtualizable	O
instructions	O
.	O
</s>
<s>
With	O
this	O
approach	O
,	O
critical	O
instructions	O
are	O
discovered	O
(	O
statically	O
or	O
dynamically	O
at	O
run-time	O
)	O
and	O
replaced	O
with	O
traps	B-Application
into	O
the	O
VMM	O
to	O
be	O
emulated	B-Application
in	O
software	O
.	O
</s>
<s>
Binary	O
translation	O
can	O
incur	O
a	O
large	O
performance	O
overhead	O
in	O
comparison	O
to	O
a	O
virtual	B-Architecture
machine	I-Architecture
running	O
on	O
natively	O
virtualized	O
architectures	O
such	O
as	O
the	O
IBM	B-Device
System/370	I-Device
.	O
</s>
<s>
VirtualBox	B-Operating_System
,	O
VMware	B-Operating_System
Workstation	I-Operating_System
(	O
for	O
32-bit	O
guests	O
only	O
)	O
,	O
and	O
Microsoft	B-Application
Virtual	I-Application
PC	I-Application
,	O
are	O
well-known	O
commercial	O
implementations	O
of	O
full	B-Architecture
virtualization	I-Architecture
.	O
</s>
<s>
Paravirtualization	O
is	O
a	O
technique	O
in	O
which	O
the	O
hypervisor	B-Operating_System
provides	O
an	O
API	O
and	O
the	O
OS	O
of	O
the	O
guest	O
virtual	B-Architecture
machine	I-Architecture
calls	O
that	O
API	O
,	O
requiring	O
OS	O
modifications	O
.	O
</s>
<s>
In	O
2005	O
and	O
2006	O
,	O
Intel	O
and	O
AMD	O
(	O
working	O
independently	O
)	O
created	O
new	O
processor	O
extensions	O
to	O
the	O
x86	B-Operating_System
architecture	I-Operating_System
called	O
Intel	O
VT-x	O
and	O
AMD-V	O
,	O
respectively	O
.	O
</s>
<s>
On	O
the	O
Itanium	B-General_Concept
architecture	O
,	O
hardware-assisted	B-General_Concept
virtualization	I-General_Concept
is	O
known	O
as	O
VT-i	O
.	O
</s>
<s>
The	O
first	O
generation	O
of	O
x86	B-Operating_System
processors	O
to	O
support	O
these	O
extensions	O
were	O
released	O
in	O
late	O
2005	O
early	O
2006	O
:	O
</s>
<s>
Well-known	O
implementations	O
of	O
hardware-assisted	O
x86	B-Operating_System
virtualization	O
include	O
VMware	B-Operating_System
Workstation	I-Operating_System
(	O
for	O
64-bit	O
guests	O
only	O
)	O
,	O
XenCenter	O
,	O
Xen	B-Operating_System
3.x	O
(	O
including	O
derivatives	O
like	O
Virtual	O
Iron	O
)	O
,	O
Linux	B-Application
KVM	I-Application
and	O
Microsoft	O
Hyper-V	O
.	O
</s>
<s>
Hardware-assisted	B-General_Concept
virtualization	I-General_Concept
reduces	O
the	O
maintenance	O
overhead	O
of	O
paravirtualization	O
as	O
it	O
reduces	O
(	O
ideally	O
,	O
eliminates	O
)	O
the	O
changes	O
needed	O
in	O
the	O
guest	B-General_Concept
operating	I-General_Concept
system	I-General_Concept
.	O
</s>
<s>
A	O
practical	O
benefit	O
of	O
hardware-assisted	B-General_Concept
virtualization	I-General_Concept
has	O
been	O
cited	O
by	O
VMware	O
engineers	O
and	O
Virtual	O
Iron	O
.	O
</s>
<s>
Hardware-assisted	B-General_Concept
virtualization	I-General_Concept
requires	O
explicit	O
support	O
in	O
the	O
host	O
CPU	O
,	O
which	O
is	O
not	O
available	O
on	O
all	O
x86/x86_64	O
processors	O
.	O
</s>
<s>
A	O
"	O
pure	O
"	O
hardware-assisted	B-General_Concept
virtualization	I-General_Concept
approach	O
,	O
using	O
entirely	O
unmodified	O
guest	B-General_Concept
operating	I-General_Concept
systems	I-General_Concept
,	O
involves	O
many	O
VM	B-Application
traps	B-Application
,	O
and	O
thus	O
high	O
CPU	O
overheads	O
,	O
limiting	O
scalability	O
and	O
the	O
efficiency	O
of	O
server	O
consolidation	O
.	O
</s>
<s>
In	O
2006	O
first-generation	O
32	O
-	O
and	O
64-bit	O
x86	B-Operating_System
hardware	O
support	O
was	O
found	O
rarely	O
to	O
offer	O
performance	O
advantages	O
over	O
software	O
virtualization	O
.	O
</s>
