<s>
A	O
hypervisor	B-Operating_System
(	O
also	O
known	O
as	O
a	O
virtual	B-Operating_System
machine	I-Operating_System
monitor	I-Operating_System
,	O
VMM	O
,	O
or	O
virtualizer	O
)	O
is	O
a	O
type	O
of	O
computer	O
software	O
,	O
firmware	B-Application
or	O
hardware	B-Architecture
that	O
creates	O
and	O
runs	O
virtual	B-Architecture
machines	I-Architecture
.	O
</s>
<s>
A	O
computer	O
on	O
which	O
a	O
hypervisor	B-Operating_System
runs	O
one	O
or	O
more	O
virtual	B-Architecture
machines	I-Architecture
is	O
called	O
a	O
host	B-Operating_System
machine	I-Operating_System
,	O
and	O
each	O
virtual	B-Architecture
machine	I-Architecture
is	O
called	O
a	O
guest	O
machine	O
.	O
</s>
<s>
The	O
hypervisor	B-Operating_System
presents	O
the	O
guest	B-General_Concept
operating	I-General_Concept
systems	I-General_Concept
with	O
a	O
virtual	B-General_Concept
operating	I-General_Concept
platform	I-General_Concept
and	O
manages	O
the	O
execution	O
of	O
the	O
guest	B-General_Concept
operating	I-General_Concept
systems	I-General_Concept
.	O
</s>
<s>
Unlike	O
an	O
emulator	B-Application
,	O
the	O
guest	O
executes	O
most	O
instructions	O
on	O
the	O
native	O
hardware	B-Architecture
.	O
</s>
<s>
Multiple	O
instances	O
of	O
a	O
variety	O
of	O
operating	B-General_Concept
systems	I-General_Concept
may	O
share	O
the	O
virtualized	O
hardware	B-Architecture
resources	O
:	O
for	O
example	O
,	O
Linux	B-Application
,	O
Windows	B-Application
,	O
and	O
macOS	B-Application
instances	O
can	O
all	O
run	O
on	O
a	O
single	O
physical	O
x86	B-Operating_System
machine	O
.	O
</s>
<s>
This	O
contrasts	O
with	O
operating-system	B-Application
–	I-Application
level	I-Application
virtualization	I-Application
,	O
where	O
all	O
instances	O
(	O
usually	O
called	O
containers	B-Application
)	O
must	O
share	O
a	O
single	O
kernel	B-Operating_System
,	O
though	O
the	O
guest	B-General_Concept
operating	I-General_Concept
systems	I-General_Concept
can	O
differ	O
in	O
user	B-Operating_System
space	I-Operating_System
,	O
such	O
as	O
different	O
Linux	B-Application
distributions	I-Application
with	O
the	O
same	O
kernel	B-Operating_System
.	O
</s>
<s>
The	O
term	O
hypervisor	B-Operating_System
is	O
a	O
variant	O
of	O
supervisor	O
,	O
a	O
traditional	O
term	O
for	O
the	O
kernel	B-Operating_System
of	O
an	O
operating	B-General_Concept
system	I-General_Concept
:	O
the	O
hypervisor	B-Operating_System
is	O
the	O
supervisor	O
of	O
the	O
supervisors	O
,	O
with	O
hyper	O
-	O
used	O
as	O
a	O
stronger	O
variant	O
of	O
super-	O
.	O
</s>
<s>
The	O
term	O
dates	O
to	O
circa	O
1970	O
;	O
IBM	O
coined	O
it	O
for	O
the	O
360/65	B-Device
and	O
later	O
used	O
it	O
for	O
the	O
DIAG	O
handler	O
of	O
CP-67	B-Application
.	O
</s>
<s>
In	O
the	O
earlier	O
CP/CMS	B-Application
(	O
1967	O
)	O
system	O
,	O
the	O
term	O
Control	O
Program	O
was	O
used	O
instead	O
.	O
</s>
<s>
In	O
his	O
1973	O
thesis	O
,	O
"	O
Architectural	O
Principles	O
for	O
Virtual	B-Architecture
Computer	I-Architecture
Systems	O
,	O
"	O
Robert	O
P	O
.	O
Goldberg	O
classified	O
two	O
types	O
of	O
hypervisor	B-Operating_System
:	O
</s>
<s>
These	O
hypervisors	B-Operating_System
run	O
directly	O
on	O
the	O
host	O
's	O
hardware	B-Architecture
to	O
control	O
the	O
hardware	B-Architecture
and	O
to	O
manage	O
guest	B-General_Concept
operating	I-General_Concept
systems	I-General_Concept
.	O
</s>
<s>
For	O
this	O
reason	O
,	O
they	O
are	O
sometimes	O
called	O
bare-metal	O
hypervisors	B-Operating_System
.	O
</s>
<s>
The	O
first	O
hypervisors	B-Operating_System
,	O
which	O
IBM	O
developed	O
in	O
the	O
1960s	O
,	O
were	O
native	O
hypervisors	B-Operating_System
.	O
</s>
<s>
These	O
included	O
the	O
test	O
software	O
SIMMON	B-Operating_System
and	O
the	O
CP/CMS	B-Application
operating	B-General_Concept
system	I-General_Concept
,	O
the	O
predecessor	O
of	O
IBM	O
z/VM	B-Application
.	O
</s>
<s>
These	O
hypervisors	B-Operating_System
run	O
on	O
a	O
conventional	O
operating	B-General_Concept
system	I-General_Concept
(	O
OS	O
)	O
just	O
as	O
other	O
computer	O
programs	O
do	O
.	O
</s>
<s>
A	O
virtual	B-Operating_System
machine	I-Operating_System
monitor	I-Operating_System
runs	O
as	O
a	O
process	B-Operating_System
on	O
the	O
host	O
.	O
</s>
<s>
Type-2	O
hypervisors	B-Operating_System
abstract	O
guest	B-General_Concept
operating	I-General_Concept
systems	I-General_Concept
from	O
the	O
host	O
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
For	O
instance	O
,	O
KVM	O
and	O
bhyve	B-Application
are	O
kernel	B-Application
modules	I-Application
that	O
effectively	O
convert	O
the	O
host	O
operating	B-General_Concept
system	I-General_Concept
to	O
a	O
type-1	O
hypervisor	B-Operating_System
.	O
</s>
<s>
At	O
the	O
same	O
time	O
,	O
since	O
Linux	B-Application
distributions	I-Application
and	O
FreeBSD	B-Operating_System
are	O
still	O
general-purpose	B-General_Concept
operating	I-General_Concept
systems	I-General_Concept
,	O
with	O
applications	O
competing	O
with	O
each	O
other	O
for	O
VM	B-Application
resources	O
,	O
KVM	O
and	O
bhyve	B-Application
can	O
also	O
be	O
categorized	O
as	O
type-2	O
hypervisors	B-Operating_System
.	O
</s>
<s>
The	O
first	O
hypervisors	B-Operating_System
providing	O
full	B-Architecture
virtualization	I-Architecture
were	O
the	O
test	O
tool	O
SIMMON	B-Operating_System
and	O
the	O
one-off	O
IBM	B-Application
CP-40	I-Application
research	O
system	O
,	O
which	O
began	O
production	O
use	O
in	O
January	O
1967	O
and	O
became	O
the	O
first	O
version	O
of	O
the	O
IBM	O
CP/CMS	B-Application
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
CP-40	B-Application
ran	O
on	O
a	O
S/360	B-Application
-40	I-Application
modified	O
at	O
the	O
Cambridge	O
Scientific	O
Center	O
to	O
support	O
dynamic	O
address	O
translation	O
,	O
a	O
feature	O
that	O
enabled	O
virtualization	B-General_Concept
.	O
</s>
<s>
Prior	O
to	O
this	O
time	O
,	O
computer	B-Architecture
hardware	I-Architecture
had	O
only	O
been	O
virtualized	O
to	O
the	O
extent	O
to	O
allow	O
multiple	O
user	O
applications	O
to	O
run	O
concurrently	O
,	O
such	O
as	O
in	O
CTSS	B-Operating_System
and	O
IBM	B-Device
M44/44X	I-Device
.	O
</s>
<s>
With	O
CP-40	B-Application
,	O
the	O
hardware	B-Architecture
's	O
supervisor	O
state	O
was	O
virtualized	O
as	O
well	O
,	O
allowing	O
multiple	O
operating	B-General_Concept
systems	I-General_Concept
to	O
run	O
concurrently	O
in	O
separate	O
virtual	B-Architecture
machine	I-Architecture
contexts	O
.	O
</s>
<s>
Programmers	O
soon	O
implemented	O
CP-40	B-Application
(	O
as	O
CP-67	B-Application
)	O
for	O
the	O
IBM	B-Device
System/360	I-Device
-67	I-Device
,	O
the	O
first	O
production	O
computer	O
system	O
capable	O
of	O
full	B-Architecture
virtualization	I-Architecture
.	O
</s>
<s>
IBM	O
shipped	O
this	O
machine	O
in	O
1966	O
;	O
it	O
included	O
page-translation-table	B-General_Concept
hardware	B-Architecture
for	O
virtual	B-Architecture
memory	I-Architecture
and	O
other	O
techniques	O
that	O
allowed	O
a	O
full	B-Architecture
virtualization	I-Architecture
of	O
all	O
kernel	B-Operating_System
tasks	O
,	O
including	O
I/O	O
and	O
interrupt	O
handling	O
.	O
</s>
<s>
(	O
Note	O
that	O
the	O
"	O
official	O
"	O
operating	B-General_Concept
system	I-General_Concept
,	O
the	O
ill-fated	O
TSS/360	B-Application
,	O
did	O
not	O
employ	O
full	B-Architecture
virtualization	I-Architecture
.	O
)	O
</s>
<s>
Both	O
CP-40	B-Application
and	O
CP-67	B-Application
began	O
production	O
use	O
in	O
1967	O
.	O
</s>
<s>
CP/CMS	B-Application
was	O
available	O
to	O
IBM	O
customers	O
from	O
1968	O
to	O
early	O
1970s	O
,	O
in	O
source	O
code	O
form	O
without	O
support	O
.	O
</s>
<s>
CP/CMS	B-Application
formed	O
part	O
of	O
IBM	O
's	O
attempt	O
to	O
build	O
robust	O
time-sharing	B-General_Concept
systems	I-General_Concept
for	O
its	O
mainframe	B-Architecture
computers	I-Architecture
.	O
</s>
<s>
By	O
running	O
multiple	O
operating	B-General_Concept
systems	I-General_Concept
concurrently	O
,	O
the	O
hypervisor	B-Operating_System
increased	O
system	O
robustness	O
and	O
stability	O
:	O
Even	O
if	O
one	O
operating	B-General_Concept
system	I-General_Concept
crashed	O
,	O
the	O
others	O
would	O
continue	O
working	O
without	O
interruption	O
.	O
</s>
<s>
IBM	O
announced	O
its	O
System/370	B-Device
series	O
in	O
1970	O
without	O
the	O
virtual	B-Architecture
memory	I-Architecture
feature	O
needed	O
for	O
virtualization	B-General_Concept
,	O
but	O
added	O
it	O
in	O
the	O
August	O
1972	O
Advanced	O
Function	O
announcement	O
.	O
</s>
<s>
Virtualization	B-General_Concept
has	O
been	O
featured	O
in	O
all	O
successor	O
systems	O
,	O
such	O
that	O
all	O
modern-day	O
IBM	O
mainframes	B-Architecture
,	O
including	O
the	O
zSeries	O
line	O
,	O
retain	O
backward	O
compatibility	O
with	O
the	O
1960s-era	O
IBM	B-Application
S/360	I-Application
line	O
.	O
</s>
<s>
The	O
1972	O
announcement	O
also	O
included	O
VM/370	B-Application
,	O
a	O
reimplementation	O
of	O
CP/CMS	B-Application
for	O
the	O
S/370	B-Device
.	O
</s>
<s>
Unlike	O
CP/CMS	B-Application
,	O
IBM	O
provided	O
support	O
for	O
this	O
version	O
(	O
though	O
it	O
was	O
still	O
distributed	O
in	O
source	O
code	O
form	O
for	O
several	O
releases	O
)	O
.	O
</s>
<s>
VM	B-Application
stands	O
for	O
Virtual	B-Architecture
Machine	I-Architecture
,	O
emphasizing	O
that	O
all	O
,	O
not	O
just	O
some	O
,	O
of	O
the	O
hardware	B-Architecture
interfaces	O
are	O
virtualized	O
.	O
</s>
<s>
Both	O
VM	B-Application
and	O
CP/CMS	B-Application
enjoyed	O
early	O
acceptance	O
and	O
rapid	O
development	O
by	O
universities	O
,	O
corporate	O
users	O
,	O
and	O
time-sharing	B-General_Concept
vendors	O
,	O
as	O
well	O
as	O
within	O
IBM	O
.	O
</s>
<s>
Users	O
played	O
an	O
active	O
role	O
in	O
ongoing	O
development	O
,	O
anticipating	O
trends	O
seen	O
in	O
modern	O
open	B-License
source	I-License
projects	O
.	O
</s>
<s>
However	O
,	O
in	O
a	O
series	O
of	O
disputed	O
and	O
bitter	O
battles	O
,	O
time-sharing	B-General_Concept
lost	O
out	O
to	O
batch	B-General_Concept
processing	I-General_Concept
through	O
IBM	O
political	O
infighting	O
,	O
and	O
VM	B-Application
remained	O
IBM	O
's	O
"	O
other	O
"	O
mainframe	B-Architecture
operating	B-General_Concept
system	I-General_Concept
for	O
decades	O
,	O
losing	O
to	O
MVS	B-Application
.	O
</s>
<s>
It	O
enjoyed	O
a	O
resurgence	O
of	O
popularity	O
and	O
support	O
from	O
2000	O
as	O
the	O
z/VM	B-Application
product	O
,	O
for	O
example	O
as	O
the	O
platform	O
for	O
Linux	B-Application
on	I-Application
IBM	I-Application
Z	I-Application
.	O
</s>
<s>
As	O
mentioned	O
above	O
,	O
the	O
VM	B-Application
control	O
program	O
includes	O
a	O
hypervisor-call	O
handler	O
that	O
intercepts	O
DIAG	O
(	O
"	O
Diagnose	O
"	O
,	O
opcode	O
x'83'	O
)	O
instructions	O
used	O
within	O
a	O
virtual	B-Architecture
machine	I-Architecture
.	O
</s>
<s>
It	O
is	O
therefore	O
available	O
for	O
use	O
as	O
a	O
signal	O
to	O
the	O
"	O
host	O
"	O
operating	B-General_Concept
system	I-General_Concept
)	O
.	O
</s>
<s>
When	O
first	O
implemented	O
in	O
CP/CMS	B-Application
release	O
3.1	O
,	O
this	O
use	O
of	O
DIAG	O
provided	O
an	O
operating	B-General_Concept
system	I-General_Concept
interface	O
that	O
was	O
analogous	O
to	O
the	O
System/360	B-Application
Supervisor	B-Operating_System
Call	I-Operating_System
instruction	I-Operating_System
(	O
SVC	O
)	O
,	O
but	O
that	O
did	O
not	O
require	O
altering	O
or	O
extending	O
the	O
system	O
's	O
virtualization	B-General_Concept
of	O
SVC	O
.	O
</s>
<s>
In	O
1985	O
IBM	O
introduced	O
the	O
PR/SM	B-Device
hypervisor	B-Operating_System
to	O
manage	O
logical	B-Device
partitions	I-Device
(	O
LPAR	B-Device
)	O
.	O
</s>
<s>
Several	O
factors	O
led	O
to	O
a	O
resurgence	O
around	O
2005	O
in	O
the	O
use	O
of	O
virtualization	B-General_Concept
technology	I-General_Concept
among	O
Unix	B-Application
,	O
Linux	B-Application
,	O
and	O
other	O
Unix-like	B-Operating_System
operating	I-Operating_System
systems	I-Operating_System
:	O
</s>
<s>
Major	O
Unix	B-Application
vendors	O
,	O
including	O
HP	O
,	O
IBM	O
,	O
SGI	O
,	O
and	O
Sun	O
Microsystems	O
,	O
have	O
been	O
selling	O
virtualized	O
hardware	B-Architecture
since	O
before	O
2000	O
.	O
</s>
<s>
These	O
have	O
generally	O
been	O
large	O
,	O
expensive	O
systems	O
(	O
in	O
the	O
multimillion-dollar	O
range	O
at	O
the	O
high	O
end	O
)	O
,	O
although	O
virtualization	B-General_Concept
has	O
also	O
been	O
available	O
on	O
some	O
low	O
-	O
and	O
mid-range	O
systems	O
,	O
such	O
as	O
IBM	O
pSeries	B-Device
servers	O
,	O
HP	B-Device
Superdome	I-Device
series	O
machines	O
,	O
and	O
Sun/Oracle	O
T-series	O
CoolThreads	O
servers	O
.	O
</s>
<s>
Although	O
Solaris	B-Application
has	O
always	O
been	O
the	O
only	O
guest	O
domain	O
OS	O
officially	O
supported	O
by	O
Sun/Oracle	O
on	O
their	O
Logical	O
Domains	O
hypervisor	B-Operating_System
,	O
,	O
Linux	B-Application
(	O
Ubuntu	O
and	O
Gentoo	O
)	O
,	O
and	O
FreeBSD	B-Operating_System
have	O
been	O
ported	O
to	O
run	O
on	O
top	O
of	O
the	O
hypervisor	B-Operating_System
(	O
and	O
can	O
all	O
run	O
simultaneously	O
on	O
the	O
same	O
processor	O
,	O
as	O
fully	O
virtualized	O
independent	O
guest	O
OSes	O
)	O
.	O
</s>
<s>
Wind	O
River	O
"	O
Carrier	B-Application
Grade	I-Application
Linux	I-Application
"	O
also	O
runs	O
on	O
Sun	O
's	O
Hypervisor	B-Operating_System
.	O
</s>
<s>
Full	B-Architecture
virtualization	I-Architecture
on	O
SPARC	B-Architecture
processors	O
proved	O
straightforward	O
:	O
since	O
its	O
inception	O
in	O
the	O
mid-1980s	O
Sun	O
deliberately	O
kept	O
the	O
SPARC	B-Architecture
architecture	O
clean	O
of	O
artifacts	O
that	O
would	O
have	O
impeded	O
virtualization	B-General_Concept
.	O
</s>
<s>
(	O
Compare	O
with	O
virtualization	B-General_Concept
on	O
x86	B-Operating_System
processors	O
below	O
.	O
)	O
</s>
<s>
HPE	O
provides	O
HP	B-Device
Integrity	I-Device
Virtual	I-Device
Machines	I-Device
(	O
Integrity	O
VM	B-Application
)	O
to	O
host	O
multiple	O
operating	B-General_Concept
systems	I-General_Concept
on	O
their	O
Itanium	B-General_Concept
powered	O
Integrity	O
systems	O
.	O
</s>
<s>
Itanium	B-General_Concept
can	O
run	O
HP-UX	B-Application
,	O
Linux	B-Application
,	O
Windows	B-Application
and	O
OpenVMS	B-Operating_System
,	O
and	O
these	O
environments	O
are	O
also	O
supported	O
as	O
virtual	O
servers	O
on	O
HP	O
's	O
Integrity	O
VM	B-Application
platform	O
.	O
</s>
<s>
The	O
HP-UX	B-Application
operating	B-General_Concept
system	I-General_Concept
hosts	O
the	O
Integrity	O
VM	B-Application
hypervisor	B-Operating_System
layer	O
that	O
allows	O
for	O
many	O
important	O
features	O
of	O
HP-UX	B-Application
to	O
be	O
taken	O
advantage	O
of	O
and	O
provides	O
major	O
differentiation	O
between	O
this	O
platform	O
and	O
other	O
commodity	O
platforms	O
-	O
such	O
as	O
processor	O
hotswap	O
,	O
memory	O
hotswap	O
,	O
and	O
dynamic	O
kernel	B-Operating_System
updates	O
without	O
system	O
reboot	O
.	O
</s>
<s>
While	O
it	O
heavily	O
leverages	O
HP-UX	B-Application
,	O
the	O
Integrity	O
VM	B-Application
hypervisor	B-Operating_System
is	O
really	O
a	O
hybrid	O
that	O
runs	O
on	O
bare-metal	O
while	O
guests	O
are	O
executing	O
.	O
</s>
<s>
Running	O
normal	O
HP-UX	B-Application
applications	O
on	O
an	O
Integrity	O
VM	B-Application
host	O
is	O
heavily	O
discouraged	O
,	O
because	O
Integrity	O
VM	B-Application
implements	O
its	O
own	O
memory	O
management	O
,	O
scheduling	O
and	O
I/O	O
policies	O
that	O
are	O
tuned	O
for	O
virtual	B-Architecture
machines	I-Architecture
and	O
are	O
not	O
as	O
effective	O
for	O
normal	O
applications	O
.	O
</s>
<s>
IBM	O
provides	O
virtualization	B-General_Concept
partition	B-Device
technology	O
known	O
as	O
logical	B-Device
partitioning	I-Device
(	O
LPAR	B-Device
)	O
on	O
System/390	B-Device
,	O
zSeries	O
,	O
pSeries	B-Device
and	O
IBM	B-Device
AS/400	I-Device
systems	O
.	O
</s>
<s>
For	O
IBM	O
's	O
Power	B-Device
Systems	O
,	O
the	O
POWER	B-Device
Hypervisor	B-Operating_System
(	O
PHYP	O
)	O
is	O
a	O
native	O
(	O
bare-metal	O
)	O
hypervisor	B-Operating_System
in	O
firmware	B-Application
and	O
provides	O
isolation	O
between	O
LPARs	B-Device
.	O
</s>
<s>
Processor	O
capacity	O
is	O
provided	O
to	O
LPARs	B-Device
in	O
either	O
a	O
dedicated	O
fashion	O
or	O
on	O
an	O
entitlement	O
basis	O
where	O
unused	O
capacity	O
is	O
harvested	O
and	O
can	O
be	O
re-allocated	O
to	O
busy	O
workloads	O
.	O
</s>
<s>
Groups	O
of	O
LPARs	B-Device
can	O
have	O
their	O
processor	O
capacity	O
managed	O
as	O
if	O
they	O
were	O
in	O
a	O
"	O
pool	O
"	O
-	O
IBM	O
refers	O
to	O
this	O
capability	O
as	O
Multiple	O
Shared-Processor	O
Pools	O
(	O
MSPPs	O
)	O
and	O
implements	O
it	O
in	O
servers	O
with	O
the	O
POWER6	B-Device
processor	O
.	O
</s>
<s>
LPAR	B-Device
and	O
MSPP	O
capacity	O
allocations	O
can	O
be	O
dynamically	O
changed	O
.	O
</s>
<s>
Memory	O
is	O
allocated	O
to	O
each	O
LPAR	B-Device
(	O
at	O
LPAR	B-Device
initiation	O
or	O
dynamically	O
)	O
and	O
is	O
address-controlled	O
by	O
the	O
POWER	B-Device
Hypervisor	B-Operating_System
.	O
</s>
<s>
For	O
real-mode	O
addressing	O
by	O
operating	B-General_Concept
systems	I-General_Concept
(	O
AIX	B-Application
,	O
Linux	B-Application
,	O
IBM	B-Application
i	I-Application
)	O
,	O
the	O
Power	B-Device
processors	O
(	O
POWER4	B-Device
onwards	O
)	O
have	O
designed	O
virtualization	B-General_Concept
capabilities	O
where	O
a	O
hardware	B-Architecture
address-offset	O
is	O
evaluated	O
with	O
the	O
OS	O
address-offset	O
to	O
arrive	O
at	O
the	O
physical	O
memory	O
address	O
.	O
</s>
<s>
Input/Output	O
(	O
I/O	O
)	O
adapters	O
can	O
be	O
exclusively	O
"	O
owned	O
"	O
by	O
LPARs	B-Device
or	O
shared	O
by	O
LPARs	B-Device
through	O
an	O
appliance	O
partition	B-Device
known	O
as	O
the	O
Virtual	O
I/O	O
Server	O
(	O
VIOS	O
)	O
.	O
</s>
<s>
The	O
Power	B-Device
Hypervisor	B-Operating_System
provides	O
for	O
high	O
levels	O
of	O
reliability	O
,	O
availability	O
and	O
serviceability	O
(	O
RAS	O
)	O
by	O
facilitating	O
hot	O
add/replace	O
of	O
many	O
parts	O
(	O
model	O
dependent	O
:	O
processors	O
,	O
memory	O
,	O
I/O	O
adapters	O
,	O
blowers	O
,	O
power	B-Device
units	O
,	O
disks	O
,	O
system	O
controllers	O
,	O
etc	O
.	O
)	O
</s>
<s>
Similar	O
trends	O
have	O
occurred	O
with	O
x86/x86	O
-64	O
server	O
platforms	O
,	O
where	O
open-source	B-License
projects	O
such	O
as	O
Xen	B-Operating_System
have	O
led	O
virtualization	B-General_Concept
efforts	O
.	O
</s>
<s>
These	O
include	O
hypervisors	B-Operating_System
built	O
on	O
Linux	B-Application
and	O
Solaris	B-Application
kernels	I-Application
as	O
well	O
as	O
custom	O
kernels	B-Operating_System
.	O
</s>
<s>
X86	B-General_Concept
virtualization	I-General_Concept
was	O
introduced	O
in	O
the	O
1990s	O
,	O
with	O
its	O
emulation	B-Application
being	O
included	O
in	O
Bochs	B-Language
.	O
</s>
<s>
Intel	O
and	O
AMD	O
released	O
their	O
first	O
x86	B-Operating_System
processors	O
with	O
hardware	B-General_Concept
virtualisation	I-General_Concept
in	O
2005	O
with	O
Intel	O
VT-x	O
(	O
code-named	O
Vanderpool	O
)	O
and	O
AMD-V	O
(	O
code-named	O
Pacifica	O
)	O
.	O
</s>
<s>
An	O
alternative	O
approach	O
requires	O
modifying	O
the	O
guest	B-General_Concept
operating	I-General_Concept
system	I-General_Concept
to	O
make	O
a	O
system	B-Operating_System
call	I-Operating_System
to	O
the	O
underlying	O
hypervisor	B-Operating_System
,	O
rather	O
than	O
executing	O
machine	O
I/O	O
instructions	O
that	O
the	O
hypervisor	B-Operating_System
simulates	O
.	O
</s>
<s>
This	O
is	O
called	O
paravirtualization	O
in	O
Xen	B-Operating_System
,	O
a	O
"	O
hypercall	B-Operating_System
"	O
in	O
Parallels	B-Operating_System
Workstation	I-Operating_System
,	O
and	O
a	O
"	O
DIAGNOSE	O
code	O
"	O
in	O
IBM	O
VM	B-Application
.	O
</s>
<s>
Some	O
microkernels	O
,	O
such	O
as	O
Mach	B-Operating_System
and	O
L4	B-Operating_System
,	O
are	O
flexible	O
enough	O
to	O
allow	O
paravirtualization	O
of	O
guest	B-General_Concept
operating	I-General_Concept
systems	I-General_Concept
.	O
</s>
<s>
Embedded	B-Application
hypervisors	I-Application
,	O
targeting	O
embedded	B-Architecture
systems	I-Architecture
and	O
certain	O
real-time	B-Operating_System
operating	I-Operating_System
system	I-Operating_System
(	O
RTOS	B-Operating_System
)	O
environments	O
,	O
are	O
designed	O
with	O
different	O
requirements	O
when	O
compared	O
to	O
desktop	O
and	O
enterprise	O
systems	O
,	O
including	O
robustness	O
,	O
security	O
and	O
real-time	B-General_Concept
capabilities	O
.	O
</s>
<s>
The	O
resource-constrained	O
nature	O
of	O
many	O
embedded	B-Architecture
systems	I-Architecture
,	O
especially	O
battery-powered	O
mobile	O
systems	O
,	O
imposes	O
a	O
further	O
requirement	O
for	O
small	O
memory-size	O
and	O
low	O
overhead	O
.	O
</s>
<s>
Finally	O
,	O
in	O
contrast	O
to	O
the	O
ubiquity	O
of	O
the	O
x86	B-Operating_System
architecture	I-Operating_System
in	O
the	O
PC	O
world	O
,	O
the	O
embedded	O
world	O
uses	O
a	O
wider	O
variety	O
of	O
architectures	O
and	O
less	O
standardized	O
environments	O
.	O
</s>
<s>
Support	O
for	O
virtualization	B-General_Concept
requires	O
memory	B-General_Concept
protection	I-General_Concept
(	O
in	O
the	O
form	O
of	O
a	O
memory	B-General_Concept
management	I-General_Concept
unit	I-General_Concept
or	O
at	O
least	O
a	O
memory	B-General_Concept
protection	I-General_Concept
unit	O
)	O
and	O
a	O
distinction	O
between	O
user	B-Operating_System
mode	I-Operating_System
and	O
privileged	B-Operating_System
mode	I-Operating_System
,	O
which	O
rules	O
out	O
most	O
microcontrollers	B-Architecture
.	O
</s>
<s>
This	O
still	O
leaves	O
x86	B-Operating_System
,	O
MIPS	B-Device
,	O
ARM	B-Architecture
and	O
PowerPC	B-Architecture
as	O
widely	O
deployed	O
architectures	O
on	O
medium	O
-	O
to	O
high-end	O
embedded	B-Architecture
systems	I-Architecture
.	O
</s>
<s>
As	O
manufacturers	O
of	O
embedded	B-Architecture
systems	I-Architecture
usually	O
have	O
the	O
source	O
code	O
to	O
their	O
operating	B-General_Concept
systems	I-General_Concept
,	O
they	O
have	O
less	O
need	O
for	O
full	B-Architecture
virtualization	I-Architecture
in	O
this	O
space	O
.	O
</s>
<s>
Instead	O
,	O
the	O
performance	O
advantages	O
of	O
paravirtualization	O
make	O
this	O
usually	O
the	O
virtualization	B-General_Concept
technology	I-General_Concept
of	O
choice	O
.	O
</s>
<s>
Nevertheless	O
,	O
ARM	B-Architecture
and	O
MIPS	B-Device
have	O
recently	O
added	O
full	B-Architecture
virtualization	I-Architecture
support	O
as	O
an	O
IP	O
option	O
and	O
has	O
included	O
it	O
in	O
their	O
latest	O
high-end	O
processors	O
and	O
architecture	O
versions	O
,	O
such	O
as	O
ARM	B-Architecture
Cortex-A15	O
MPCore	O
and	O
ARMv8	O
EL2	O
.	O
</s>
<s>
Other	O
differences	O
between	O
virtualization	B-General_Concept
in	O
server/desktop	O
and	O
embedded	O
environments	O
include	O
requirements	O
for	O
efficient	O
sharing	O
of	O
resources	O
across	O
virtual	B-Architecture
machines	I-Architecture
,	O
high-bandwidth	O
,	O
low-latency	O
inter-VM	O
communication	O
,	O
a	O
global	O
view	O
of	O
scheduling	O
and	O
power	B-Device
management	O
,	O
and	O
fine-grained	O
control	O
of	O
information	O
flows	O
.	O
</s>
<s>
The	O
use	O
of	O
hypervisor	B-Operating_System
technology	O
by	O
malware	O
and	O
rootkits	O
installing	O
themselves	O
as	O
a	O
hypervisor	B-Operating_System
below	O
the	O
operating	B-General_Concept
system	I-General_Concept
,	O
known	O
as	O
hyperjacking	B-General_Concept
,	O
can	O
make	O
them	O
more	O
difficult	O
to	O
detect	O
because	O
the	O
malware	O
could	O
intercept	O
any	O
operations	O
of	O
the	O
operating	B-General_Concept
system	I-General_Concept
(	O
such	O
as	O
someone	O
entering	O
a	O
password	O
)	O
without	O
the	O
anti-malware	O
software	O
necessarily	O
detecting	O
it	O
(	O
since	O
the	O
malware	O
runs	O
below	O
the	O
entire	O
operating	B-General_Concept
system	I-General_Concept
)	O
.	O
</s>
<s>
However	O
,	O
such	O
assertions	O
have	O
been	O
disputed	O
by	O
others	O
who	O
claim	O
that	O
it	O
would	O
be	O
possible	O
to	O
detect	O
the	O
presence	O
of	O
a	O
hypervisor-based	O
rootkit	O
.	O
</s>
<s>
In	O
2009	O
,	O
researchers	O
from	O
Microsoft	O
and	O
North	O
Carolina	O
State	O
University	O
demonstrated	O
a	O
hypervisor-layer	O
anti-rootkit	O
called	O
Hooksafe	B-General_Concept
that	O
can	O
provide	O
generic	O
protection	O
against	O
kernel-mode	B-Operating_System
rootkits	O
.	O
</s>
