<s>
An	O
embedded	B-Operating_System
hypervisor	I-Operating_System
is	O
a	O
hypervisor	B-Operating_System
that	O
supports	O
the	O
requirements	O
of	O
embedded	B-Architecture
systems	I-Architecture
.	O
</s>
<s>
The	O
requirements	O
for	O
an	O
embedded	B-Operating_System
hypervisor	I-Operating_System
are	O
distinct	O
from	O
hypervisors	B-Operating_System
targeting	O
server	O
and	O
desktop	O
applications	O
.	O
</s>
<s>
An	O
embedded	B-Operating_System
hypervisor	I-Operating_System
is	O
designed	O
into	O
the	O
embedded	B-Architecture
device	I-Architecture
from	O
the	O
outset	O
,	O
rather	O
than	O
loaded	O
subsequent	O
to	O
device	O
deployment	O
.	O
</s>
<s>
While	O
desktop	O
and	O
enterprise	O
environments	O
use	O
hypervisors	B-Operating_System
to	O
consolidate	O
hardware	O
and	O
isolate	O
computing	O
environments	O
from	O
one	O
another	O
,	O
in	O
an	O
embedded	B-Architecture
system	I-Architecture
,	O
the	O
various	O
components	O
typically	O
function	O
collectively	O
to	O
provide	O
the	O
device	O
's	O
functionality	O
.	O
</s>
<s>
Mobile	B-Application
virtualization	I-Application
overlaps	O
with	O
embedded	B-Architecture
system	I-Architecture
virtualization	O
,	O
and	O
shares	O
some	O
use	O
cases	O
.	O
</s>
<s>
Software	O
virtualization	O
has	O
been	O
a	O
major	O
topic	O
in	O
the	O
enterprise	O
space	O
since	O
the	O
late	O
1960s	O
,	O
but	O
only	O
since	O
the	O
early	O
2000s	O
has	O
its	O
use	O
appeared	O
in	O
embedded	B-Architecture
systems	I-Architecture
.	O
</s>
<s>
The	O
use	O
of	O
virtualization	O
and	O
its	O
implementation	O
in	O
the	O
form	O
of	O
a	O
hypervisor	B-Operating_System
in	O
embedded	B-Architecture
systems	I-Architecture
are	O
very	O
different	O
from	O
enterprise	O
applications	O
.	O
</s>
<s>
An	O
effective	O
implementation	O
of	O
an	O
embedded	B-Operating_System
hypervisor	I-Operating_System
must	O
deal	O
with	O
a	O
number	O
of	O
issues	O
specific	O
to	O
such	O
applications	O
.	O
</s>
<s>
These	O
issues	O
include	O
the	O
highly	O
integrated	O
nature	O
of	O
embedded	B-Architecture
systems	I-Architecture
,	O
the	O
requirement	O
for	O
isolated	O
functional	O
blocks	O
within	O
the	O
system	O
to	O
communicate	O
rapidly	O
,	O
the	O
need	O
for	O
real-time/deterministic	O
performance	O
,	O
the	O
resource-constrained	O
target	O
environment	O
and	O
the	O
wide	O
range	O
of	O
security	O
and	O
reliability	O
requirements	O
.	O
</s>
<s>
A	O
hypervisor	B-Operating_System
provides	O
one	O
or	O
more	O
software	O
virtualization	O
environments	O
in	O
which	O
other	O
software	O
,	O
including	O
operating	B-General_Concept
systems	I-General_Concept
,	O
can	O
run	O
with	O
the	O
appearance	O
of	O
full	O
access	O
to	O
the	O
underlying	O
system	O
hardware	O
,	O
where	O
in	O
fact	O
such	O
access	O
is	O
under	O
the	O
complete	O
control	O
of	O
the	O
hypervisor	B-Operating_System
.	O
</s>
<s>
These	O
virtual	O
environments	O
are	O
called	O
virtual	B-Architecture
machines	I-Architecture
(	O
VM	O
)	O
s	O
,	O
and	O
a	O
hypervisor	B-Operating_System
will	O
typically	O
support	O
multiple	O
VMs	O
managed	O
simultaneously	O
.	O
</s>
<s>
Hypervisors	B-Operating_System
are	O
generally	O
classed	O
as	O
either	O
type	O
1	O
or	O
type	O
2	O
,	O
depending	O
on	O
whether	O
the	O
hypervisor	B-Operating_System
runs	O
exclusively	O
in	O
supervisor	O
mode	O
or	O
privileged	O
mode	O
(	O
type	O
1	O
)	O
or	O
is	O
itself	O
hosted	O
by	O
an	O
operating	B-General_Concept
system	I-General_Concept
as	O
a	O
regular	O
application	O
(	O
type	O
2	O
)	O
.	O
</s>
<s>
Type	O
1	O
hypervisors	B-Operating_System
manage	O
key	O
system	O
resources	O
required	O
to	O
maintain	O
control	O
over	O
the	O
virtual	B-Architecture
machines	I-Architecture
,	O
and	O
facilitate	O
a	O
minimal	O
trusted	O
computing	O
base	O
(	O
TCB	O
)	O
.	O
</s>
<s>
Type	O
2	O
hypervisors	B-Operating_System
typically	O
run	O
as	O
an	O
application	O
within	O
a	O
more	O
general	O
purpose	O
operating	B-General_Concept
system	I-General_Concept
,	O
relying	O
on	O
services	O
of	O
the	O
OS	O
to	O
manage	O
system	O
resources	O
.	O
</s>
<s>
An	O
embedded	B-Operating_System
hypervisor	I-Operating_System
is	O
most	O
often	O
a	O
type	O
1	O
hypervisor	B-Operating_System
which	O
supports	O
the	O
requirements	O
of	O
embedded	B-Architecture
systems	I-Architecture
development	O
.	O
</s>
<s>
A	O
small	O
,	O
fast	O
hypervisor	B-Operating_System
with	O
support	O
for	O
multiple	O
isolated	O
VMs	O
;	O
</s>
<s>
An	O
embedded	B-Operating_System
hypervisor	I-Operating_System
typically	O
provides	O
multiple	O
VMs	O
,	O
each	O
of	O
which	O
emulates	O
a	O
hardware	O
platform	O
on	O
which	O
the	O
virtualised	O
software	O
executes	O
.	O
</s>
<s>
The	O
VM	O
may	O
emulate	O
the	O
underlying	O
native	O
hardware	O
,	O
in	O
which	O
case	O
embedded	O
code	O
that	O
runs	O
on	O
the	O
real	O
machine	O
will	O
run	O
on	O
the	O
virtual	B-Architecture
machine	I-Architecture
and	O
vice	O
versa	O
.	O
</s>
<s>
An	O
emulation	O
of	O
the	O
native	O
hardware	O
is	O
not	O
always	O
possible	O
or	O
desired	O
,	O
and	O
a	O
virtual	B-Architecture
platform	I-Architecture
may	O
be	O
defined	O
instead	O
.	O
</s>
<s>
When	O
a	O
VM	O
provides	O
a	O
virtual	B-Architecture
platform	I-Architecture
,	O
guest	O
software	O
has	O
to	O
be	O
ported	O
to	O
run	O
in	O
this	O
environment	O
,	O
however	O
since	O
a	O
virtual	B-Architecture
platform	I-Architecture
can	O
be	O
defined	O
without	O
reliance	O
on	O
the	O
native	O
hardware	O
,	O
guest	O
software	O
supporting	O
a	O
virtual	B-Architecture
platform	I-Architecture
can	O
be	O
run	O
unmodified	O
across	O
various	O
distinct	O
hardware	O
platforms	O
supported	O
by	O
the	O
hypervisor	B-Operating_System
.	O
</s>
<s>
Embedded	B-Operating_System
hypervisors	I-Operating_System
employ	O
either	O
paravirtualization	O
or	O
use	O
virtualization	O
features	O
of	O
the	O
underlying	O
CPU	B-General_Concept
.	O
</s>
<s>
Emulation	O
of	O
hardware	O
at	O
the	O
register	O
level	O
is	O
rarely	O
seen	O
in	O
embedded	B-Operating_System
hypervisors	I-Operating_System
as	O
this	O
is	O
very	O
complex	O
and	O
slow	O
.	O
</s>
<s>
The	O
custom	O
nature	O
of	O
embedded	B-Architecture
systems	I-Architecture
means	O
that	O
the	O
need	O
to	O
support	O
unmodified	O
binary-only	O
guest	O
software	O
which	O
require	O
these	O
techniques	O
is	O
rare	O
.	O
</s>
<s>
The	O
size	O
and	O
efficiency	O
of	O
the	O
implementation	O
is	O
also	O
an	O
issue	O
for	O
an	O
embedded	B-Operating_System
hypervisor	I-Operating_System
,	O
as	O
embedded	B-Architecture
systems	I-Architecture
are	O
often	O
much	O
more	O
resource	O
constrained	O
than	O
desktop	O
and	O
server	O
platforms	O
.	O
</s>
<s>
It	O
is	O
also	O
desirable	O
for	O
the	O
hypervisor	B-Operating_System
to	O
maintain	O
,	O
as	O
closely	O
as	O
possible	O
,	O
the	O
native	O
speed	O
,	O
real-time	O
response	O
and	O
determinism	O
and	O
power	O
efficiency	O
of	O
the	O
underlying	O
hardware	O
platform	O
.	O
</s>
<s>
Implementations	O
for	O
embedded	B-Architecture
systems	I-Architecture
applications	O
have	O
most	O
commonly	O
been	O
based	O
on	O
small	O
microkernel	B-Operating_System
and	O
separation	B-Operating_System
kernel	I-Operating_System
designs	O
,	O
with	O
virtualization	O
built-in	O
as	O
an	O
integral	O
capability	O
.	O
</s>
<s>
This	O
was	O
introduced	O
with	O
PikeOS	B-Operating_System
in	O
2005	O
.	O
</s>
<s>
Examples	O
of	O
these	O
approaches	O
have	O
been	O
produced	O
by	O
companies	O
such	O
as	O
Open	O
Kernel	O
Labs	O
(	O
microkernel	B-Operating_System
followed	O
by	O
a	O
separation	B-Operating_System
kernel	I-Operating_System
)	O
and	O
LynuxWorks	B-Operating_System
(	O
separation	B-Operating_System
kernel	I-Operating_System
)	O
.	O
</s>
<s>
VirtualLogix	O
appears	O
to	O
take	O
the	O
position	O
that	O
an	O
approach	O
based	O
on	O
a	O
dedicated	O
Virtual	B-Operating_System
Machine	I-Operating_System
Monitor	I-Operating_System
(	O
VMM	O
)	O
would	O
be	O
even	O
smaller	O
and	O
more	O
efficient	O
.	O
</s>
<s>
For	O
example	O
:	O
"	O
...	O
hypervisors	B-Operating_System
for	O
embedded	O
use	O
must	O
be	O
real-time	O
capable	O
,	O
as	O
well	O
as	O
resource-miserly.	O
"	O
</s>
<s>
Embedded	B-Architecture
systems	I-Architecture
are	O
typically	O
highly	O
resource	O
constrained	O
due	O
to	O
cost	O
and	O
technical	O
limitations	O
of	O
the	O
hardware	O
.	O
</s>
<s>
It	O
is	O
therefore	O
important	O
for	O
an	O
embedded	B-Operating_System
hypervisor	I-Operating_System
to	O
be	O
as	O
efficient	O
as	O
possible	O
.	O
</s>
<s>
The	O
microkernel	B-Operating_System
and	O
separation	B-Operating_System
kernel	I-Operating_System
based	O
designs	O
allow	O
for	O
small	O
and	O
efficient	O
hypervisors	B-Operating_System
.	O
</s>
<s>
Thus	O
embedded	B-Operating_System
hypervisors	I-Operating_System
usually	O
have	O
a	O
memory	O
footprint	O
from	O
several	O
tens	O
to	O
several	O
hundred	O
kilobytes	O
,	O
depending	O
on	O
the	O
efficiency	O
of	O
the	O
implementation	O
and	O
the	O
level	O
of	O
functionality	O
provided	O
.	O
</s>
<s>
With	O
the	O
small	O
TCB	O
of	O
a	O
type	O
1	O
embedded	B-Operating_System
hypervisor	I-Operating_System
,	O
the	O
system	O
can	O
be	O
made	O
highly	O
secure	O
&	O
reliable	O
.	O
</s>
<s>
Standard	O
software-engineering	O
techniques	O
,	O
such	O
as	O
code	O
inspections	O
and	O
systematic	O
testing	O
,	O
can	O
be	O
used	O
to	O
reduce	O
the	O
number	O
of	O
bugs	O
in	O
such	O
a	O
small	O
code	O
base	O
to	O
a	O
tiny	O
fraction	O
of	O
the	O
defects	O
that	O
must	O
be	O
expected	O
for	O
a	O
hypervisor	B-Operating_System
and	O
guest	O
OS	O
combination	O
that	O
may	O
be	O
100,000	O
–	O
300,000	O
lines	O
in	O
total	O
.	O
</s>
<s>
One	O
of	O
the	O
most	O
important	O
functions	O
required	O
in	O
an	O
embedded	B-Operating_System
hypervisor	I-Operating_System
is	O
a	O
secure	O
message-passing	O
mechanism	O
,	O
which	O
is	O
needed	O
to	O
support	O
real-time	O
communication	O
between	O
processes	O
.	O
</s>
<s>
In	O
a	O
virtualized	O
environment	O
,	O
the	O
embedded	B-Operating_System
hypervisor	I-Operating_System
will	O
support	O
and	O
enforce	O
this	O
isolation	O
between	O
multiple	O
VMs	O
.	O
</s>
<s>
An	O
inter-process	B-Operating_System
communication	I-Operating_System
(	O
IPC	O
)	O
mechanism	O
can	O
be	O
used	O
to	O
provide	O
these	O
functions	O
,	O
as	O
well	O
as	O
invoking	O
all	O
system	O
services	O
,	O
and	O
implemented	O
in	O
a	O
manner	O
which	O
ensures	O
that	O
the	O
desired	O
level	O
of	O
VM	O
isolation	O
is	O
maintained	O
.	O
</s>
<s>
An	O
embedded	B-Operating_System
hypervisor	I-Operating_System
needs	O
to	O
be	O
in	O
complete	O
control	O
of	O
system	O
resources	O
,	O
including	O
memory	O
accesses	O
,	O
to	O
ensure	O
that	O
software	O
cannot	O
break	O
out	O
of	O
the	O
VM	O
.	O
</s>
<s>
A	O
hypervisor	B-Operating_System
therefore	O
requires	O
the	O
target	O
CPU	B-General_Concept
to	O
provide	O
memory	B-General_Concept
management	I-General_Concept
support	O
(	O
typically	O
using	O
an	O
MMU	B-General_Concept
)	O
.	O
</s>
<s>
Many	O
embedded	B-Architecture
processors	I-Architecture
including	O
such	O
as	O
ARM	B-Architecture
,	O
MIPS	O
and	O
PowerPC	B-Architecture
have	O
followed	O
desktop	O
and	O
server	O
chip	O
vendors	O
in	O
adding	O
hardware	O
support	O
for	O
virtualization	O
.	O
</s>
<s>
There	O
are	O
still	O
a	O
large	O
proportion	O
of	O
embedded	B-Architecture
processors	I-Architecture
however	O
which	O
do	O
not	O
provide	O
such	O
support	O
and	O
a	O
hypervisor	B-Operating_System
supporting	O
paravirtualization	O
is	O
required	O
.	O
</s>
<s>
ARM	B-Architecture
processors	I-Architecture
are	O
notable	O
in	O
that	O
most	O
of	O
their	O
application	O
class	O
processor	O
designs	O
support	O
a	O
technology	O
called	O
ARM	B-Architecture
TrustZone	O
,	O
which	O
provides	O
essentially	O
hardware	O
support	O
for	O
one	O
privileged	O
and	O
one	O
unprivileged	O
VM	O
.	O
</s>
<s>
Some	O
of	O
the	O
most	O
common	O
use	O
cases	O
for	O
an	O
embedded	B-Operating_System
hypervisor	I-Operating_System
are	O
:	O
</s>
<s>
Designers	O
of	O
embedded	B-Architecture
systems	I-Architecture
may	O
have	O
many	O
hardware	O
drivers	O
and	O
system	O
services	O
which	O
are	O
specific	O
to	O
a	O
target	O
platform	O
.	O
</s>
<s>
If	O
support	O
for	O
more	O
than	O
one	O
OS	O
is	O
required	O
on	O
the	O
platform	O
,	O
either	O
concurrently	O
or	O
consecutively	O
using	O
a	O
common	O
hardware	O
design	O
,	O
an	O
embedded	B-Operating_System
hypervisor	I-Operating_System
can	O
greatly	O
simplify	O
the	O
task	O
.	O
</s>
<s>
Typically	O
this	O
is	O
used	O
to	O
run	O
a	O
real-time	B-Operating_System
operating	I-Operating_System
system	I-Operating_System
(	O
RTOS	B-Operating_System
)	O
for	O
low-level	O
real-time	O
functionality	O
(	O
such	O
as	O
the	O
communication	O
stack	O
)	O
while	O
at	O
the	O
same	O
time	O
running	O
a	O
general	O
purpose	O
OS	O
,	O
(	O
GPOS	B-General_Concept
)	O
like	O
Linux	B-Application
or	O
Windows	B-Application
,	O
to	O
support	O
user	O
applications	O
,	O
such	O
as	O
a	O
web	O
browser	O
or	O
calendar	O
.	O
</s>
<s>
An	O
embedded	B-Operating_System
hypervisor	I-Operating_System
is	O
able	O
to	O
provide	O
secure	O
encapsulation	O
for	O
any	O
subsystem	O
defined	O
by	O
the	O
developer	O
,	O
so	O
that	O
a	O
compromised	O
subsystem	O
cannot	O
interfere	O
with	O
other	O
subsystems	O
.	O
</s>
<s>
As	O
the	O
embedded	B-Operating_System
hypervisor	I-Operating_System
can	O
encapsulate	O
a	O
subsystem	O
in	O
a	O
VM	O
,	O
it	O
can	O
then	O
enforce	O
the	O
required	O
security	O
policies	O
for	O
communication	O
to	O
and	O
from	O
that	O
subsystem	O
.	O
</s>
<s>
This	O
can	O
be	O
particularly	O
important	O
for	O
embedded	B-Architecture
device	I-Architecture
drivers	O
,	O
as	O
this	O
is	O
where	O
the	O
highest	O
density	O
of	O
fault	O
conditions	O
is	O
seen	O
to	O
occur	O
,	O
and	O
is	O
thus	O
the	O
most	O
common	O
cause	O
of	O
OS	O
failure	O
and	O
system	O
instability	O
.	O
</s>
<s>
It	O
also	O
allows	O
the	O
encapsulation	O
of	O
operating	B-General_Concept
systems	I-General_Concept
that	O
were	O
not	O
necessarily	O
built	O
to	O
the	O
reliability	O
standards	O
demanded	O
of	O
the	O
new	O
system	O
design	O
.	O
</s>
<s>
Where	O
access	O
to	O
the	O
operating	B-General_Concept
system	I-General_Concept
source	O
code	O
is	O
available	O
,	O
paravirtualization	O
is	O
commonly	O
used	O
to	O
virtualize	O
the	O
OS	O
’s	O
on	O
processors	O
without	O
hardware	O
virtualization	O
support	O
,	O
and	O
thus	O
the	O
applications	O
supported	O
by	O
the	O
OS	O
can	O
also	O
run	O
unmodified	O
and	O
without	O
re-compilation	O
in	O
new	O
hardware	O
platform	O
designs	O
.	O
</s>
<s>
Even	O
without	O
source	O
access	O
,	O
legacy	O
binary	O
code	O
can	O
be	O
executed	O
in	O
systems	O
running	O
on	O
processors	O
with	O
hardware	O
virtualization	O
support	O
such	O
as	O
the	O
AMD-V	O
,	O
Intel	O
VT	O
technologies	O
and	O
the	O
latest	O
ARM	B-Architecture
processors	I-Architecture
with	O
virtualization	O
support	O
.	O
</s>
<s>
The	O
legacy	O
binary	O
code	O
could	O
run	O
completely	O
unmodified	O
in	O
a	O
VM	O
with	O
all	O
resource	O
mapping	O
handled	O
by	O
the	O
embedded	B-Operating_System
hypervisor	I-Operating_System
,	O
assuming	O
the	O
system	O
hardware	O
provides	O
equivalent	O
functionality	O
.	O
</s>
<s>
An	O
embedded	B-Operating_System
hypervisor	I-Operating_System
makes	O
it	O
possible	O
to	O
restrict	O
access	O
by	O
other	O
system	O
software	O
components	O
to	O
a	O
specific	O
part	O
of	O
the	O
system	O
containing	O
IP	O
that	O
needs	O
to	O
be	O
protected	O
.	O
</s>
<s>
For	O
example	O
,	O
the	O
embedded	B-Operating_System
hypervisor	I-Operating_System
can	O
provide	O
an	O
isolated	O
execution	O
environment	O
for	O
proprietary	O
software	O
sharing	O
the	O
processor	O
with	O
open	O
source	O
software	O
subject	O
to	O
the	O
GPL	O
.	O
</s>
<s>
As	O
new	O
processors	O
utilise	O
multi-core	O
architectures	O
to	O
increase	O
performance	O
,	O
the	O
embedded	B-Operating_System
hypervisor	I-Operating_System
can	O
manage	O
the	O
underlying	O
architecture	O
and	O
present	O
a	O
uni-processor	O
environment	O
to	O
legacy	O
applications	O
and	O
operating	B-General_Concept
systems	I-General_Concept
while	O
efficiently	O
using	O
the	O
new	O
multiprocessor	O
system	O
design	O
.	O
</s>
<s>
Cross-OS	O
Hypervisor	B-Operating_System
-	O
Allows	O
applications	O
to	O
run	O
natively	O
on	O
a	O
single	O
OS	O
platform	O
from	O
MapuSoft	O
Technologies	O
,	O
Inc	O
.	O
</s>
<s>
OKL4	O
Hypervisor	B-Operating_System
-	O
Supports	O
ARM	B-Architecture
based	O
smart	O
connected	O
devices	O
(	O
embedded	O
,	O
mobile	O
)	O
.	O
</s>
