<s>
x86	B-General_Concept
virtualization	I-General_Concept
is	O
the	O
use	O
of	O
hardware-assisted	B-General_Concept
virtualization	I-General_Concept
capabilities	O
on	O
an	O
x86/x86	O
-64	O
CPU	O
.	O
</s>
<s>
In	O
the	O
late	O
1990s	O
x86	B-General_Concept
virtualization	I-General_Concept
was	O
achieved	O
by	O
complex	O
software	O
techniques	O
,	O
necessary	O
to	O
compensate	O
for	O
the	O
processor	O
's	O
lack	O
of	O
hardware-assisted	B-General_Concept
virtualization	I-General_Concept
capabilities	O
while	O
attaining	O
reasonable	O
performance	O
.	O
</s>
<s>
The	O
following	O
discussion	O
focuses	O
only	O
on	O
virtualization	O
of	O
the	O
x86	O
architecture	O
protected	B-Application
mode	I-Application
.	O
</s>
<s>
In	O
protected	B-Application
mode	I-Application
the	O
operating	O
system	O
kernel	B-Operating_System
runs	O
at	O
a	O
higher	O
privilege	O
such	O
as	O
ring	B-Operating_System
0	I-Operating_System
,	O
and	O
applications	O
at	O
a	O
lower	O
privilege	O
such	O
as	O
ring	B-Operating_System
3	I-Operating_System
.	O
</s>
<s>
One	O
approach	O
used	O
in	O
x86	O
software-based	O
virtualization	O
to	O
overcome	O
this	O
limitation	O
is	O
called	O
ring	B-General_Concept
deprivileging	I-General_Concept
,	O
which	O
involves	O
running	O
the	O
guest	O
OS	O
at	O
a	O
ring	B-Operating_System
higher	O
(	O
lesser	O
privileged	O
)	O
than	O
0	O
.	O
</s>
<s>
Three	O
techniques	O
made	O
virtualization	O
of	O
protected	B-Application
mode	I-Application
possible	O
:	O
</s>
<s>
Binary	O
translation	O
is	O
used	O
to	O
rewrite	O
certain	O
ring	B-Operating_System
0	I-Operating_System
instructions	O
in	O
terms	O
of	O
ring	B-Operating_System
3	I-Operating_System
instructions	O
,	O
such	O
as	O
POPF	O
,	O
that	O
would	O
otherwise	O
fail	O
silently	O
or	O
behave	O
differently	O
when	O
executed	O
above	O
ring	B-Operating_System
0	I-Operating_System
,	O
making	O
the	O
classic	O
trap-and-emulate	B-General_Concept
virtualization	I-General_Concept
impossible	O
.	O
</s>
<s>
To	O
improve	O
performance	O
,	O
the	O
translated	O
basic	B-Application
blocks	I-Application
need	O
to	O
be	O
cached	O
in	O
a	O
coherent	O
way	O
that	O
detects	O
code	O
patching	O
(	O
used	O
in	O
VxDs	B-Application
for	O
instance	O
)	O
,	O
the	O
reuse	O
of	O
pages	O
by	O
the	O
guest	O
OS	O
,	O
or	O
even	O
self-modifying	B-Application
code	I-Application
.	O
</s>
<s>
A	O
number	O
of	O
key	O
data	B-General_Concept
structures	I-General_Concept
used	O
by	O
a	O
processor	O
need	O
to	O
be	O
shadowed	B-General_Concept
.	O
</s>
<s>
Because	O
most	O
operating	O
systems	O
use	O
paged	O
virtual	O
memory	O
,	O
and	O
granting	O
the	O
guest	O
OS	O
direct	O
access	O
to	O
the	O
MMU	B-General_Concept
would	O
mean	O
loss	O
of	O
control	O
by	O
the	O
virtualization	B-Operating_System
manager	I-Operating_System
,	O
some	O
of	O
the	O
work	O
of	O
the	O
x86	O
MMU	B-General_Concept
needs	O
to	O
be	O
duplicated	O
in	O
software	O
for	O
the	O
guest	O
OS	O
using	O
a	O
technique	O
known	O
as	O
shadow	O
page	O
tables	O
.	O
</s>
<s>
This	O
involves	O
denying	O
the	O
guest	O
OS	O
any	O
access	O
to	O
the	O
actual	O
page	O
table	O
entries	O
by	O
trapping	O
access	O
attempts	O
and	O
emulating	B-Application
them	O
instead	O
in	O
software	O
.	O
</s>
<s>
The	O
x86	O
architecture	O
uses	O
hidden	O
state	O
to	O
store	O
segment	B-Operating_System
descriptors	I-Operating_System
in	O
the	O
processor	O
,	O
so	O
once	O
the	O
segment	B-Operating_System
descriptors	I-Operating_System
have	O
been	O
loaded	O
into	O
the	O
processor	O
,	O
the	O
memory	O
from	O
which	O
they	O
have	O
been	O
loaded	O
may	O
be	O
overwritten	O
and	O
there	O
is	O
no	O
way	O
to	O
get	O
the	O
descriptors	O
back	O
from	O
the	O
processor	O
.	O
</s>
<s>
I/O	O
device	O
emulation	B-Application
:	O
Unsupported	O
devices	O
on	O
the	O
guest	O
OS	O
must	O
be	O
emulated	B-Application
by	O
a	O
device	B-Application
emulator	I-Application
that	O
runs	O
in	O
the	O
host	O
OS	O
.	O
</s>
<s>
These	O
techniques	O
incur	O
some	O
performance	O
overhead	O
due	O
to	O
lack	O
of	O
MMU	B-General_Concept
virtualization	O
support	O
,	O
as	O
compared	O
to	O
a	O
VM	O
running	O
on	O
a	O
natively	O
virtualizable	O
architecture	O
such	O
as	O
the	O
IBM	B-Device
System/370	I-Device
.	O
</s>
<s>
On	O
traditional	O
mainframes	O
,	O
the	O
classic	O
type	O
1	O
hypervisor	B-Operating_System
was	O
self-standing	O
and	O
did	O
not	O
depend	O
on	O
any	O
operating	O
system	O
or	O
run	O
any	O
user	O
applications	O
itself	O
.	O
</s>
<s>
In	O
contrast	O
,	O
the	O
first	O
x86	B-General_Concept
virtualization	I-General_Concept
products	O
were	O
aimed	O
at	O
workstation	O
computers	O
,	O
and	O
ran	O
a	O
guest	O
OS	O
inside	O
a	O
host	O
OS	O
by	O
embedding	O
the	O
hypervisor	B-Operating_System
in	O
a	O
kernel	B-Operating_System
module	O
that	O
ran	O
under	O
the	O
host	O
OS	O
(	O
type	O
2	O
hypervisor	B-Operating_System
)	O
.	O
</s>
<s>
There	O
has	O
been	O
some	O
controversy	O
whether	O
the	O
x86	O
architecture	O
with	O
no	O
hardware	O
assistance	O
is	O
virtualizable	O
as	O
described	O
by	O
Popek	B-Architecture
and	I-Architecture
Goldberg	I-Architecture
.	O
</s>
<s>
VMware	O
researchers	O
pointed	O
out	O
in	O
a	O
2006	O
ASPLOS	O
paper	O
that	O
the	O
above	O
techniques	O
made	O
the	O
x86	O
platform	O
virtualizable	O
in	O
the	O
sense	O
of	O
meeting	O
the	O
three	O
criteria	O
of	O
Popek	B-Architecture
and	I-Architecture
Goldberg	I-Architecture
,	O
albeit	O
not	O
by	O
the	O
classic	O
trap-and-emulate	O
technique	O
.	O
</s>
<s>
A	O
different	O
route	O
was	O
taken	O
by	O
other	O
systems	O
like	O
Denali	O
,	O
L4	B-Operating_System
,	O
and	O
Xen	B-Operating_System
,	O
known	O
as	O
paravirtualization	O
,	O
which	O
involves	O
porting	O
operating	O
systems	O
to	O
run	O
on	O
the	O
resulting	O
virtual	B-Architecture
machine	I-Architecture
,	O
which	O
does	O
not	O
implement	O
the	O
parts	O
of	O
the	O
actual	O
x86	O
instruction	O
set	O
that	O
are	O
hard	O
to	O
virtualize	O
.	O
</s>
<s>
The	O
paravirtualized	O
I/O	O
has	O
significant	O
performance	O
benefits	O
as	O
demonstrated	O
in	O
the	O
original	O
SOSP'03	O
Xen	B-Operating_System
paper	O
.	O
</s>
<s>
The	O
initial	O
version	O
of	O
x86-64	B-Device
(	O
AMD64	B-Device
)	O
did	O
not	O
allow	O
for	O
a	O
software-only	O
full	O
virtualization	O
due	O
to	O
the	O
lack	O
of	O
segmentation	O
support	O
in	O
long	B-Application
mode	I-Application
,	O
which	O
made	O
the	O
protection	O
of	O
the	O
hypervisor	B-Operating_System
's	O
memory	O
impossible	O
,	O
in	O
particular	O
,	O
the	O
protection	O
of	O
the	O
trap	B-Application
handler	O
that	O
runs	O
in	O
the	O
guest	O
kernel	B-Operating_System
address	O
space	O
.	O
</s>
<s>
Revision	O
D	O
and	O
later	O
64-bit	O
AMD	O
processors	O
(	O
as	O
a	O
rule	O
of	O
thumb	O
,	O
those	O
manufactured	O
in	O
90nm	O
or	O
less	O
)	O
added	O
basic	O
support	O
for	O
segmentation	O
in	O
long	B-Application
mode	I-Application
,	O
making	O
it	O
possible	O
to	O
run	O
64-bit	O
guests	O
in	O
64-bit	O
hosts	O
via	O
binary	O
translation	O
.	O
</s>
<s>
Intel	O
did	O
not	O
add	O
segmentation	O
support	O
to	O
its	O
x86-64	B-Device
implementation	O
(	O
Intel	O
64	O
)	O
,	O
making	O
64-bit	O
software-only	O
virtualization	O
impossible	O
on	O
Intel	O
CPUs	O
,	O
but	O
Intel	O
VT-x	O
support	O
makes	O
64-bit	O
hardware	O
assisted	O
virtualization	O
possible	O
on	O
the	O
Intel	O
platform	O
.	O
</s>
<s>
The	O
issue	O
of	O
low	O
performance	O
of	O
virtualized	O
system	O
memory	O
was	O
addressed	O
with	O
MMU	B-General_Concept
virtualization	O
that	O
was	O
added	O
to	O
the	O
chipset	B-Device
later	O
.	O
</s>
<s>
Based	O
on	O
painful	O
experiences	O
with	O
the	O
80286	B-General_Concept
protected	B-Application
mode	I-Application
,	O
which	O
by	O
itself	O
was	O
not	O
suitable	O
enough	O
to	O
run	O
concurrent	O
DOS	B-Device
applications	O
well	O
,	O
Intel	O
introduced	O
the	O
virtual	B-Application
8086	I-Application
mode	I-Application
in	O
their	O
80386	B-General_Concept
chip	O
,	O
which	O
offered	O
virtualized	O
8086	O
processors	O
on	O
the	O
386	B-General_Concept
and	O
later	O
chips	O
.	O
</s>
<s>
Hardware	O
support	O
for	O
virtualizing	O
the	O
protected	B-Application
mode	I-Application
itself	O
,	O
however	O
,	O
became	O
available	O
20	O
years	O
later	O
.	O
</s>
<s>
AMD	O
developed	O
its	O
first	O
generation	O
virtualization	O
extensions	O
under	O
the	O
code	O
name	O
"	O
Pacifica	O
"	O
,	O
and	O
initially	O
published	O
them	O
as	O
AMD	O
Secure	O
Virtual	B-Architecture
Machine	I-Architecture
(	O
SVM	O
)	O
,	O
but	O
later	O
marketed	O
them	O
under	O
the	O
trademark	O
AMD	O
Virtualization	O
,	O
abbreviated	O
AMD-V	O
.	O
</s>
<s>
AMD-V	O
capability	O
also	O
features	O
on	O
the	O
Athlon	O
64	O
and	O
Athlon	O
64	O
X2	O
family	O
of	O
processors	O
with	O
revisions	O
"	O
F	O
"	O
or	O
"	O
G	O
"	O
on	O
socket	O
AM2	O
,	O
Turion	O
64	O
X2	O
,	O
and	O
Opteron	B-General_Concept
2nd	O
generation	O
and	O
third-generation	O
,	O
Phenom	O
and	O
Phenom	O
II	O
processors	O
.	O
</s>
<s>
The	O
APU	B-Architecture
Fusion	I-Architecture
processors	O
support	O
AMD-V	O
.	O
AMD-V	O
is	O
not	O
supported	O
by	O
any	O
Socket	O
939	O
processors	O
.	O
</s>
<s>
AMD	B-General_Concept
Opteron	I-General_Concept
CPUs	O
beginning	O
with	O
the	O
Family	O
0x10	O
Barcelona	O
line	O
,	O
and	O
Phenom	O
II	O
CPUs	O
,	O
support	O
a	O
second	O
generation	O
hardware	O
virtualization	B-General_Concept
technology	I-General_Concept
called	O
Rapid	O
Virtualization	O
Indexing	O
(	O
formerly	O
known	O
as	O
Nested	B-Device
Page	I-Device
Tables	I-Device
during	O
its	O
development	O
)	O
,	O
later	O
adopted	O
by	O
Intel	O
as	O
Extended	O
Page	O
Tables	O
(	O
EPT	O
)	O
.	O
</s>
<s>
This	O
may	O
be	O
checked	O
in	O
BSD	B-Operating_System
derivatives	I-Operating_System
via	O
dmesg	B-Device
or	O
sysctl	B-Application
and	O
in	O
Linux	B-Operating_System
via	O
/proc/cpuinfo	O
.	O
</s>
<s>
With	O
some	O
motherboards	B-Device
,	O
users	O
must	O
enable	O
AMD	O
SVM	O
feature	O
in	O
the	O
BIOS	B-Operating_System
setup	O
before	O
applications	O
can	O
make	O
use	O
of	O
it	O
.	O
</s>
<s>
On	O
November	O
13	O
,	O
2005	O
,	O
Intel	O
released	O
two	O
models	O
of	O
Pentium	B-General_Concept
4	I-General_Concept
(	O
Model	O
662	O
and	O
672	O
)	O
as	O
the	O
first	O
Intel	O
processors	O
to	O
support	O
VT-x	O
.	O
</s>
<s>
The	O
CPU	O
flag	O
for	O
VT-x	O
capability	O
is	O
"	O
vmx	O
"	O
;	O
in	O
Linux	B-Operating_System
,	O
this	O
can	O
be	O
checked	O
via	O
/proc/cpuinfo	O
,	O
or	O
in	O
macOS	B-Application
via	O
sysctl	B-Application
machdep.cpu.features	O
.	O
</s>
<s>
"	O
VMX	O
"	O
stands	O
for	O
Virtual	B-Architecture
Machine	I-Architecture
Extensions	O
,	O
which	O
adds	O
13	O
new	O
instructions	O
:	O
VMPTRLD	O
,	O
VMPTRST	O
,	O
VMCLEAR	O
,	O
VMREAD	O
,	O
VMWRITE	O
,	O
VMCALL	O
,	O
VMLAUNCH	O
,	O
VMRESUME	O
,	O
VMXOFF	O
,	O
VMXON	O
,	O
INVEPT	O
,	O
INVVPID	O
,	O
and	O
VMFUNC	O
.	O
</s>
<s>
These	O
instructions	O
permit	O
entering	O
and	O
exiting	O
a	O
virtual	O
execution	O
mode	O
where	O
the	O
guest	O
OS	O
perceives	O
itself	O
as	O
running	O
with	O
full	O
privilege	O
(	O
ring	B-Operating_System
0	I-Operating_System
)	O
,	O
but	O
the	O
host	O
OS	O
remains	O
protected	O
.	O
</s>
<s>
,	O
almost	O
all	O
newer	O
server	O
,	O
desktop	O
and	O
mobile	O
Intel	O
processors	O
support	O
VT-x	O
,	O
with	O
some	O
of	O
the	O
Intel	B-Device
Atom	I-Device
processors	O
as	O
the	O
primary	O
exception	O
.	O
</s>
<s>
With	O
some	O
motherboards	B-Device
,	O
users	O
must	O
enable	O
Intel	O
's	O
VT-x	O
feature	O
in	O
the	O
BIOS	B-Operating_System
setup	O
before	O
applications	O
can	O
make	O
use	O
of	O
it	O
.	O
</s>
<s>
Intel	O
started	O
to	O
include	O
Extended	O
Page	O
Tables	O
(	O
EPT	O
)	O
,	O
a	O
technology	O
for	O
page-table	O
virtualization	O
,	O
since	O
the	O
Nehalem	B-Device
architecture	O
,	O
released	O
in	O
2008	O
.	O
</s>
<s>
In	O
2010	O
,	O
Westmere	B-Device
added	O
support	O
for	O
launching	O
the	O
logical	O
processor	O
directly	O
in	O
real	B-Application
mode	I-Application
a	O
feature	O
called	O
"	O
unrestricted	O
guest	O
"	O
,	O
which	O
requires	O
EPT	O
to	O
work	O
.	O
</s>
<s>
Since	O
the	O
Haswell	B-Device
microarchitecture	I-Device
(	O
announced	O
in	O
2013	O
)	O
,	O
Intel	O
started	O
to	O
include	O
VMCS	O
shadowing	O
as	O
a	O
technology	O
that	O
accelerates	O
nested	O
virtualization	O
of	O
VMMs	O
.	O
</s>
<s>
The	O
virtual	B-Architecture
machine	I-Architecture
control	O
structure	O
(	O
VMCS	O
)	O
is	O
a	O
data	B-General_Concept
structure	I-General_Concept
in	O
memory	O
that	O
exists	O
exactly	O
once	O
per	O
VM	O
,	O
while	O
it	O
is	O
managed	O
by	O
the	O
VMM	O
.	O
</s>
<s>
In	O
such	O
cases	O
,	O
VMCS	O
needs	O
to	O
be	O
shadowed	B-General_Concept
multiple	O
times	O
(	O
in	O
case	O
of	O
nesting	O
)	O
and	O
partially	O
implemented	O
in	O
software	O
in	O
case	O
there	O
is	O
no	O
hardware	O
support	O
by	O
the	O
processor	O
.	O
</s>
<s>
VIA	B-Device
Nano	I-Device
3000	O
Series	O
Processors	O
and	O
higher	O
support	O
VIA	O
VT	O
virtualization	B-General_Concept
technology	I-General_Concept
compatible	O
with	O
Intel	O
VT-x	O
.	O
</s>
<s>
EPT	O
is	O
present	O
in	O
Zhaoxin	B-Device
ZX-C	I-Device
,	O
a	O
descendant	O
of	O
VIA	O
QuadCore-E	O
&	O
Eden	O
X4	O
similar	O
to	O
Nano	O
C4350AL	O
.	O
</s>
<s>
In	O
2012	O
,	O
AMD	O
announced	O
their	O
Advanced	O
Virtual	O
Interrupt	B-Application
Controller	O
(	O
AVIC	O
)	O
targeting	O
interrupt	B-Application
overhead	O
reduction	O
in	O
virtualization	O
environments	O
.	O
</s>
<s>
This	O
technology	O
,	O
as	O
announced	O
,	O
does	O
not	O
support	O
x2APIC	B-Device
.	O
</s>
<s>
Also	O
in	O
2012	O
,	O
Intel	O
announced	O
a	O
similar	O
technology	O
for	O
interrupt	B-Application
and	O
APIC	B-Device
virtualization	O
,	O
which	O
did	O
not	O
have	O
a	O
brand	O
name	O
at	O
its	O
announcement	O
time	O
.	O
</s>
<s>
and	O
it	O
became	O
commercially	O
available	O
in	O
the	O
Ivy	B-Device
Bridge	I-Device
EP	I-Device
series	O
of	O
Intel	O
CPUs	O
,	O
which	O
is	O
sold	O
as	O
Xeon	O
E5-26xx	O
v2	O
(	O
launched	O
in	O
late	O
2013	O
)	O
and	O
as	O
Xeon	O
E5-46xx	O
v2	O
(	O
launched	O
in	O
early	O
2014	O
)	O
.	O
</s>
<s>
Intel	O
Graphics	O
Virtualization	B-General_Concept
Technology	I-General_Concept
(	O
GVT	O
)	O
provides	O
graphics	O
virtualization	O
as	O
part	O
of	O
more	O
recent	O
Gen	O
graphics	O
architectures	O
.	O
</s>
<s>
Although	O
AMD	B-Architecture
APUs	I-Architecture
implement	O
the	O
x86-64	B-Device
instruction	O
set	O
,	O
they	O
implement	O
AMD	O
's	O
own	O
graphics	O
architectures	O
(	O
TeraScale	B-Architecture
,	O
GCN	B-Architecture
and	O
RDNA	O
)	O
which	O
do	O
not	O
support	O
graphics	O
virtualization	O
.	O
</s>
<s>
Larrabee	B-Architecture
was	O
the	O
only	O
graphics	O
microarchitecture	B-General_Concept
based	O
on	O
x86	O
,	O
but	O
it	O
likely	O
did	O
not	O
include	O
support	O
for	O
graphics	O
virtualization	O
.	O
</s>
<s>
Memory	O
and	O
I/O	B-Protocol
virtualization	I-Protocol
is	O
performed	O
by	O
the	O
chipset	B-Device
.	O
</s>
<s>
Typically	O
these	O
features	O
must	O
be	O
enabled	O
by	O
the	O
BIOS	B-Operating_System
,	O
which	O
must	O
be	O
able	O
to	O
support	O
them	O
and	O
also	O
be	O
set	O
to	O
use	O
them	O
.	O
</s>
<s>
An	O
input/output	O
memory	B-General_Concept
management	I-General_Concept
unit	I-General_Concept
(	O
IOMMU	O
)	O
allows	O
guest	O
virtual	B-Architecture
machines	I-Architecture
to	O
directly	O
use	O
peripheral	O
devices	O
,	O
such	O
as	O
Ethernet	O
,	O
accelerated	O
graphics	O
cards	O
,	O
and	O
hard-drive	O
controllers	O
,	O
through	O
DMA	B-General_Concept
and	O
interrupt	B-Application
remapping	O
.	O
</s>
<s>
This	O
is	O
sometimes	O
called	O
PCI	B-Protocol
passthrough	O
.	O
</s>
<s>
At	O
the	O
same	O
time	O
,	O
an	O
IOMMU	O
also	O
allows	O
operating	O
systems	O
and	O
hypervisors	B-Operating_System
to	O
prevent	O
buggy	O
or	O
malicious	O
hardware	O
from	O
compromising	O
memory	O
security	O
.	O
</s>
<s>
Intel	O
's	O
"	O
Virtualization	B-General_Concept
Technology	I-General_Concept
for	O
Directed	O
I/O	O
"	O
(	O
VT-d	O
)	O
,	O
included	O
in	O
most	O
high-end	O
(	O
but	O
not	O
all	O
)	O
newer	O
Intel	O
processors	O
since	O
the	O
Core	O
2	O
architecture	O
.	O
</s>
<s>
In	O
addition	O
to	O
the	O
CPU	O
support	O
,	O
both	O
motherboard	B-Device
chipset	B-Device
and	O
system	O
firmware	O
(	O
BIOS	B-Operating_System
or	O
UEFI	B-Architecture
)	O
need	O
to	O
fully	O
support	O
the	O
IOMMU	O
I/O	B-Protocol
virtualization	I-Protocol
functionality	O
for	O
it	O
to	O
be	O
usable	O
.	O
</s>
<s>
Only	O
the	O
PCI	B-Protocol
or	O
PCI	B-Protocol
Express	O
devices	O
supporting	O
function	O
level	O
reset	O
(	O
FLR	O
)	O
can	O
be	O
virtualized	O
this	O
way	O
,	O
as	O
it	O
is	O
required	O
for	O
reassigning	O
various	O
device	O
functions	O
between	O
virtual	B-Architecture
machines	I-Architecture
.	O
</s>
<s>
If	O
a	O
device	O
to	O
be	O
assigned	O
does	O
not	O
support	O
Message	B-Architecture
Signaled	I-Architecture
Interrupts	I-Architecture
(	O
MSI	O
)	O
,	O
it	O
must	O
not	O
share	O
interrupt	B-Application
lines	I-Application
with	O
other	O
devices	O
for	O
the	O
assignment	O
to	O
be	O
possible	O
.	O
</s>
<s>
All	O
conventional	B-Protocol
PCI	I-Protocol
devices	O
routed	O
behind	O
a	O
PCI/PCI	O
-X-to-PCI	O
Express	O
bridge	O
can	O
be	O
assigned	O
to	O
a	O
guest	O
virtual	B-Architecture
machine	I-Architecture
only	O
all	O
at	O
once	O
;	O
PCI	B-Protocol
Express	O
devices	O
have	O
no	O
such	O
restriction	O
.	O
</s>
<s>
Intel	O
's	O
"	O
Virtualization	B-General_Concept
Technology	I-General_Concept
for	O
Connectivity	O
"	O
(	O
VT-c	O
)	O
.	O
</s>
<s>
PCI-SIG	O
Single	O
Root	O
I/O	B-Protocol
Virtualization	I-Protocol
(	O
SR-IOV	B-Protocol
)	O
provides	O
a	O
set	O
of	O
general	O
(	O
non-x86	O
specific	O
)	O
I/O	B-Protocol
virtualization	I-Protocol
methods	O
based	O
on	O
PCI	B-Protocol
Express	O
(	O
PCIe	O
)	O
native	O
hardware	O
,	O
as	O
standardized	O
by	O
PCI-SIG	O
:	O
</s>
<s>
Address	O
translation	O
services	O
(	O
ATS	O
)	O
supports	O
native	O
IOV	O
across	O
PCI	B-Protocol
Express	O
via	O
address	O
translation	O
.	O
</s>
<s>
Single-root	B-Protocol
IOV	I-Protocol
(	O
SR-IOV	B-Protocol
or	O
SRIOV	B-Protocol
)	O
supports	O
native	O
IOV	O
in	O
existing	O
single-root	O
complex	O
PCI	B-Protocol
Express	O
topologies	O
.	O
</s>
<s>
Multi-root	O
IOV	O
(	O
MR-IOV	O
)	O
supports	O
native	O
IOV	O
in	O
new	O
topologies	O
(	O
for	O
example	O
,	O
blade	O
servers	O
)	O
by	O
building	O
on	O
SR-IOV	B-Protocol
to	O
provide	O
multiple	O
root	O
complexes	O
which	O
share	O
a	O
common	O
PCI	B-Protocol
Express	O
hierarchy	O
.	O
</s>
<s>
In	O
SR-IOV	B-Protocol
,	O
the	O
most	O
common	O
of	O
these	O
,	O
a	O
host	O
VMM	O
configures	O
supported	O
devices	O
to	O
create	O
and	O
allocate	O
virtual	O
"	O
shadows	O
"	O
of	O
their	O
configuration	O
spaces	O
so	O
that	O
virtual	B-Architecture
machine	I-Architecture
guests	O
can	O
directly	O
configure	O
and	O
access	O
such	O
"	O
shadow	O
"	O
device	O
resources	O
.	O
</s>
<s>
With	O
SR-IOV	B-Protocol
enabled	O
,	O
virtualized	O
network	O
interfaces	O
are	O
directly	O
accessible	O
to	O
the	O
guests	O
,	O
</s>
<s>
avoiding	O
involvement	O
of	O
the	O
VMM	O
and	O
resulting	O
in	O
high	O
overall	O
performance	O
;	O
for	O
example	O
,	O
SR-IOV	B-Protocol
achieves	O
over	O
95%	O
of	O
the	O
bare	O
metal	O
network	O
bandwidth	O
in	O
NASA	O
's	O
virtualized	O
datacenter	O
and	O
in	O
the	O
Amazon	B-Application
Public	I-Application
Cloud	I-Application
.	O
</s>
