<s>
Intel	B-Device
5-level	I-Device
paging	I-Device
,	O
referred	O
to	O
simply	O
as	O
5-level	B-Device
paging	I-Device
in	O
Intel	O
documents	O
,	O
is	O
a	O
processor	O
extension	O
for	O
the	O
x86-64	B-Device
line	O
of	O
processors	O
.	O
</s>
<s>
It	O
extends	O
the	O
size	O
of	O
virtual	O
addresses	O
from	O
48bits	O
to	O
57bits	O
,	O
increasing	O
the	O
addressable	O
virtual	B-Architecture
memory	I-Architecture
from	O
256TB	O
to	O
128PB	O
.	O
</s>
<s>
The	O
extension	O
was	O
first	O
implemented	O
in	O
the	O
Ice	B-Device
Lake	I-Device
processors	O
,	O
and	O
the	O
4.14	O
Linux	B-Operating_System
kernel	I-Operating_System
adds	O
support	O
for	O
it	O
.	O
</s>
<s>
It	O
is	O
provided	O
by	O
a	O
separate	O
kernel	B-Operating_System
of	O
the	O
system	O
called	O
ntkrla57.exe	O
.	O
</s>
<s>
x86-64	B-Device
processors	O
without	O
this	O
feature	O
use	O
a	O
four-level	O
page	B-General_Concept
table	I-General_Concept
structure	O
when	O
operating	O
in	O
64-bit	O
mode	O
.	O
</s>
<s>
A	O
similar	O
situation	O
arose	O
when	O
the	O
32	O
bit	O
IA-32	B-Device
processors	O
used	O
two	O
levels	O
,	O
allowing	O
up	O
to	O
four	O
GB	O
of	O
memory	O
(	O
both	O
virtual	O
and	O
physical	O
)	O
.	O
</s>
<s>
To	O
support	O
more	O
than	O
4GB	O
of	O
RAM	B-Architecture
,	O
an	O
additional	O
mode	O
of	O
address	O
translation	O
called	O
Physical	B-General_Concept
Address	I-General_Concept
Extension	I-General_Concept
(	O
PAE	O
)	O
was	O
defined	O
,	O
involving	O
a	O
third	O
level	O
.	O
</s>
<s>
Likewise	O
,	O
the	O
new	O
extension	O
is	O
enabled	O
by	O
setting	O
bit	O
12	O
of	O
the	O
CR4	O
register	O
(	O
known	O
as	O
LA57	B-Device
)	O
.	O
</s>
<s>
As	O
adding	O
another	O
page	B-General_Concept
table	I-General_Concept
multiplies	O
the	O
address	O
space	O
by	O
512	O
,	O
the	O
virtual	O
limit	O
has	O
increased	O
from	O
256TB	O
to	O
128PB	O
.	O
</s>
<s>
An	O
extra	O
nine	O
bits	O
of	O
the	O
virtual	B-General_Concept
address	I-General_Concept
index	O
the	O
new	O
table	O
,	O
so	O
while	O
formerly	O
bits0	O
through47	O
were	O
in	O
use	O
,	O
now	O
bits0	O
through56	O
are	O
in	O
use	O
.	O
</s>
<s>
As	O
with	O
four	O
level	O
paging	O
,	O
the	O
high-order	O
bits	O
of	O
a	O
virtual	B-General_Concept
address	I-General_Concept
that	O
do	O
not	O
participate	O
in	O
address	O
translation	O
must	O
be	O
the	O
same	O
as	O
the	O
most	O
significant	O
implemented	O
bit	O
.	O
</s>
<s>
Intel	O
has	O
renamed	O
the	O
existing	O
paging	O
system	O
as	O
"	O
4-level	O
paging	O
"	O
,	O
which	O
used	O
to	O
be	O
known	O
as	O
IA-32e	B-Device
paging	O
.	O
</s>
<s>
Extending	O
page	B-General_Concept
table	I-General_Concept
entry	O
to	O
128	O
bits	O
allows	O
full	O
64-bit	O
address	O
space	O
in	O
4-level	O
paging	O
scheme	O
,	O
and	O
makes	O
possible	O
arbitrary	O
allocation	O
unit	O
sizes	O
in	O
5-level	O
scheme	O
.	O
</s>
<s>
5-level	B-Device
paging	I-Device
is	O
implemented	O
by	O
the	O
Ice	B-Device
Lake	I-Device
microarchitecture	B-General_Concept
.	O
</s>
<s>
Support	O
for	O
the	O
extension	O
was	O
submitted	O
as	O
a	O
set	O
of	O
patches	O
to	O
the	O
Linux	B-Operating_System
kernel	I-Operating_System
on	O
8	O
December	O
2016	O
.	O
</s>
<s>
As	O
was	O
reported	O
on	O
the	O
Linux	B-Application
kernel	I-Application
mailing	I-Application
list	I-Application
,	O
it	O
consisted	O
of	O
extending	O
the	O
Linux	B-Operating_System
memory	O
model	O
to	O
use	O
five	O
levels	O
rather	O
than	O
four	O
.	O
</s>
<s>
This	O
is	O
because	O
,	O
although	O
Linux	B-Operating_System
abstracts	B-Application
the	O
details	O
of	O
the	O
page	B-General_Concept
tables	I-General_Concept
,	O
it	O
still	O
depends	O
on	O
having	O
a	O
number	O
of	O
levels	O
in	O
its	O
own	O
representation	O
.	O
</s>
<s>
When	O
an	O
architecture	B-General_Concept
supports	O
fewer	O
levels	O
,	O
Linux	B-Operating_System
emulates	O
extra	O
levels	O
that	O
do	O
nothing	O
.	O
</s>
<s>
Adding	O
another	O
level	O
of	O
indirection	O
makes	O
page	B-General_Concept
table	I-General_Concept
"	O
walks	O
"	O
longer	O
.	O
</s>
<s>
A	O
page	B-General_Concept
table	I-General_Concept
walk	O
occurs	O
when	O
either	O
the	O
processor	O
's	O
memory	B-General_Concept
management	I-General_Concept
unit	I-General_Concept
or	O
the	O
memory	O
management	O
code	O
in	O
the	O
operating	O
system	O
navigates	O
the	O
tree	O
of	O
page	B-General_Concept
tables	I-General_Concept
to	O
find	O
the	O
page	B-General_Concept
table	I-General_Concept
entry	O
corresponding	O
to	O
a	O
virtual	B-General_Concept
address	I-General_Concept
.	O
</s>
<s>
This	O
means	O
that	O
,	O
in	O
the	O
worst	O
case	O
,	O
the	O
processor	O
or	O
the	O
memory	O
manager	O
has	O
to	O
access	O
physical	O
memory	O
six	O
times	O
for	O
a	O
single	O
virtual	B-Architecture
memory	I-Architecture
access	O
,	O
rather	O
than	O
five	O
for	O
the	O
previous	O
iteration	O
of	O
x86-64	B-Device
processors	O
.	O
</s>
<s>
In	O
practice	O
this	O
cost	O
is	O
greatly	O
mitigated	O
by	O
caches	O
such	O
as	O
the	O
translation	B-Architecture
lookaside	I-Architecture
buffer	I-Architecture
(	O
TLB	O
)	O
.	O
</s>
<s>
Further	O
extensions	O
may	O
reduce	O
page	O
walks	O
by	O
using	O
4096	O
128-bit	O
page	B-General_Concept
table	I-General_Concept
entries	O
,	O
and	O
allow	O
a	O
larger	O
64KB	O
page	O
allocation	O
size	O
and	O
backward	O
compatibility	O
with	O
4KB	O
page	O
operations	O
.	O
</s>
