<s>
User-mode	B-Application
Linux	I-Application
(	O
UML	O
)	O
is	O
an	O
architectural	O
port	O
of	O
the	O
Linux	B-Operating_System
kernel	I-Operating_System
to	O
its	O
own	O
system	B-Operating_System
call	I-Operating_System
interface	O
,	O
which	O
enables	O
multiple	O
virtual	O
Linux	B-Operating_System
kernel-based	O
operating	O
systems	O
(	O
known	O
as	O
guests	O
)	O
to	O
run	O
as	O
an	O
application	O
within	O
a	O
normal	O
Linux	B-Operating_System
system	O
(	O
known	O
as	O
the	O
host	O
)	O
.	O
</s>
<s>
A	O
Linux	B-Operating_System
kernel	I-Operating_System
compiled	O
for	O
the	O
um	O
architecture	O
can	O
then	O
boot	O
as	O
a	O
process	O
under	O
another	O
Linux	B-Operating_System
kernel	I-Operating_System
,	O
entirely	O
in	O
user	B-Operating_System
space	I-Operating_System
,	O
without	O
affecting	O
the	O
host	O
environment	O
's	O
configuration	O
or	O
stability	O
.	O
</s>
<s>
This	O
method	O
gives	O
the	O
user	O
a	O
way	O
to	O
run	O
many	O
virtual	O
Linux	B-Operating_System
machines	O
on	O
a	O
single	O
piece	O
of	O
hardware	O
,	O
allowing	O
some	O
isolation	O
,	O
typically	O
without	O
changing	O
the	O
configuration	O
or	O
stability	O
of	O
the	O
host	O
environment	O
because	O
each	O
guest	O
is	O
just	O
a	O
regular	O
application	O
running	O
as	O
a	O
process	O
in	O
user	B-Operating_System
space	I-Operating_System
.	O
</s>
<s>
One	O
can	O
run	O
network	B-Architecture
services	O
from	O
a	O
UML	O
environment	O
and	O
remain	O
totally	O
sequestered	O
from	O
the	O
main	O
Linux	B-Operating_System
system	O
in	O
which	O
the	O
UML	O
environment	O
runs	O
.	O
</s>
<s>
Administrators	O
can	O
use	O
UML	O
to	O
set	O
up	O
honeypots	O
,	O
which	O
allow	O
one	O
to	O
test	O
the	O
security	O
of	O
one	O
's	O
computers	O
or	O
network	B-Architecture
.	O
</s>
<s>
UML	O
can	O
also	O
be	O
used	O
for	O
teaching	O
and	O
research	O
,	O
providing	O
a	O
realistic	O
Linux	B-Operating_System
networked	O
environment	O
with	O
a	O
high	O
degree	O
of	O
safety	O
.	O
</s>
<s>
In	O
UML	O
environments	O
,	O
host	O
and	O
guest	O
kernel	B-Operating_System
versions	O
do	O
n't	O
need	O
to	O
match	O
,	O
so	O
it	O
is	O
entirely	O
possible	O
to	O
test	O
a	O
"	O
bleeding	B-Application
edge	I-Application
"	O
version	O
of	O
Linux	B-Operating_System
in	O
User-mode	O
on	O
a	O
system	O
running	O
a	O
much	O
older	O
kernel	B-Operating_System
.	O
</s>
<s>
UML	O
also	O
allows	O
kernel	B-Operating_System
debugging	O
to	O
be	O
performed	O
on	O
one	O
machine	O
,	O
where	O
other	O
kernel	B-Operating_System
debugging	O
tools	O
(	O
such	O
as	O
kgdb	B-Application
)	O
require	O
two	O
machines	O
connected	O
with	O
a	O
null	O
modem	O
cable	O
.	O
</s>
<s>
Some	O
web	O
hosting	O
providers	O
offer	O
UML-powered	O
virtual	B-General_Concept
servers	I-General_Concept
for	O
lower	O
prices	O
than	O
true	O
dedicated	O
servers	O
.	O
</s>
<s>
Each	O
customer	O
has	O
root	B-Application
access	I-Application
on	O
what	O
appears	O
to	O
be	O
their	O
own	O
system	O
,	O
while	O
in	O
reality	O
one	O
physical	O
computer	O
is	O
shared	O
between	O
many	O
people	O
.	O
</s>
<s>
libguestfs	B-Application
has	O
supported	O
a	O
UML	O
backend	O
since	O
version	O
1.24	O
as	O
an	O
alternative	O
to	O
using	O
QEMU	O
or	O
KVM	O
.	O
</s>
<s>
The	O
UML	O
guest	O
application	O
(	O
a	O
Linux	B-Operating_System
binary	O
ELF	O
)	O
was	O
originally	O
available	O
as	O
a	O
patch	O
for	O
some	O
Kernel	B-Operating_System
versions	O
above	O
2.2.x	O
,	O
and	O
the	O
host	O
with	O
any	O
kernel	B-Operating_System
version	O
above	O
2.2.x	O
supported	O
it	O
easily	O
in	O
the	O
thread	O
mode	O
(	O
i.e.	O
,	O
non-SKAS3	O
)	O
.	O
</s>
<s>
As	O
of	O
Linux	O
2.6.0	O
,	O
it	O
is	O
integrated	O
into	O
the	O
main	O
kernel	B-Operating_System
source	O
tree	O
.	O
</s>
<s>
A	O
method	O
of	O
running	O
a	O
separate	O
kernel	B-Operating_System
address	O
space	O
(	O
SKAS	O
)	O
that	O
does	O
not	O
require	O
host	O
kernel	B-Operating_System
patching	O
has	O
been	O
implemented	O
.	O
</s>
<s>
This	O
improves	O
performance	O
and	O
security	O
over	O
the	O
old	O
Traced	O
Thread	O
approach	O
,	O
in	O
which	O
processes	O
running	O
in	O
the	O
UML	O
share	O
the	O
same	O
address	O
space	O
from	O
the	O
host	O
's	O
point	O
of	O
view	O
,	O
which	O
leads	O
the	O
memory	O
inside	O
the	O
UML	O
to	O
not	O
be	O
protected	O
by	O
the	O
memory	B-General_Concept
management	I-General_Concept
unit	I-General_Concept
.	O
</s>
<s>
Unlike	O
the	O
current	O
UML	O
using	O
SKAS	O
,	O
buggy	O
or	O
malicious	O
software	O
inside	O
a	O
UML	O
running	O
on	O
a	O
non-SKAS	O
host	O
could	O
be	O
able	O
to	O
read	O
the	O
memory	O
space	O
of	O
other	O
UML	O
processes	O
or	O
even	O
the	O
UML	O
kernel	B-Operating_System
memory	O
.	O
</s>
<s>
User-mode	B-Application
Linux	I-Application
is	O
generally	O
considered	O
to	O
have	O
lower	O
performance	O
than	O
some	O
competing	O
technologies	O
,	O
such	O
as	O
Xen	B-Operating_System
and	O
OpenVZ	B-Language
.	O
</s>
<s>
Future	O
work	O
in	O
adding	O
support	O
for	O
x86	B-General_Concept
virtualization	I-General_Concept
to	O
UML	O
may	O
reduce	O
this	O
disadvantage	O
.	O
</s>
<s>
Often	O
cited	O
as	O
a	O
strength	O
of	O
Xen	B-Operating_System
(	O
a	O
competing	O
technology	O
)	O
is	O
support	O
for	O
thread-local	O
storage	O
(	O
TLS	O
)	O
.	O
</s>
<s>
This	O
is	O
now	O
also	O
supported	O
in	O
the	O
latest	O
UML	O
kernels	B-Operating_System
.	O
</s>
<s>
Xen	B-Operating_System
concentrates	O
on	O
virtualizing	B-General_Concept
the	O
whole	O
machine	O
,	O
and	O
thus	O
all	O
systems	O
running	O
on	O
a	O
Xen	B-Operating_System
machine	O
are	O
really	O
virtual	O
machines	O
.	O
</s>
<s>
In	O
UML	O
,	O
the	O
host	O
machine	O
is	O
not	O
virtualized	B-General_Concept
in	O
any	O
way	O
,	O
and	O
only	O
guest	O
systems	O
are	O
true	O
virtual	O
machines	O
.	O
</s>
<s>
UML	O
was	O
originally	O
designed	O
for	O
the	O
x86	B-Operating_System
instruction	O
set	O
,	O
but	O
has	O
also	O
been	O
ported	O
to	O
others	O
including	O
IA-64	B-General_Concept
and	O
PowerPC	B-Architecture
.	O
</s>
