User-mode Linux (UML) is a

virtualization In computing, virtualization (abbreviated v12n) is a series of technologies that allows dividing of physical computing resources into a series of virtual machines, operating systems, processes or containers. Virtualization began in the 1960s wit ...

system for the

Linux Linux ( ) is a family of open source Unix-like operating systems based on the Linux kernel, an kernel (operating system), operating system kernel first released on September 17, 1991, by Linus Torvalds. Linux is typically package manager, pac ...

operating system based on an architectural

port A port is a maritime facility comprising one or more wharves or loading areas, where ships load and discharge cargo and passengers. Although usually situated on a sea coast or estuary, ports can also be found far inland, such as Hamburg, Manch ...

of the

Linux kernel The Linux kernel is a Free and open-source software, free and open source Unix-like kernel (operating system), kernel that is used in many computer systems worldwide. The kernel was created by Linus Torvalds in 1991 and was soon adopted as the k ...

to its own

system call In computing, a system call (syscall) is the programmatic way in which a computer program requests a service from the operating system on which it is executed. This may include hardware-related services (for example, accessing a hard disk drive ...

interface, which enables multiple virtual Linux kernel-based operating systems (known as guests) to run as an application within a normal Linux system (known as the host). A Linux kernel compiled for the ''um'' architecture can then boot as a process under another Linux kernel, entirely in

user space A modern computer operating system usually uses virtual memory to provide separate address spaces or regions of a single address space, called user space and kernel space. This separation primarily provides memory protection and hardware prote ...

, without affecting the host environment's configuration or stability. This method gives the user a way to run many virtual Linux machines on a single piece of hardware, allowing some isolation, typically without changing the configuration or stability of the host environment because each guest is just a regular application running as a process in user space.

Applications

Numerous things become possible through the use of UML. One can run

network Network, networking and networked may refer to: Science and technology * Network theory, the study of graphs as a representation of relations between discrete objects * Network science, an academic field that studies complex networks Mathematics ...

services from a UML environment and remain totally sequestered from the main Linux system in which the UML environment runs. Administrators can use UML to set up honeypots, which allow one to test the security of one's computers or network. UML can serve to test and debug new software without adversely affecting the host system. UML can also be used for teaching and research, providing a realistic Linux networked environment with a high degree of safety. In UML environments, host and guest kernel versions don't need to match, so it is entirely possible to test a "

bleeding edge Emerging technologies are technologies whose development, practical applications, or both are still largely unrealized. These technologies are generally new but also include old technologies finding new applications. Emerging technologies are o ...

" version of Linux in User-mode on a system running a much older kernel. UML also allows kernel debugging to be performed on one machine, where other kernel debugging tools (such as kgdb) require two machines connected with a

null modem Null modem is a communication method to directly connect two data terminal equipment, DTEs (computer, computer terminal, terminal, printer (computing), printer, etc.) using an RS-232 serial cable. The name stems from the historical use of RS-232 ...

cable. Some

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providers offer UML-powered virtual servers for lower prices than true

dedicated servers A dedicated hosting service, dedicated server, or managed hosting service is a type of Internet hosting in which the client leases an entire server not shared with anyone else. This is more flexible than shared hosting, as organizations have ...

. Each customer has

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access on what appears to be their own system, while in reality one physical computer is shared between many people. libguestfs has supported a UML backend since version 1.24 as an alternative to using QEMU or KVM.

Integration into the Linux kernel

The UML guest application (a

binary

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) was originally available as a patch for some Kernel versions above 2.2.x, and the host with any kernel version above 2.2.x supported it easily in the thread mode (i.e., non-SKAS3). As of

2.6.0, it is integrated into the main kernel source tree. A method of running a separate kernel address space (SKAS) that does not require host kernel patching has been implemented. This improves performance and security over the old Traced Thread approach, in which processes running in the UML share the same address space from the host's point of view, which leads the memory inside the UML to not be protected by the

memory management unit A memory management unit (MMU), sometimes called paged memory management unit (PMMU), is a computer hardware unit that examines all references to computer memory, memory, and translates the memory addresses being referenced, known as virtual mem ...

. Unlike the current UML using SKAS, buggy or malicious software inside a UML running on a non-SKAS host could be able to read the memory space of other UML processes or even the UML kernel memory.

Comparison with other technologies

User-mode Linux is generally considered to have lower performance than some competing technologies, such as Xen and

OpenVZ OpenVZ (Open Virtuozzo) is an operating-system-level virtualization technology for Linux. It allows a physical server to run multiple isolated operating system instances, called containers, virtual private servers (VPSs), or virtual environments ...

. Future work in adding support for

x86 virtualization x86 virtualization is the use of hardware-assisted virtualization capabilities on an x86/x86-64 CPU. In the late 1990s x86 virtualization was achieved by complex software techniques, necessary to compensate for the processor's lack of hardware ...

to UML may reduce this disadvantage. Often cited as a strength of Xen (a competing technology) is support for

thread-local storage In computer programming, thread-local storage (TLS) is a memory management method that uses static memory allocation, static or global computer storage, memory local to a thread (computing), thread. The concept allows storage of data that appear ...

(TLS). This is now also supported in the latest UML kernels. Xen concentrates on virtualizing the whole machine, and thus all systems running on a Xen machine are really virtual machines. In UML, the host machine is not virtualized in any way, and only guest systems are true virtual machines. This allows UML guest direct access to host filesystems and hardware, where it is common to map a host directory (e.g., → ).

Supported platforms

UML was originally designed for the

x86 x86 (also known as 80x86 or the 8086 family) is a family of complex instruction set computer (CISC) instruction set architectures initially developed by Intel, based on the 8086 microprocessor and its 8-bit-external-bus variant, the 8088. Th ...

instruction set, but has also been ported to others including

IA-64 IA-64 (Intel Itanium architecture) is the instruction set architecture (ISA) of the discontinued Itanium family of 64-bit Intel microprocessors. The basic ISA specification originated at Hewlett-Packard (HP), and was subsequently implemented by ...

and

PowerPC PowerPC (with the backronym Performance Optimization With Enhanced RISC – Performance Computing, sometimes abbreviated as PPC) is a reduced instruction set computer (RISC) instruction set architecture (ISA) created by the 1991 Apple Inc., App ...

References

External links