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    1: **Contents**
    3: [[!toc levels=2]]
    5: # The Anykernel and Rump Kernels
    7: ## About
    9: A driver abstracts an underlying entity. For example, a TCP/IP driver
   10: abstracts the details required to perform networking, the Fast File
   11: System (FFS) driver abstracts how the file system blocks are laid out on
   12: the storage medium, and a PCI network card driver abstracts how to
   13: access device registers to send and receive packets. The kernel
   14: architecture controls how kernel drivers run with respect to other
   15: system components. Some examples of kernel architectures are the
   16: monolithic kernel, microkernel and exokernel. In contrast to the above
   17: architectures, NetBSD is an *anykernel*. This means that some kernel
   18: drivers can be run according to any kernel architecture instead of being
   19: limited to a single one.
   21: <!--
   22:     TODO
   23:     ![](rumparch.png)
   24: -->
   26: When a driver is not run as part of the monolithic kernel, e.g. when it
   27: is run as a microkernel style server, the driver is hosted in a *rump
   28: kernel*. A rump kernel is an ultralightweight virtualized kernel running
   29: on top of high-level hypercall interface. Instead of a low-level
   30: hypercall API typically seen with operating systems with operations such
   31: as "modify page table", the rump kernel hypercall API provides
   32: high-level operations such as "run this code in a thread".
   34: Currently, three implementations of the rump kernel hypercall interface
   35: exist.
   37: -   The POSIX implementation is included in the NetBSD tree and makes
   38:     rump kernels to run as userspace processes on most operating systems
   39:     such as NetBSD, Linux and Solaris.
   40: -   The Xen implementation allows running rump kernels directly as Xen
   41:     DomU's without an intermediate operating system.
   42: -   The Linux kernel hypervisor allows rump kernels to run inside the
   43:     Linux kernel.
   45: Rump kernels are radically different from OS virtualization technologies
   46: such as KVM, containers and usermode operating systems. A rump kernel
   47: does not support hosting application processes because a rump kernel is
   48: aimed at virtualizing kernel drivers and application virtualization
   49: would be pure overhead. Instead, existing entities such as processes
   50: from a hosting OS are used as clients for the rump kernel ("application"
   51: in the figure).
   53: As a result of the above design choices, rump kernels are extremely
   54: lightweight. The bootstrap time for rump kernels on POSIX hosts is
   55: measured in milliseconds and memory footprint in 100kB's. This means
   56: that a rump kernel can be bootstrapped for example as part of a command
   57: line tool for virtually no cost or user impact. Rump kernels also
   58: mandate very little from the hypercall implementation meaning that rump
   59: kernels, and by extension NetBSD kernel drivers, can be hosted in
   60: virtually any environment.
   62: Use cases for rump kernels include:
   64: -   **Code reuse**: kernel drivers can be reused without having to run a
   65:     whole OS. For example, a full-featured TCP/IP stack (IPv6, IPSec,
   66:     etc.) can be included in an embedded appliance without having to
   67:     write the stack from scratch or waste resources on running an entire
   68:     OS.
   69: -   **Kernel driver virtualization**: every rump kernel has its own
   70:     state. Furthermore, the functionality offered by multiple rump
   71:     kernels running on the same host does not need to be equal. For
   72:     example, multiple different networking stacks optimized for
   73:     different purposes are possible.
   74: -   **Security**: when hosted on a POSIX system, a rump kernel runs in
   75:     its own instance of a userspace process. For example, it is widely
   76:     published that file system drivers are vulnerable to untrusted file
   77:     system images. Unlike on other general purpose operating systems, on
   78:     NetBSD it is possible to mount untrusted file systems, such as those
   79:     on a USB stick, in an isolated server with the kernel file system
   80:     driver. This isolates attacks and prevents kernel compromises while
   81:     not requiring to maintain separate userspace implementations of the
   82:     file system drivers or use other resource-intensive approaches such
   83:     as virtual machines.
   84: -   **Easy prototyping and development**: kernel code can be developed
   85:     as a normal userspace application. Once development is finished, the
   86:     code can simply be complied into the kernel. This is a much more
   87:     convenient and straightforward approach to kernel development than
   88:     the use of virtual machines.
   89: -   **Safe testing**: kernel code can be tested in userspace on any host
   90:     without risk of the test host being affected. Again, virtual
   91:     machines are not required.
   93: ## Further Reading
   95: ### Dissertation
   97: The following is the definitive guide to the anykernel and rump kernels
   98: and supercedes all earlier publications and terminology on the subject.
  100: -   [Flexible Operating System Internals: The Design and Implementation
  101:     of the Anykernel and Rump
  102:     Kernels](
  104: ### Software using rump kernels
  106: These links are interesting for people who want to use rump kernels in
  107: addition to reading about them.
  109: -   [Scripts for building rump kernels for POSIX
  110:     systems](
  111: -   [Rump kernel hypercall implementation for Xen; rump kernels as Xen
  112:     DomU's](
  113: -   [fs-utils: File system image access
  114:     utilities](
  115: -   Fast userspace packet processing: TCP/IP stack for use with
  116:     [DPDK]( or
  117:     [netmap](
  119: ### Articles, Tutorials & Howtos
  121: -   [Running rump kernels and applications on Xen without a full
  122:     OS](
  123: -   [PCI device driver support in rump kernels on
  124:     Xen](
  125: -   [Experiment with a rump kernel hypervisor for the Linux
  126:     kernel](
  127:     (allows rump kernels to run *in* the Linux kernel)
  128: -   [Experiment on compiling rump kernels to javascript and running them
  129:     in the
  130:     browser](
  131: -   [Kernel Servers using
  132:     Rump](
  133: -   [Tutorial On Rump Kernel Servers and
  134:     Clients](
  135: -   [Revolutionizing Kernel Development: Testing With
  136:     Rump](
  138: ### Conference publications and talks
  140: -   "The Anykernel and Rump Kernels" gives a general overview and
  141:     demonstrates rump kernels on Windows and in Firefox. The
  142:     [video](,
  143:     [slides](
  144:     and an
  145:     [interview](
  146:     are available. Presented at FOSDEM 2013 (Operating Systems track).
  147: -   "Rump Device Drivers: Shine On You Kernel Diamond" describes device
  148:     driver and USB. The
  149:     [paper](
  150:     and [video presentation](
  151:     are available. Presented at AsiaBSDCon 2010.
  152: -   "Fs-utils: File Systems Access Tools for Userland" describes
  153:     fs-utils, an mtools-like utility kit which uses rump kernel file
  154:     systems as a backend. The
  155:     [paper](
  156:     is available. Presented at EuroBSDCon 2009.
  157: -   "Rump File Systems: Kernel Code Reborn" describes kernel file system
  158:     code and its uses in userspace. The
  159:     [paper](
  160:     and
  161:     [slides](
  162:     are available. Presented at the 2009 USENIX Annual Technical
  163:     Conference.
  164: -   "Kernel Development in Userspace - The Rump Approach" describes
  165:     doing kernel development with rump kernels. The
  166:     [paper](
  167:     and
  168:     [slides](
  169:     are available. Presented at BSDCan 2009.
  170: -   "Environmental Independence: BSD Kernel TCP/IP in Userspace"
  171:     describes networking in rump kernels. The
  172:     [paper]( and
  173:     [video presentation]( are
  174:     available. Presented at AsiaBSDCon 2009.
  176: ### Manual pages
  178: The manpages provide the usual type of information. Start from
  179: [rump.3]( and
  180: follow the cross-references in "SEE ALSO".
  182: ## Discuss
  184: Any topic related to rump kernels can be discussed on the
  185: [rumpkernel-users mailing
  186: list](
  187: Alternatively, you can use a NetBSD mailing which is related to a
  188: specific subtopic.
  190: The IRC channel for rump kernels is **\#rumpkernel** on
  191: ****.
  193: ## Availability
  195: The anykernel and rump kernels were first introduced as a prototype in
  196: NetBSD 5.0. A stable version with numerous new features and improvements
  197: was shipped with NetBSD 6.0.
  199: ## Source Code
  201: All of the source code is available from the NetBSD source tree and can
  202: be obtained with the usual methods.
  204: You can also [browse]( the
  205: source code history online. Code is found from all areas of the source
  206: tree. Some examples of where to look include
  207: [src/lib](,
  208: [src/usr.bin]( and
  209: [src/sys/rump](

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