RUMP(3) BSD Library Functions Manual RUMP(3)
rump -- The Rump Anykernel
rump Library (librump, -lrump)
rump is part of the realization of a flexible anykernel architecture for NetBSD. An anyker-
nel architecture enables using kernel code in a number of different kernel models. These
models include, but are not limited to, the original monolithic kernel, a microkernel
server, or an exokernel style application library. rump itself makes it possible to run
unmodified kernel components in a regular userspace process. Most of the time "unmodified"
means unmodified source code, but some architectures can also execute unmodified kernel mod-
ule binaries in userspace. Examples of different use models are running file system drivers
as userspace servers (see p2k(3)) and being able to write standalone applications which
understand file system images.
Regardless of the kernel model used, a rump kernel is a fullfledged kernel with its own vir-
tual namespaces, including a file system hierarchy, CPUs, TCP/UDP ports, device driver
attachments and file descriptors. This means that any modification to the system state on
the host running the rump kernel will not show up in the rump kernel and vice versa. A rump
kernel may also be significantly more lightweight than the host, and might not include for
example file system support at all.
Clients using services provided by rump kernels can exist either in the same process as the
rump kernel or in other processes. Local clients access the rump kernel through direct
function calls. They also naturally have access to the kernel memory space. This document
is geared towards local clients. For more information on remote clients, see rump_sp(7).
It is also possible to use unmodified application binaries as remote clients with
A rump kernel is bootstrapped by calling rump_init(). Before bootstrapping the kernel, it
is possible to control its functionality by setting various environment variables:
RUMP_NCPU If set, indicates the number of virtual CPUs configured into a rump kernel.
The default is the number of host CPUs. The number of virtual CPUs con-
trols how many threads can enter the rump kernel simultaneously.
RUMP_VERBOSE If set to non-zero, activates bootverbose.
RUMP_THREADS If set to 0, prevents the rump kernel from creating any kernel threads.
This is possible usually only for file systems, as other subsystems depend
on threads to work.
RUMP_MEMLIMIT If set, indicates how many bytes of memory a rump kernel will allocate
before attempting to purge caches. The default is as much as the host
RUMP_NVNODES Sets the value of the kern.maxvnodes sysctl node to the indicated amount.
Adjusting this may be useful for example when testing vnode reclaim code
paths. While the same value can be set by means of sysctl, the env vari-
able is often more convenient for quick testing. As expected, this option
has effect only in rump kernels which support VFS. The current default is
A number of interfaces are available for requesting services from a rump kernel. The most
commonly used ones are the rump system calls. They are exactly like regular system calls
but with the exception that they target the rump kernel of the current process instead of
the host kernel. For example, rump_sys_socket() takes the same parameters as socket() and
will open a socket in the rump kernel. The resulting file descriptor may be used only in
other rump system calls and will have undefined results if passed to the host kernel.
Another set of interfaces specifically crafted for rump kernels are the rump public calls.
These calls reside in the rump_pub namespace. An example is rump_pub_module_init() which
initializes a prelinked kernel module.
A rump kernel is constructed at build time by linking a set of libraries with application
level code. The mandatory libraries are the kernel base (librump) and the rump hypercall
library (librumpuser) which a rump kernel uses to request services from the host. Beyond
that, there are three factions which define the flavour of a rump kernel (librumpdev,
librumpnet and librumpvfs) and driver components which use features provided by the base and
factions. Notably, components may have interdependencies. For example, a rump kernel pro-
viding a virtual IP router requires the following components: rumpnet_netinet, rumpnet_net,
rumpnet, rumpnet_virtif, rump, and rumpuser. A rump kernel providing an NFS client requires
the above and additionally rumpfs_nfs and rumpvfs.
In addition to defining the configuration at link time, it is also possible to load compo-
nents at runtime. There are two ways of doing this: using dlopen() to link a shared library
into a rump kernel and initializing with rump_pub_module_init() or specifying a module on
the file system to rump_sys_modctl() and letting the rump kernel do the linking. Notably,
in the latter case debugging with symbols is not possible since the host gdb does not know
about symbols loaded by the rump kernel. Generally speaking, dynamically loadable compo-
nents must follow kernel module boundaries.
rump_server(1), p2k(3), rump_etfs(3), rump_lwproc(3), rumpclient(3), rumphijack(3),
rumpuser(3), ukfs(3), rump_sp(7)
Antti Kantee, "Environmental Independence: BSD Kernel TCP/IP in Userspace", Proceedings of
AsiaBSDCon 2009, pp. 71-80, March 2009.
Antti Kantee, "Kernel Development in Userspace - The Rump Approach", BSDCan 2009, May 2009.
Antti Kantee, "Rump File Systems: Kernel Code Reborn", Proceedings of the 2009 USENIX Annual
Technical Conference, pp. 201-214, June 2009.
Arnaud Ysmal and Antti Kantee, "Fs-utils: File Systems Access Tools for Userland",
EuroBSDCon 2009, September 2009.
Antti Kantee, "Rump Device Drivers: Shine On You Kernel Diamond", Proceedings of AsiaBSDCon
2010, pp. 75-84, March 2010.
rump appeared as an experimental concept in NetBSD 5.0. The first stable version was
released in NetBSD 6.0.
Antti Kantee <firstname.lastname@example.org>
BSD March 25, 2011 BSD