RUMP(3)

NAME
rump - The Rump Anykernel

LIBRARY
rump Library (librump, -lrump)

SYNOPSIS
#include <rump/rump.h>
#include <rump/rump_syscalls.h>

DESCRIPTION
rump is part of the realization of a flexible anykernel architecture for NetBSD. An anykernel 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 module 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 virtual 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 rumphijack(3).

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 controls 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 allows.

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 variable is often more convenient for quick testing. As expected, this option has effect only in rump kernels which support VFS. The current default is 1024 vnodes.

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 providing 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 components 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 components must follow kernel module boundaries.

SEE ALSO
rump_server(1), p2k(3), rump_etfs(3), rump_lwproc(3), rumpclient(3), rumphijack(3), rumpuser(3), ukfs(3), rump_sp(7) Antti Kantee, March 2009, Proceedings of AsiaBSDCon 2009, pp. 71-80, Environmental Independence: BSD Kernel TCP/IP in Userspace. Antti Kantee, May 2009, BSDCan 2009, Kernel Development in Userspace - The Rump Approach. Antti Kantee, June 2009, Proceedings of the 2009 USENIX Annual Technical Conference, pp. 201-214, Rump File Systems: Kernel Code Reborn. Arnaud Ysmal, Antti Kantee, September 2009, EuroBSDCon 2009, Fs-utils: File Systems Access Tools for Userland. Antti Kantee, March 2010, Proceedings of AsiaBSDCon 2010, pp. 75-84, Rump Device Drivers: Shine On You Kernel Diamond.

HISTORY
rump appeared as an experimental concept in NetBSD 5.0. The first stable version was released in NetBSD 6.0.

AUTHORS
Antti Kantee <pooka@iki.fi>