The Design and Implementation of the Anykernel and Rump Kernels

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source loading linking initiated by
builtin external external external toolchain
bootloader external kernel bootloader
file system kernel kernel syscall, kernel autoload
Table 3.3: Kernel module classification. These categories represent the types
of kernel modules that were readily present in NetBSD independent of this work.
being converted. A modular driver knows how to attach to the kernel and detach
from the kernel both when it is statically included and when it is loaded at runtime.
NetBSD divides kernel modules into three classes depending on their source and
when they are loaded. These classes are summarized in Table 3.3. Builtin modules
are linked into the kernel image when the kernel is built. The bootloader can load
kernel modules into memory at the same time as it loads the kernel image. These
modules must later be linked by the kernel during the bootstrap process. Finally,
at runtime modules must be both loaded and linked by the kernel.
The fundamental steps of loading a kernel module on NetBSD at runtime are:
1. The kernel module is loaded into the kernel’s address space.
2. The loaded code is linked with the kernel’s symbol table.
3. The module’s init routine is run. This routine informs other kernel subsystems
that a new module is present. For example, a file system module at a
minimum informs the VFS layer that it is possible to mount a new type of
file system.
onward are incompatible with the old LKM scheme. The reasons why LKM was retired in favor
of the new system are available from mailing list archives and beyond the scope of this document.