The Design and Implementation of the Anykernel and Rump Kernels

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interrupts from hardware clock interrupts to run periodic tasks (used e.g. by TCP
timers).
The users of the kernel softint facility expect them to operate exactly according
to the principles we listed. Initially, for simplicity, softints were implemented as
regular threads. The use of regular threads resulted in a number of problems. For
example, when the Ethernet code schedules a soft interrupt to do IP level processing
for a received frame, code first schedules the softint and only later adds the frame
to the processing queue. When softints were implemented as regular threads, the
host could run the softint thread before the Ethernet interrupt handler had put the
frame on the processing queue. If the softint ran before the packet was queued, the
packet would not be delivered until the next incoming packet was handled.
Soft interrupts are implemented in sys/rump/librump/rumpkern/intr.c according
to the principles we listed earlier. The standard NetBSD implementation was
not usable in a rump kernel since that implementation is based on direct interaction
with the NetBSD scheduler.
3.4 Virtual Memory Subsystem
The main purpose of the NetBSD virtual memory subsystem is to manage memory
address spaces and the mappings to the backing content [20]. While the memory
address spaces of a rump kernel and its clients are managed by their respective hosts,
the virtual memory subsystem is conceptually exposed throughout the kernel. For
example, file systems are tightly built around being able to use virtual memory
subsystem data structures to cache file data. To illustrate, the standard way the
kernel reads data from a file system is to memory map the file, access the mapped
range, and possibly fault in missing data [101].