Day 2: Conference: User Mode Linux

Jeff Dike gave us an overview of his User Mode Linux work.

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The idea is that the OS and its processes run as processes on the host kernel. The hardware is emulated, typically with files.

Processes are mapped to processes on the host kernel and the PID and owner are renumbered/renamed as needed. The main reason for this is to speed up context switching, but it does complicate context switching, debugging and signal handling.
Faults are implemented with signals. Memory faults for instance are handled with SIGSEGV and interrupts are handled with SIGIO.

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What works?

Almost everything
All Filesystems and network protocols
X-Window apps (through Xnest)
Kernel Modules

What doesn't work are priviledged instructions and nesting.

Applications

Kernel debugging with gdb via ptrace proxy
gprof and gcov
Security: as a jail or sandbox
Resource allocation: virtual hosting (user level code runs at almost native speed, but I/O intensive code (apache) can take a performance hit of 3).
Prototyping (testing before commiting to a virtual environment)

Future work

SMP and clustering
hostfs extensions
security: full memory protection
performance: tracing thread, block driver, host kernel
Ports to other architectures

Picture library

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2001/01/28 (12:29): Version 1.0