In a microkernel OS, a few services that we think would normally be at the kernel level are run as regular processes outside the microkernel. The user level contains both regular processes and important OS services.
Design philosophy is that the kernel code should be as small as possible, keeping the abstractions small, clean, and logical.
The kernel includes tasks and threads, virtual memory, and interprocess communication. Everything else (networking, FS, etc.) is at the user-level.
Experience is mixed.
Very extensible, easy to add new OS functionality
kernel doesn't impose a particular OS design, so you can have different OS personalities, i.e. run different servers each of which emulates a different OS, but each running on the same microkernel. Allows apps to use their own customized OS.
mostly hardware agnostic, since the servers don't need to know about what hardware they're running on
strongly protected, even with parts of the OS
performance: microkernel killer. System calls can require a lot of protection mode changes.
expensive to reimplement everything with a new model. Moving your OS to a microkernel can actually be more work than moving to new hardware, since at least with new hardware you'll know what subsystems will be affected.
bad past: look up IBM Workplace OS
Each message requires trapping. Yikes.
Research project approached in a few steps:
The multi-server part was never completed, but it was a hugely influential project that got people excited and talking about microkernels.
Also provided device handling and a concept of virtual memory
What's really nice about the abstraction of messages is that you can easily have a much more powerful multiprocessor computer take care of a workload by transmitting it via messages and receiving the results back.
It explored a lot of ideas, but left a few things to be desired in the implementation. Notably because they started with a monolithic system and broke it out, there were still a lot of dependencies, and the microkernel was larger than one would want.
Starts mostly from scratch, proper microkernel. Has two basic abstractions:
Address spaces, the unit of protection. Initially empty, and populated by privleged mapping operations.
Threads, the unit of execution. Kernel-scheduled, but managed at the user level.
and two mechanisms:
Drastically improved performance over Mach. L3 whitepaper showed that Mach's performance issues weren't necessarily from the microkernel idea, but from the implementation.
The main performance benefit comes from a very small cache footprint. Mach had a complex API, but more importantly, its large cache footprint meant that it was memory bandwidth-limited. L4, instead, is CPU-limited.
L4Ka is extremely small these days (only 10k lines!). This is sweet, because the kernel can be tested and formally proven to be correct/stable.
The microkernel design philosophy said that we should reduce the number of abstractions. The exokernel research project from MIT claims that all OS abstractions are bad, since in the process of hiding detail you prevent applications from doing things that could be cool. Discourages innovation.
So the solution an exokernel proposes is to separate protection from abstraction and management. It's a resource multiplexer. It gives physical hardware access to processes, and that's it. It's also very fine-grained about how it gives out resources, e.g. provides individual disk blocks, instead of disk partitions.
To deal with things like file permissions that are difficult for the exokernel to keep track of, the user can provide code for the kernel to download that can be executed in a privleged mode. Complicated, since you have to avoid things like infinite loops when executing arbitrary code.
In a virtual machine, you have a thin layer of software that resides over the hardware, with monitoring tools like the hypervisor that simulate the hardware, so the OS isn't aware that it's actually running on the software level.
However, there are some virtualization systems that actually make it clear to the OS that it's being virtualized, so it can handle some situations more efficiently.
A virtual machine monitor provides an efficient, isolated and duplicated version of the real machine.
The original motivation behind virtual machines in the 1960s was that you needed to share large, expensive computers with many users, and different groups wanted or needed different operating systems. So it was convient for timesharing.
Today, large scale servers have similar issues as the machines that originally motivated VMs. VMs also provide security, portability/compatibility(since your hypervisor can schedule things on multiple CPUs while the OS only sees a single CPU), reliability/fault tolerance(try out code you don't trust in a VM), migration(not dependent on physical hardware), and innovation(allows us to try out new ideas safely).
Conventional software is developed for a specific OS and instruction set architecture(ISA). This combination is considered the ABI. Gives us two types of VMs: process VMs and system VMs. The process VM supports a single process, while the system VM works on an operating system.
In a classic system VM (VMM), the VM runs on bare hardware. Most privleged form of software. In a hosted VM, the VM is like a regular application.
There are a few architecture requirements for a VM to function:
None of these things are particularly difficult/esoteric on today's architectures.
There are also a few generic VM operations that need to be implemented.
A privleged instruction is required to trap when it's not executed in supervisor mode. A sensitive instruction affects the operation of the system in some way. Pretty vague, but generally, there's a theorem that as long as the sensitive instructions form a subset of the privleged instructions, we can make an efficient VMM, since the OS won't know the difference. This means that we need to avoid instructions that have different effects depending on whether they are executed in user mode or kernel mode.
Different approaches between VMs of how this is handled. VMware uses binary rewriting, whereas Xen uses paravirtualization.
Adding new functions to OS "on the fly", at runtime. Allows for fixing mistakes, supporting new features or hardware. For implementation, you could give everyone their own machine (VMs), allow some OS functions to run outside (ukernel), or we could alow users to modify the OS directly (loadable kernel modules).
The last solution is popular because giving everyone their own VM is pretty drastic for a potentially small change. So this solution allows parts of the kernel to be dynamically loaded and unloaded, replacing the small parts accordingly. This is for example how drivers could be implemented.
But this also means that if the loaded part is buggy, then the kernel will crash. Responsible for a large majority of BSODs.