What would this kernel architecture be classified as?

[saltlamp]

Hello, I am not sure as to what this type of kernel architecture would be classified as. I'm guessing it's a hybrid kernel, more-so on the micro-kernel side, but I am not sure. This is the type of kernel that I am building, so if anyone has any additional critiques, am more than happy to hear them! ^^

My kernel is structures as follows:

There is a distinction between user-mode and kernel-mode device drivers. User-mode device drivers are used to extended ALREADY EXISTING kernel functionality to the user, and kernel-mode drivers are used to implement NEW functionality to the kernel.

EDIT
User-mode device drivers are the same as a standard user-process, as I call them, and not a kernel-process. Hidden from standard user-mode processes, and user-mode drivers do communicate to the kernel via messages.

So, you would have kernel-mode drivers such as, a FAT (12, 16, 32) kernel-mode device driver, an EXT (1, 2, 3, 4) kernel-mode device driver, etc, etc.

And then you would have a user-mode device driver which is the file-system (the virtual file system, I suppose), and it uses the kernel to handle files, which has extended functionality thanks to the kernel-mode device drivers.

You can go even further by having one kernel-mode graphic-card device driver to allow the kernel to work with graphics-cards, and then user-mode device drivers for certain things, I guess. Idk, just giving examples.

Again, I cannot tell if this a hybrid-kernel, or a micro-kernel. And critiques and (constructive, hopefully) criticisms with regards to my way of doing things are much appreciated.

Kind Regards,
Joe

[bzt]

Hi,

I think this is most likely a monolithic kernel, as you have kernel-mode drivers to do the hard work. If I'm correct what you call user-mode drivers are nothing more than shared libraries.

Micro-kernel would require to push as much as possible into user-mode. A micro-kernel typically implements context-switching (with address space management probably), low level IRQ handlers, message passing, and has no drivers at all. Therefore a micro-kernel simply NEVER uses the kernel to handle files, and it does not have FAT or ext drivers of any kind (handling files is the job of the user-mode FS server).

Cheers,
bzt

[saltlamp]

Hi,

I think this is most likely a monolithic kernel, as you have kernel-mode drivers to do the hard work. If I'm correct what you call user-mode drivers are nothing more than shared libraries.

Micro-kernel would require to push as much as possible into user-mode. A micro-kernel typically implements context-switching (with address space management probably), low level IRQ handlers, message passing, and has no drivers at all. Therefore a micro-kernel simply NEVER uses the kernel to handle files, and it does not have FAT or ext drivers of any kind (handling files is the job of the user-mode FS server).

Cheers,
bzt

Hey there, thanks for the reply! I am afraid that I forgot to add a very important piece of information..... When I was talking about user-mode drivers, I forgot to mention that they are the nothing more than 'hidden' normal user-mode processes, and communicate with the kernel via messages. I edited the OP so no one sees this mistake. I will mention, though, that there is a kernel has a DLL subsystem which manages DLLs for user-mode processes and kernel-mode processes, but that's no the same.

[nullplan]

Well, what you are describing there has too much going on in userspace to call it monolithic, and too much going on in kernelspace to call it microkernel. So let's compromise and call it a hybrid.

[Combuster]

too much going on in userspace to call it monolithic

So a kernel becomes hybrid by virtue of running something big, say... firefox? After all it uses network and filesystem drivers plus some internal logic to expose "files" addressable by "url". It doesn't need to have an UI - most bigger linux desktops have various userland services that do things at this level.

There's absolutely nothing in userspace with any knowledge of specific hardware. The driver model is therefore by all means monolithic. There are other orthogonal directions available towards modular kernels and exokernels, but the key selling points of those principles have not been mentioned.

[Schol-R-LEA]

Keep in mind that the classifications are mainly a way to talk about and compare different designs - it is a pattern language, not a set of checkboxes.

It's that same problem one gets whenever someone tries to use a descriptive language as a prescriptive one. It comes up all the time when talking about design patterns (which, keep in mind, were mainly meant for understanding existing code when refactoring, not for building new software).

Hell, I've seen it come up in homebrewing when discussing beer styles, where you get arguments over whether a given beer 'is a' New England IPA versus an English IPA versus a Northwestern IPA (or whatever style it is closest to and/or meant to fit into). The important part is what it actually is - categorization is mostly to make it easier to discuss with others.

As for this particular question, the matter might seems to mostly be 'do the low-level drivers run in Supervisor/System privilege, or in User privilege?', but it is a lot more complex than that.

If there are no runtime (as opposed to boot-up) drivers other those needed to multiplex the CPU, interrupts, and memory, and userland processes can access the hardware directly when allowed to by the kernel, then it is definitely an exokernel. Strictly speaking, exokernels don't have drivers at all; the given process is free to access the hardware with any sort of device code it chooses, so long as it follows the system's rules about device and memory sharing. They can use shared libraries, but they don't need to; there can be unique driver code that is specific to a given program. The idea here is that each application can optimize the device-driving code to its exact needs, rather than having generalized, abstracted device drivers that aren't optimal for any one program but must be used by all.

One can view a type-I hypervisor as a specialized exokernel which a) runs in a separate 'hypervisor' mode, and b) can multiplex entire operating systems (with their own supervisor mode), rather than just individual userland processes.

If the OS has true drivers which (aside from any boot-up drivers) all run as separate userland processes, and uses a message-passing system for both driver calls and system calls, it is definitely a microkernel. Indeed, the use of message-passing for IPC and synchronization - and only message-passing, even if other abstractions are built on top of that - is more definitive of micros than userspace drivers is, as the original micro concept was intended for systems without separate privilege levels. Equally implicit in this is that the drivers are themselves Actors (in the Actor Model sense), and do not share memory with the kernel or other userland processes. The whole question of where the driver runs is secondary in a micro-kernel, as the term was initially defined, as there was no kernel space for most of the early ones. If it doesn't exclusively use message-passing for the underlying IPC and mutex systems, and the drivers aren't separate processes which the caller sends messages to and receives messages from, then it isn't a 'real' microkernel.

If, conversely, the drivers are linked into the kernel when the kernel is built, and all run in kernel space, it is definitely a monolithic kernel - if the drivers aren't part of the kernel binary itself, it can't truly be a monolithic kernel. Early Unix systems were monolithic, but later ones went away from that.

If the drivers can be loaded at boot time, or dynamically during runtime, but run in kernel space (or a separate driver space, which some hardware still supports but has fallen out of favor since the 1980s with the rise of Unix), then it is a modular kernel. This includes systems such as Linux, later versions of Unix, and Multics, as well as any system that uses a separate driver privilege level (which was actually the classic kernel model; while some very early experimental OSes were monolithic, once hardware memory protection with separate privilege levels was introduced, modular kernels became the dominant design until Unix - with its strictly two-level privilege model - came along and pushed them aside for unrelated reasons).

If the device-driving code is generated dynamically by the kernel at runtime from a complex of driver templates, with optimizations applied to the specific circumstance of the call, then it is a synthesizing kernel. So far, there is only one notable example of this, and even that wasn't a 'pure' system - it kinda-sorta virtualized each process (the so-called 's-machines'), so that the 'supervisor mode' was specialized on a per-process basis. A modern interpretation would probably use a containerizing hypervisor for the primary kernel, to get a result somewhere between an exokernel and a containerizing system with very minimal 'rump' OSes for each process.

If some drivers run in kernel/driver space, and others in user space, or if the drivers are split between a low-level kernel section and a high-level userspace section (as sounds like is the case here), then it could reasonably be called a hybrid kernel. However, that term is a lot 'fuzzier' than most of the others, being more of a term if inclusion than exclusion.

But again, these are just ways of describing different design philosophies; reality is usually a lot messier. Very few OS kernels actually fit neatly into any one of these pigeonholes.

[eekee]

Plan 9 has much the same choice of drivers in kernel oe user space, so congratulations! your kernel has something in common with one of Ken Thompson's!

I had a friend named Uriel who was a vociferous proponent of Plan 9. I ended up writing these lines about some of his experiences:

One day, Uriel met a man who argued convincingly that Plan 9 is a microkernel. On another day, Uriel met a man who argued convincingly that Plan 9 is a macrokernel. Uriel was enlightened.

I don't really know if Uriel was enlightened, but I certainly was: I stopped caring about the distinction between microkernels and macrokernels.

Schol-R-LEA's comments here are insightful.

Separately:

Sounds like exokernels are the oldest kind of kernel of all. Compare MS-DOS or Atari 800, and I'm sure this kind of resource allocation with TSRs (MS-DOS) or device drivers (Atari) was going on in the 50s.

Firefox and other modern web browsers are operating systems, they just parasitize lower-level operating systems to survive. I suppose you could call it symbiosis because operating systems which support them are more popular. (You wouldn't believe how regularly Uriel's IRC channel was flooded by know-nothings demanding to know why Plan 9 didn't support Firefox or Chrome, or just assuming that it did because they didn't understand what an operating system is!)