Even in Windows 10, the Windows kernel does a LOT of stuff that's GUI centric. Indeed the entire Windows API is totally GUI centric. The fundamental IPC primitive is window messages, which is a microkernel-esque very fast inter-thread/process context switch tool. Like in seL4 or similar you get to send two numbers this way and not more, and it's optimised heavily. Other IPC systems you'd expect to be GUI independent like DCOM end up using this under the hood.
It used to be that the entire GUI subsystem ran in kernel mode, because context switching was very slow and it made a big difference. In Vista+ large parts of it moved out into a separate privileged process, but the kernel is still involved in IPC, I believe. So when Tavis says "the kernel is creating a new window on behalf of the process" that's not quite correct, I think. Various subsystems get involved in creating windows and mostly not running in kernel mode anymore.
That is not remotely true. Windows GUI messages are implemented as part of the win32k.sys device driver via traditional IPC and synchronization mechanisms.
> Other IPC systems you'd expect to be GUI independent like DCOM end up using this under the hood.
That's not entirely accurate either. Yes, DCOM on GUI threads use Windows messaging for IPC for compatibility reasons that go back to OLE on 16-bit Windows.
But the multithreaded apartment (ie, COM for a modern OS) is completely independent from the GUI; it is layered atop Microsoft RPC, which itself uses the NT kernel's ALPC mechanism for IPC.
If you're not creating and working with COM objects on a GUI thread, then you should not be using old-style single-threaded apartment COM. Yes, even if your program is only single threaded, you should still be initializing COM to use the multithreaded apartment.
Moreover this is the default mode. The CoInitialize API puts you in a STA by default. To get the GUIless MTA you must use the replacement CoInitializeEx API. Obviously COM programmers know this and it's not a big deal, but I don't understand this resistance to accepting that an OS literally called Windows might make windows and window messages an important part of the API.
And I did that window messages run in the kernel, which you accept - even in latest versions the (mandatory) win32k.sys driver implements the message syscalls. I'm not sure how you know what the implementation looks like, but I'm pretty sure it used to be well optimised. It'd be odd if it no longer was.
Now I haven't claimed every IPC primitive in Windows is GUI based, have I? But fundamental features use it, including features like RPC that you wouldn't imagine do so, and that supports my point that it's called Windows for a reason. The GUI aspects historically integrate deeply into everything. As yet one more example, if the dynamic linker encounters a problem during program startup, you get a GUI message box telling you so, even if the program was started from a console.
A fun way to think about Windows, is that it's what Erlang would be if the kernel expected to be able to store GUI state in the process dictionary, such that every actor-process can potentially play a secondary role as a GUI element; and every GUI element is necessarily a first-class actor-process. (These actor-processes being a tree originally rooted in one actor-process holding the kernel state for an OS process—that is, until COM multi-threaded apartments came along and wrecked the abstraction.)
I think this is fundamentally incorrect. Windows certainly allows processes and threads to exist with no GUI. The process doesn't need to have message loop, etc.
I suggest you check your other facts as well. Windows Internals is a good book to start.
Also just look at the definition of the PEB and TEB. There's plenty of slots for GUI stuff that's just left empty for headless apps, but the space is still allocated. In the end Windows is GUI first with non GUI apps a second class citizen.
As you can see the primary OS per thread structure has fields like "GDI pen" and "GDI brush". There's no separation of GUI specific stuff from other kinds of apps. Even the EXE file format distinguishes between "an app that will open a window to run in the background" and "a command line app". There's no notion of a program that could be used as both a GUI or a command line app depending on how it's run.
You make that sound like that was some kind of accidental leaky abstraction, when in fact it was by design. COM STAs were designed to work with GUI threads, full stop. If you aren't a GUI thread, you shouldn't be initializing yourself with STA COM.
> Also just look at the definition of the PEB and TEB. There's plenty of slots for GUI stuff that's just left empty for headless apps, but the space is still allocated.
I am perfectly fine with that, as it essentially removes thread-local GUI data from the application's TLS namespace. So a few extra pointers exist in the PEB and TEB, big deal.
> In the end Windows is GUI first with non GUI apps a second class citizen.
I don't really see how you can conclude that from STA COM and a couple of TLS slots reserved for the GUI.
But the whole thing is nevertheless backed in and coupled with low layers, including the kernel. There is just a kind of lazy init of GDI resources and the like so that the init is skipped for those processes which don't use them ever. It is fundamentally different from a general purpose OS like Linux which does not care that much (or even at all? I haven't checked) about graphic shits for processes.
The Windows NT kernel has been carefully designed like that from the start. In fact, the Windows part of NT is called a Windows subsystem. Windows kernel DOES NOT do windowing. The Executive, Kernel, Device drivers and the HAL are cleanly separated.
In fact, Microsoft provides Windows Nano Server which is a totally headless installation with no GUI components.
So of course it is unlikely there is something that e.g. draws pixels in the scheduler.
But for example there is still some space reserved in the TEB for GDI things. And kernel space code for graphic purposes related to processes and threads -- a kind of graphic "kernel" if you want. I mean: you just cannot take Windows and change all the low level graphics support code to a kind of WinWayland or WinAndroid. Even just programming raw NT processes is not officially supported IIUC, so you are bound to using e.g. Win32, and there definitively are some pieces of code all over Win32 (and not just in trivially graphic related APIs) which is aware of the existence of graphics related features on the OS.
So while it might be possible to recompile and/or rewrite parts of NT if you work at Microsoft to actually obtain a graphic agnostic OS (which is not even exactly what Nano Server is despite the re-engineering effort, because it is explicitly graphicless, not just graphic agnostic, and actually it now does not even exist anymore in a standalone form but only for containers, so you stick with your regular host kernel), that's not what I had in mind.
It depends what level of the OS you’re talking about, but the clean separation is not so clear.
CreateWindowEx does not run in the Kernel, yet it has parts that run in kernel space.
Linux also implements most features as separate modules that are linked together, that doesn't mean they aren't all part of the kernel.