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Like 75% of those lines of code are in drivers or architecture-specific code (code that only runs for x86 or ARM or SPARC or POWER etc.)

The amount of kernel code actually executing on any given machine at any given point in time is more likely to be around 9-12 million lines than anywhere near 40 million.

And a replacement kernel won't eliminate the need for hardware drivers for a very wide range of hardware. Again, that's where the line count ramps up.not

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Yes, of course. But apart from the (current) disadvantage that those drivers don't exist yet, those are all positives in favor of microkernel architectures. All of the massive SLOC codebases run in usermode and with full process isolation, require no specific language compatibility and can be written in any language, do not require upstreaming, and do not require extensive security evaluations from highly capable maintainers who have their focus scattered across 40m lines of code.

The ADMgpu driver alone was over 5 million loc in 2023.

Most of these are header files. I suspect most of its contents are constants and blobs autogenerated with some tool by AMD.

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