The headers in kernel-headers are sanitized headers meant for userspace. Putting raw kernel headers, like gentoo does, in /usr/include is a no-go. If you want to do in-kernel development, you have to use the real kernelheaders, not the sanitized headers from /usr/include.

I think this will also fix problems with compiling UML.

There are no conclusive arguments NOT to to use raw kernel headers.
Problematic areas are protected with __KERNEL__ - if someone realize problem than
is this an kernel issue!

But to shrink the kernel-headers make archlinux unusable for developers who interact
with the kernel just a little bit more.

BTW: to use the raw-kernel headers is an no-go: libc is linked against the current
version of arch-headers.

TIM

things defined in __KERNEL__ are useless in userspace, that's what the kernel developers say, so they stop you from using it from userspace by removing it when you export the userspace kernelheaders. If you need stuff that is inside __KERNEL__, then two things could be wrong:

1. You're not developing a userspace application but kernelspace, go use raw kernelheaders
2. The part the userspace kernelheader needs should not be inside __KERNEL__ but should be exported to userspace

When compiling an application that isn't running in kernelspace, you should never ever compile with -D__KERNEL__. If your app has to use that define, your application isn't a userspace application and should not use the headers in /usr/include.