[kazinator]

Running Linux under something lower level is old hat, see RTLinux:

http://www.yodaiken.com/papers/BarabanovThesis.pdf [1997]

Well, that's not exactly a microkernel, but took lots of inspiration from microkernels.

I take it that you may be alluding to the debate between Torvalds and Tannenbaum regarding microkernels.

https://en.wikipedia.org/wiki/Tanenbaum%E2%80%93Torvalds_deb...

[naasking]

The first PowerPC linux for the Power Mac ran under the Mach microkernel [1].

Then there was also L4Linux, which I believe was the first low-overhead virtualization of Linux using the L4 microkernel. Papers demonstrated < 10% overhead I believe, where previous virtualization efforts were somewhat higher. I'd say it kicked off the whole paravirtualization/virtualization craze.

[1] http://www.mklinux.org/