Created attachment 597566 [details, diff]
patch to compile with kernel 5.4.0
The attached patch works for me. A better solution would be to create an ebuild for Virtualbox 6.1
Created attachment 597838 [details, diff]
fix fall_through defines
Here are official patches:
And I needed the attached fall-through.patch from https://lkml.org/lkml/2019/10/15/1009.
BTW: The modules vboxguest and vboxvideo are in linux 5.4 now. The last one (vboxsf) gets in there maybe with linux 5.5.