RISC-V International
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Work is in progress adding the port, with the first commit made on 23rd April 2021.
There are multiple RISC-V ports of xv6, including one for the Allwinner D1: https://github.com/michaelengel/xv6-d1
Plan 9 ports exist; see also: https://ntnuopen.ntnu.no/ntnu-xmlui/bitstream/handle/11250/2902876/no.ntnu:inspera:74730513:31541262.pdf?sequence=1
Haiku has been ported to RISC-V: https://www.haiku-os.org/blog/kallisti5/2021-11-07_booting_our_risc-v_images/
There is an Oberon port for RISC-V, e.g., for the Bouffalo Lab BL808: http://oberon.wikidot.com/project-oberon-v