OpenBSD supports the 64-core RISC-V Milk-V Pioneer board since version 7.6.
However, this is not mentioned on the riscv64 port page (https://www.openbsd.org/riscv64.html), there is no information on installing OpenBSD on the Pioneer in the INSTALL.riscv64 docs and there also seems to be no RISC-V-specific mailing list for OpenBSD.
Does anyone have more information on running OpenBSD on this specific piece of hardware?