Comment by onjectic

The MMU leads to horribly leaky operating system abstractions. IME it’s leaky due to the lack of separation between address space remapping(preventing fragmentation) and memory protection(security).

Perhaps unintentionally, RISC-V provides more flexibility to kernel developers by also including a physical memory protection unit that can run underneath and simultaneously with the MMU. This can make it far cheaper to switch memory protection on and off for arbitrarily sized areas of physical memory since this capability is no longer needlessly coupled to expensive memory remapping operations. Kernels can move away from the typical “process is its own virtual computer” model, and towards things closer to single address space designs or some middle ground between the two.