M-mode only design is not scalable to better CPUs. Use U-mode only design.

[sam-falvo]

(idea from sorear on IRC #riscv)

The 53000 was originally designed prior to v1.9 priv spec, and thus is currently designed to offer an M-mode only environment for software to run in. However, this makes drop-in CPU upgrades difficult without also replacing the system software to match the new CPU. To improve the ability to upgrade a Kestrel-3 design, the processor should be built around an all U-mode design. This lets a superior processor architecture emulate the base configuration Kestrel-3 using a combination of delegated interrupts and traps, and perhaps paging to simulate the address space.

New CPU modules can come with M-mode code intended to provide an abstraction layer for the existing U-mode firmware and software, allowing portability between the 53000 and, say, a Rocket or BOOM core, for instance, without requiring any recompiles of Forth, Oberon, or whatever else is in Kestrel-3's firmware.

[sam-falvo]

• [ ] Resolve kestrelcomputer/kestrel#253
• [ ] Expose ustatus register as user-mode image of mstatus.
• [ ] Move mstatus.MIE flag bit to ustatus.UIE.
• [ ] Make sure MPP, HPP, and SPP fields = 0.
• [ ] Move M-mode IRQ enable, pending bits in mie, mip to their corresponding places in U-mode uip, uie.
• [ ] Move trap setup and trap handling CSRs into user-space.
• [ ] Make sure SMG code has no reference to m* CSRs, fields, or triggers.
• [x] Review existing open issues, update them if incompatible with v1.9.1 of the priv spec.