RISCV PMP port

[Blebowski]

Hi,

it would be nice if CMRX supported also RISCV PMP (instead of ARMs MPU only).

E.g. to get it working on cores like IBEX or CVA6.

[ventZl]

This is indeed possible and would very well be welcome. Yet I myself don't have the bandwidth to make code improvements in the core and create another port. If anyone was interested in working on this port, I can provide all the assistance and guidance on how ports are meant to be integrated with the core.

[Blebowski]

Thanks @ventZl , TBH, I would love to do that, but I am afraid my time is also very limited.

[tobiaguiar08]

Hi.

I would be interested in contributing to this port.

However, I have no prior experience with the RISC-V architecture or PMP, so the work would take some time.

If you’re not in a hurry and are open to helping with design questions, troubleshooting, and general guidance along the way, I’d be happy to give it a try.

It would also help to know which hardware you recommend for testing, and any links to PMP documentation or reference material that could speed up the learning curve.

Let me know if this approach works for you.

[ventZl]

Cool!

Sure, this support can be delivered step by step, first by providing working basic RISC-V port mostly ignoring PMP and then activating PMP support. This is an approach used by the Linux port so far.

Following is a core dump of my memory regarding my research on RISC-V feasibility:

PMP is composed of blocks. A device may contain 0, 1, 2, ... blocks of PMP units. These then cascade. If no PMP block is provided, then PMP is not supported at all. One PMP block contains 4 protection regions. This is smaller count compared to Cortex-M MPU and as such is considered insufficient for reasonable execution of CMRX.

Thus, only parts containing 2 PMP blocks should be considered as feasible.

One part that seems to be feasible is Bouffalo BL-602 used in stuff like Pine64 PineCone. Another good candidate is Raspberry Pi RP2350 and its Hazard3 core which is present in configuration with 8 regions, which is on par with ARMv6M and ARMv7M.

I would probably go with Raspberry Pi as its SDK is known to be working with CMRX.

[tobiaguiar08]

Hi,

I opened my first PR for this port : #137

THere is almost nothing there, just my attempt to make a basic context switch working for now. Since I have no OS design experience, I am not confident that code is good and I asked AI for help in the implementation. So I would appreciate any feedback to help move this port foward