Re: PMP shared permissions for S and U
toggle quoted messageShow quoted text
Not all OSes require or desire VM address mapping, especially in the embedded space (or so I've been led to believe) - yet they still need some level of privilege protection layering.
That's where this comes SPMP (or SMPU) comes in. M-mode above the Smode is for security monitors, boot firmware, etc. - and to protect the Smode OS, so that doesn't go away.
On Tue, Aug 23, 2022 at 7:06 AM Anthony Coulter <riscv@...> wrote:
Out of curiosity, what's the point of supporting S-mode if not to use paged virtual memory? My understanding of things was that S-mode provides virtual memory, a secondary level of interrupt/exception handling beneath M-mode (which can already handle interrupts and exceptions by itself), and nothing else. I also understood that S-mode had restricted access to the machine's hardware mainly to support virtualization---the idea is that it uses an interface that's designed to be easily intercepted (per the Popek-Goldberg rules on virtualization). I also assume (which may be wrong) that MMU-less systems are MMU-less because they can't afford the transistors and power, so I would not expect them to run hypervisors.