Specify required memory type / caching behavior for `map_physical_region`

[ChocolateLoverRaj]

I'm guessing Write-Back should be okay (and preferred, because it is the most performant), but I'm not 100% sure. Am I right? Either way, can the requirements for this be mentioned in the docs for map_physical_region?

Also I am assuming the memory protection needs to be readable, and should be not writable and not executable. Can this also be added to the docs?

[martin-hughes]

I'm not sure that Write-Back caching is always sufficient - for example, I assume that the write in wake_aps needs to be uncached.

For the caching behaviour, I think we'd need to specify it to map_physical_region as an argument. But is it a priority? My feeling is no - I assume that the system's MTRRs are set up correctly to cope with OSs that don't use PATs? Which would mean the correct caching behaviour is applied. That is an assumption though - I'm not in a position to test it. I also don't have a good understanding of whether it's necessary to OSs to specify cache behaviour or ARM or RISC-V.

So please feel free to add a PR if you need it.

One thing that I can see straight away is memory protection - writeable is needed (sometimes) but AFAICT executable is not. Again, this could be added as a parameter to map_physical_region.