arm64: support identity mapped area >512GiB

[mail_patrick-wildt.de]

Summary

A machine in my possesion has memory starting at 0x8000000000
which is just shortly after what 39 bits of virtual address
space can handle. IO ports, like UART, are inside of those
first 39 bits though.

Our bootup code requires an identity mapped TTBR0 until it
jumps into kernel VA space. To be able to cope with those areas
we need to use 48 bit VA space for TTBR0 and add another
pagetable level by starting the translation at L0.

This way we can create 2 512GiB identitiy mappings to handle
the first 39 bits and the space our kernel runs in.

[andrew]

Can you rebase this after rS289581? I've committed the locore.S and armreg.h parts.

sys/arm64/arm64/pmap.c

448

This should be at the top of the file

573

kernstart isn't the correct value here, it should be based on what physical memory is available. I think this could be moved after the loop, and calculate the minimum PA based on that, with kernstart the fallback value.

sys/arm64/include/vmparam.h

178

Would it not be better to use the delta between DMAP_MIN_PHYSADDR and dmap_phys_base? i.e. have in pmap.c:

dmap_phys_delta = DMAP_MIN_ADDRESS - dmap_phys_base;

Then make this:

#define PHYS_TO_DMAP(pa)    \
  ({                        \
    ...
    (pa) + dmap_phys_delta; \
  })

(assuming I have the subtraction correct)