riscv: Always use the EFI-provided FDT for boot-hartid

[jrtc27]

Summary

When booting via SBI, the hartid is available in a1. When booting via
EFI, the hartid is present in the FDT as /chosen/boot-hartid. There is
no other way to retrieve this information, as mhartid is an M-mode CSR
and no S-mode equivalent exists (nor is there an SBI call to get it).

Currently we extract boot-hartid from the FDT in the kernel. This works
if the FDT provided to the kernel is the one from EFI, but if a custom
FDT is loaded then it will no longer have /chosen/boot-hartid, causing
the kernel to be unable to get the hartid and thus panic. Instead, pass
it in metadata from loader (and put the SBI-provided value in the fake
metadata for SBI booting) as it always has access to the EFI-provided
FDT.

[jrtc27]

stand/efi/loader/bootinfo.c

330

This whole block being here and using FDT functions is really ugly, but I don't see a better place to hook this. Suggestions welcome.

sys/riscv/riscv/machdep.c

840

I have one concern with this and that's that MD_FETCH returns 0 for unknown metadata, so if you use an old loader with a new kernel it'll start thinking boot_hart is 0 even when it might not be. I don't know if that's something we're really concerned about though.

[mhorne]

I have not messed around with loading a new DTB from loader myself, but I have thought a bit about this problem of maintaining boot-hartid before. I guess you did something like load -t dtb /path/to/dtb?

u-boot solves this problem by copying fixups from the previous stage's DTB to any new DTB it loads. In practice, I believe they copy the entirety of the /chosen node, but you might want to check this. Did you consider an approach like this here? It would eliminate the need to modify the kernel.

I can see the idea of copying fixups from the EFI DTB being generally useful, not only for riscv. If there were changes made under /chosen in an earlier boot stage it would be good not to discard them.

[imp]

sys/riscv/include/machdep.h

46

why insert this here? It seems to break the ABI between boot loader and kernel...

[jrtc27]

sys/riscv/include/machdep.h

46

This isn't from loader, this is from locore so there's no ABI issue