syscallarg_t: Add a type for system call arguments

[brooks]

Summary

This more clearly differentiates system call arguments from integer
registers and return values. On current architectures it has no effect,
but on architectures where pointers are not integers (CHERI) and may
not even share registers (CHERI-MIPS) it is necessiary to differentiate
between system call arguments (syscallarg_t) and integer register values
(register_t).

Obtained from: CheriBSD

[imp]

I like this.

[emaste]

👍

[andrew]

Do you plan to also update other architectures and ABIs?

[brooks]

• Update amd64 and arm

[brooks]

In D33780#764080, @andrew wrote:

Do you plan to also update other architectures and ABIs?

I've done arm and amd64. I've not touched powerpc because it follows a unique model.

[kib]

sys/sys/types.h

273

Can we have it named less verbose, e.g. scarg_t ?

sys/vm/vm_mmap.c

423

Is this cast needed at all?

[brooks]

Remove needless cast

[brooks]

sys/vm/vm_mmap.c

423

It doesn't appear to be. I've removed it.

[brooks]

sys/sys/types.h

273

I don't see the value in trading a more obfuscated name for 5 bytes per use of a rarely used type.

[kib]

sys/sys/types.h

273

The lines where syscallarg_t needs to be spelled, get too long. Typically if you need to cast, you need to repeat the type name in the same expression one more time, so it is 5-10 chars lost out of 72.