D056 - The guest-to-host call boundary works, and needs extern "sysv64"

decided · 2026-08-19 · phase 0b, answered on both platforms

The guest calls with System V: integer arguments in rdi, rsi, rdx, rcx, r8, r9, return in rax.

On Windows the host's native convention is different - rcx, rdx, r8, r9 plus a 32-byte shadow space. A guest calling an ordinary extern "C" Rust function on Windows would pass arguments in the wrong registers, and the failure would be silent: the callee reads whatever happened to be in rcx. That is the worst possible shape for a bug at this layer, and it is why the spike was worth doing before phase 4 rather than during it.

Host functions the guest can reach are therefore declared extern "sysv64". Proved by executing hand-assembled machine code that loads six argument registers, calls through rax, and returns - with every argument asserted on arrival. Passes on Windows and Linux from the same source.

orbistoun-abi is kept rather than thrown away: it grows into phase 4's thunk mechanism, and the tests are the regression guard for a convention mismatch.

W^X is respected in the executable buffer - written while writable, then flipped to read-execute, never both at once. Some platforms enforce that, and code assuming RWX fails there in a way that looks like a corrupt instruction stream rather than a permissions problem.