WIP linux multiarch support

src/libraries/j6/j6.module

@@ -21,6 +21,9 @@ j6 = module("j6",
         "amd64": [
             "syscalls.s.cog",
         ],
+        "linux": [
+            "linux/syscalls.s.cog",
+        ],
     },
     public_headers = [
         "j6/cap_flags.h.cog",

src/libraries/j6/linux/syscalls.s.cog

@@ -0,0 +1,49 @@
+; vim: ft=asm
+
+%macro define_syscall 2
+    global j6_%1: function (j6_%1.end - j6_%1)
+    j6_%1:
+        push rbp
+        mov rbp, rsp
+
+        ; if the syscall has more than 6 arguments, the rest
+        ; will be pushed on the stack. in that case, we'd need
+        ; to pass this stack pointer to the kernel, so stash
+        ; off rbx (callee-saved) and pass the pointer to the
+        ; arguments there.
+        push rbx
+        mov rbx, rbp
+        add rbx, 16 ; account for stack frame
+
+        ; args should already be in rdi, etc, but rcx will
+        ; get stomped, so stash it in r10, which isn't a
+        ; callee-saved register, but also isn't used in the
+        ; function call ABI.
+        mov r10, rcx
+
+        mov rax, %2
+        syscall
+        ; result is now already in rax, so just return
+
+        pop rbx
+        pop rbp
+        ret
+    .end:
+%endmacro
+
+; [[[cog code generation
+;   from definitions.context import Context
+;
+;   ctx = Context(definitions_path)
+;   ctx.parse("syscalls.def")
+;   syscalls = ctx.interfaces['syscalls']
+;
+;   if target != "kernel":
+;       for id, scope, method in syscalls.methods:
+;           if scope:
+;               name = f"{scope.name}_{method.name}"
+;           else:
+;               name = method.name
+;           cog.outl(f"define_syscall {name:20}, {id}")
+; ]]]
+; [[[end]]]