x86_64: add common trampoline code written in C, port to it

arch/common/common-trampoline.c

@@ -0,0 +1,28 @@
+/*
+ * Copyright (c) 2020 Ariadne Conill <ariadne@dereferenced.org>
+ *
+ * Permission to use, copy, modify, and/or distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * This software is provided 'as is' and without any warranty, express or
+ * implied.  In no event shall the authors be liable for any damages arising
+ * from the use of this software.
+ */
+
+#include <stddef.h>
+#include <stdlib.h>
+#include <stdio.h>
+
+void
+libucontext_trampoline(void)
+{
+	libucontext_ucontext_t *uc_link;
+
+	FETCH_LINKPTR(uc_link);
+
+	if (uc_link == NULL)
+		exit(0);
+
+	libucontext_setcontext(uc_link);
+}

arch/x86_64/defs.h

@@ -31,6 +31,9 @@
 
 #define REG_SZ		(8)
 
+#define FETCH_LINKPTR(dest) \
+	asm("movq (%%rbx), %0" : "=rm" ((dest)));
+
 #include "common-defs.h"
 
 #endif

arch/x86_64/startcontext.S

@@ -1,34 +0,0 @@
-/*
- * Copyright (c) 2018, 2020 Ariadne Conill <ariadne@dereferenced.org>
- *
- * Permission to use, copy, modify, and/or distribute this software for any
- * purpose with or without fee is hereby granted, provided that the above
- * copyright notice and this permission notice appear in all copies.
- *
- * This software is provided 'as is' and without any warranty, express or
- * implied.  In no event shall the authors be liable for any damages arising
- * from the use of this software.
- */
-
-#include "defs.h"
-
-FUNC(libucontext_trampoline)
-	/* get the proper context into position and test for NULL */
-	movq	%rbx, %rsp
-	movq	(%rsp), %rdi
-	testq	%rdi, %rdi
-
-	/* if we have no linked context, lets get out of here */
-	je	no_linked_context
-
-	/* call setcontext to switch to the linked context */
-	call	libucontext_setcontext@plt
-	movq	%rax, %rdi
-
-no_linked_context:
-	/* we are returning into a null context, it seems, so maybe we should exit */
-	call	exit@plt
-
-	/* something is really hosed, call hlt to force termination */
-	hlt
-END(libucontext_trampoline)

arch/x86_64/trampoline.c

@@ -0,0 +1,2 @@
+#include "defs.h"
+#include "common-trampoline.c"