Index: sys/amd64/amd64/trap.c
===================================================================
--- sys/amd64/amd64/trap.c
+++ sys/amd64/amd64/trap.c
@@ -970,8 +970,8 @@
 	panic("double fault");
 }
 
-int
-cpu_fetch_syscall_args(struct thread *td)
+int __noinline
+cpu_fetch_syscall_args_fallback(struct thread *td)
 {
 	struct proc *p;
 	struct trapframe *frame;
@@ -1022,6 +1022,54 @@
 	return (error);
 }
 
+int
+cpu_fetch_syscall_args(struct thread *td)
+{
+	struct proc *p;
+	struct trapframe *frame;
+	register_t *argp;
+	struct syscall_args *sa;
+	int reg, regcnt;
+
+	p = td->td_proc;
+	frame = td->td_frame;
+	sa = &td->td_sa;
+	reg = 0;
+	regcnt = 6;
+
+	sa->code = frame->tf_rax;
+
+	if (__predict_false(sa->code == SYS_syscall))
+		goto fallback;
+
+	if (__predict_false(sa->code == SYS___syscall))
+		goto fallback;
+
+	if (__predict_false(sa->code >= p->p_sysent->sv_size))
+		goto fallback;
+
+	sa->callp = &p->p_sysent->sv_table[sa->code];
+	sa->narg = sa->callp->sy_narg;
+	KASSERT(sa->narg <= sizeof(sa->args) / sizeof(sa->args[0]),
+	    ("Too many syscall arguments!"));
+
+	if (p->p_sysent->sv_mask)
+		sa->code &= p->p_sysent->sv_mask;
+
+	if (__predict_false(sa->narg > regcnt))
+		goto fallback;
+
+	argp = &frame->tf_rdi;
+	memcpy(sa->args, argp, sizeof(sa->args[0]) * 6);
+
+	td->td_retval[0] = 0;
+	td->td_retval[1] = frame->tf_rdx;
+
+	return (0);
+fallback:
+	return (cpu_fetch_syscall_args_fallback(td));
+}
+
 #include "../../kern/subr_syscall.c"
 
 /*