Index: head/sys/riscv/include/frame.h
===================================================================
--- head/sys/riscv/include/frame.h
+++ head/sys/riscv/include/frame.h
@@ -74,4 +74,7 @@
 
 #endif /* !LOCORE */
 
+/* Definitions for syscalls */
+#define	NARGREG		8				/* 8 args in regs */
+
 #endif /* !_MACHINE_FRAME_H_ */
Index: head/sys/riscv/riscv/trap.c
===================================================================
--- head/sys/riscv/riscv/trap.c
+++ head/sys/riscv/riscv/trap.c
@@ -95,7 +95,7 @@
 	register_t *ap;
 	int nap;
 
-	nap = 8;
+	nap = NARGREG;
 	p = td->td_proc;
 	ap = &td->td_frame->tf_a[0];
 
@@ -116,7 +116,7 @@
 	sa->narg = sa->callp->sy_narg;
 	memcpy(sa->args, ap, nap * sizeof(register_t));
 	if (sa->narg > nap)
-		panic("TODO: Could we have more then 8 args?");
+		panic("TODO: Could we have more then %d args?", NARGREG);
 
 	td->td_retval[0] = 0;
 	td->td_retval[1] = 0;
Index: head/usr.bin/truss/riscv64-freebsd.c
===================================================================
--- head/usr.bin/truss/riscv64-freebsd.c
+++ head/usr.bin/truss/riscv64-freebsd.c
@@ -0,0 +1,106 @@
+/*-
+ * Copyright 2017 Li-Wen Hsu <lwhsu@FreeBSD.org>
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
+ * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+ * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+ * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+ * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ */
+
+#include <sys/cdefs.h>
+__FBSDID("$FreeBSD$");
+
+/* FreeBSD/riscv64-specific system call handling. */
+
+#include <sys/ptrace.h>
+#include <sys/syscall.h>
+
+#include <machine/frame.h>
+#include <machine/reg.h>
+
+#include <stdbool.h>
+#include <stdio.h>
+#include <sysdecode.h>
+
+#include "truss.h"
+
+static int
+riscv64_fetch_args(struct trussinfo *trussinfo, u_int narg)
+{
+	struct reg regs;
+	struct current_syscall *cs;
+	lwpid_t tid;
+	u_int i, reg, syscall_num;
+
+	tid = trussinfo->curthread->tid;
+	cs = &trussinfo->curthread->cs;
+	if (ptrace(PT_GETREGS, tid, (caddr_t)&regs, 0) < 0) {
+		fprintf(trussinfo->outfile, "-- CANNOT READ REGISTERS --\n");
+		return (-1);
+	}
+
+	/*
+	 * FreeBSD has two special kinds of system call redirections --
+	 * SYS_syscall, and SYS___syscall.  The former is the old syscall()
+	 * routine, basically; the latter is for quad-aligned arguments.
+	 *
+	 * The system call argument count and code from ptrace() already
+	 * account for these, but we need to skip over the first argument.
+	 */
+	syscall_num = regs.t[0];
+	if (syscall_num == SYS_syscall || syscall_num == SYS___syscall) {
+		reg = 1;
+		syscall_num = regs.a[0];
+	} else {
+		reg = 0;
+	}
+
+	for (i = 0; i < narg && reg < NARGREG; i++, reg++)
+		cs->args[i] = regs.a[reg];
+	return (0);
+}
+
+static int
+riscv64_fetch_retval(struct trussinfo *trussinfo, long *retval, int *errorp)
+{
+	struct reg regs;
+	lwpid_t tid;
+
+	tid = trussinfo->curthread->tid;
+	if (ptrace(PT_GETREGS, tid, (caddr_t)&regs, 0) < 0) {
+		fprintf(trussinfo->outfile, "-- CANNOT READ REGISTERS --\n");
+		return (-1);
+	}
+
+	retval[0] = regs.a[0];
+	retval[1] = regs.a[1];
+	*errorp = !!(regs.t[0]);
+	return (0);
+}
+
+static struct procabi riscv64_freebsd = {
+	"FreeBSD ELF64",
+	SYSDECODE_ABI_FREEBSD,
+	riscv64_fetch_args,
+	riscv64_fetch_retval,
+	STAILQ_HEAD_INITIALIZER(riscv64_freebsd.extra_syscalls),
+	{ NULL }
+};
+
+PROCABI(riscv64_freebsd);