diff --git a/sys/cddl/contrib/opensolaris/uts/common/sys/dtrace.h b/sys/cddl/contrib/opensolaris/uts/common/sys/dtrace.h
--- a/sys/cddl/contrib/opensolaris/uts/common/sys/dtrace.h
+++ b/sys/cddl/contrib/opensolaris/uts/common/sys/dtrace.h
@@ -51,6 +51,12 @@
 
 #include <sys/param.h>
 #include <sys/stdint.h>
+#include <sys/types.h>
+
+#if defined(__i386) || defined(__amd64) || defined (__riscv)
+extern int dtrace_instr_size(uint8_t *instr);
+#endif
+
 #ifdef _KERNEL
 #include <sys/endian.h>
 #endif
@@ -71,7 +77,6 @@
 #include <sys/ioccom.h>
 #include <sys/cred.h>
 #include <sys/proc.h>
-#include <sys/types.h>
 #include <sys/ucred.h>
 #endif
 typedef int model_t;
@@ -2399,7 +2404,6 @@
 extern int dtrace_mach_aframes(void);
 
 #if defined(__i386) || defined(__amd64)
-extern int dtrace_instr_size(uint8_t *instr);
 extern int dtrace_instr_size_isa(uint8_t *, model_t, int *);
 extern void dtrace_invop_callsite(void);
 #endif
diff --git a/sys/cddl/dev/dtrace/riscv/instr_size.c b/sys/cddl/dev/dtrace/riscv/instr_size.c
new file mode 100644
--- /dev/null
+++ b/sys/cddl/dev/dtrace/riscv/instr_size.c
@@ -0,0 +1,21 @@
+/*
+ * SPDX-License-Identifier: CDDL 1.0
+ *
+ * Copyright 2023 Christos Margiolis <christos@FreeBSD.org>
+ */
+
+#include <machine/encoding.h>
+
+#include <dtrace.h>
+
+#define RVC_MASK 0x03
+
+int
+dtrace_instr_size(uint8_t *instr)
+{
+	/* Detect compressed instructions. */
+	if ((~(*instr) & RVC_MASK) == 0)
+		return (INSN_SIZE);
+	else
+		return (INSN_C_SIZE);
+}