Index: sys/riscv/include/memdev.h
===================================================================
--- sys/riscv/include/memdev.h
+++ sys/riscv/include/memdev.h
@@ -35,6 +35,6 @@
 d_open_t	memopen;
 d_read_t	memrw;
 d_ioctl_t	memioctl_md;
-#define		memmmap		(d_mmap_t *)NULL
+d_mmap_t	memmmap;
 
 #endif /* _MACHINE_MEMDEV_H_ */
Index: sys/riscv/riscv/mem.c
===================================================================
--- sys/riscv/riscv/mem.c
+++ sys/riscv/riscv/mem.c
@@ -122,6 +122,22 @@
 	return (error);
 }
 
+/*
+ * allow user processes to MMAP some memory sections
+ * instead of going through read/write
+ */
+/* ARGSUSED */
+int
+memmmap(struct cdev *dev, vm_ooffset_t offset, vm_paddr_t *paddr,
+    int prot __unused, vm_memattr_t *memattr __unused)
+{
+	if (dev2unit(dev) == CDEV_MINOR_MEM) {
+		*paddr = offset;
+		return (0);
+	}
+	return (-1);
+}
+
 int
 memioctl_md(struct cdev *dev __unused, u_long cmd __unused,
     caddr_t data __unused, int flags __unused, struct thread *td __unused)