diff --git a/sys/compat/linuxkpi/common/include/linux/io.h b/sys/compat/linuxkpi/common/include/linux/io.h
index d4376efee78d..f6c9405f7c00 100644
--- a/sys/compat/linuxkpi/common/include/linux/io.h
+++ b/sys/compat/linuxkpi/common/include/linux/io.h
@@ -171,6 +171,14 @@ readl(const volatile void *addr)
 	return *(const volatile uint32_t *)addr;
 }
 
+#if defined(__i386__) || defined(__amd64__)
+static inline void
+_outb(u_char data, u_int port)
+{
+	__asm __volatile("outb %0, %w1" : : "a" (data), "Nd" (port));
+}
+#endif
+
 #if defined(__i386__) || defined(__amd64__)
 void *_ioremap_attr(vm_paddr_t phys_addr, unsigned long size, int attr);
 #else