diff --git a/include/hw/boards.h b/include/hw/boards.h
index 24eb6f0..5da4fb00e 100644
--- a/include/hw/boards.h
+++ b/include/hw/boards.h
@@ -51,6 +51,11 @@
  *    used to provide @cpu_index to socket number mapping, allowing
  *    a machine to group CPU threads belonging to the same socket/package
  *    Returns: socket number given cpu_index belongs to.
+ * @hw_version:
+ *    Value of QEMU_VERSION when the machine was added to QEMU.
+ *    Set only by old machines because they need to keep
+ *    compatibility on code that exposed QEMU_VERSION to guests in
+ *    the past (and now use qemu_hw_version()).
  */
 struct MachineClass {
     /*< private >*/
diff --git a/include/qemu/osdep.h b/include/qemu/osdep.h
index 861d84b..84e84ac 100644
--- a/include/qemu/osdep.h
+++ b/include/qemu/osdep.h
@@ -256,6 +256,10 @@
 
 void qemu_set_cloexec(int fd);
 
+/* QEMU "hardware version" setting. Used to replace code that exposed
+ * QEMU_VERSION to guests in the past and need to keep compatibilty.
+ * Do not use qemu_hw_version() in new code.
+ */
 void qemu_set_hw_version(const char *);
 const char *qemu_hw_version(void);
 
diff --git a/util/osdep.c b/util/osdep.c
index 80c6bfe..534b511 100644
--- a/util/osdep.c
+++ b/util/osdep.c
@@ -52,7 +52,14 @@
 
 static bool fips_enabled = false;
 
-static const char *hw_version = QEMU_VERSION;
+/* Starting on QEMU 2.5, qemu_hw_version() returns "2.5+" by default
+ * instead of QEMU_VERSION, so setting hw_version on MachineClass
+ * is no longer mandatory.
+ *
+ * Do NOT change this string, or it will break compatibility on all
+ * machine classes that don't set hw_version.
+ */
+static const char *hw_version = "2.5+";
 
 int socket_set_cork(int fd, int v)
 {