xen/public: add comments regarding interface version bumps

domctl.h and sysctl.h have an interface version, which needs to be
bumped in case of incompatible modifications of the interface.

In order to avoid misunderstandings, add a comment to both headers
specifying in which cases a bump is needed.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Julien Grall <jgrall@amazon.com>

xen/include/public/domctl.h

@@ -21,6 +21,16 @@
 #include "hvm/save.h"
 #include "memory.h"
 
+/*
+ * The interface version needs to be incremented by 1 in case the interface
+ * is modified in an incompatible way AND if the version hasn't been
+ * incremented in the current development cycle already.
+ * Pure additions (e.g. new sub-commands) or compatible interface changes
+ * (e.g. adding semantics to 0-checked input fields or data to zeroed output
+ * fields) don't require a change of the version.
+ *
+ * Last version bump: Xen 4.19
+ */
 #define XEN_DOMCTL_INTERFACE_VERSION 0x00000017
 
 /*

xen/include/public/sysctl.h

@@ -18,6 +18,16 @@
 #include "domctl.h"
 #include "physdev.h"
 
+/*
+ * The interface version needs to be incremented by 1 in case the interface
+ * is modified in an incompatible way AND if the version hasn't been
+ * incremented in the current development cycle already.
+ * Pure additions (e.g. new sub-commands) or compatible interface changes
+ * (e.g. adding semantics to 0-checked input fields or data to zeroed output
+ * fields) don't require a change of the version.
+ *
+ * Last version bump: Xen 4.17
+ */
 #define XEN_SYSCTL_INTERFACE_VERSION 0x00000015
 
 /*