compat: add a little bit of description to xlat.lst

Requested-by: Roger Pau Monné <roger.pau@citrix.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>

xen/include/xlat.lst

@@ -1,3 +1,22 @@
+# There are a few fundamentally different strategies for handling compat
+# (sub-)hypercalls:
+#
+# 1) Wrap a translation layer around the native hypercall. Structures involved
+# in this model should use translation (xlat) macros generated by adding
+# !-prefixed lines here.
+#
+# 2) Compile the entire hypercall function a second time, arranging for the
+# compat structures to get used in place of the native ones. There are no xlat
+# macros involved here, all that's needed are correctly translated structures.
+#
+# 3) Adhoc translation, which may or may not involve adding entries here.
+#
+# 4) Any mixture of the above.
+#
+# In all models any structures re-used in their native form should have
+# ?-mark prefixed lines added here, with the resulting checking macros invoked
+# somewhere in the code handling the hypercall or its translation.
+#
 # First column indicator:
 # ! - needs translation
 # ? - needs checking