diff --git a/src/libxl/libxl_conf.c b/src/libxl/libxl_conf.c
index 2565f6473cb576cf0b4eefd0cf27fd8d22b8d311..2053ed332754ac1f7f801b83a44ca734e956407f 100644
--- a/src/libxl/libxl_conf.c
+++ b/src/libxl/libxl_conf.c
@@ -424,6 +424,12 @@ libxlMakeDomBuildInfo(virDomainDefPtr def,
             libxl_defbool_set(&b_info->u.hvm.nested_hvm, hasHwVirt);
         }
 
+        if (def->cpu && def->cpu->mode == VIR_CPU_MODE_CUSTOM) {
+            VIR_WARN("Ignoring CPU with mode=custom, update your config to "
+                     "mode=host-passthrough to avoid risk of changed guest "
+                     "semantics when mode=custom is supported in the future");
+        }
+
         if (def->nsounds > 0) {
             /*
              * Use first sound device.  man xl.cfg(5) describes soundhw as