diff --git a/xen/arch/x86/spec_ctrl.c b/xen/arch/x86/spec_ctrl.c
index 7a4550db83..ba64a09048 100644
--- a/xen/arch/x86/spec_ctrl.c
+++ b/xen/arch/x86/spec_ctrl.c
@@ -122,6 +122,7 @@ static int __init cf_check parse_spec_ctrl(const char *s)
             opt_l1d_flush = 0;
             opt_branch_harden = false;
             opt_srb_lock = 0;
+            opt_unpriv_mmio = false;
         }
         else if ( val > 0 )
             rc = -EINVAL;