diff --git a/xen/Makefile b/xen/Makefile
index b923235c53..7b869f4b30 100644
--- a/xen/Makefile
+++ b/xen/Makefile
@@ -256,7 +256,8 @@ export YACC = $(if $(BISON),$(BISON),bison)
 export LEX = $(if $(FLEX),$(FLEX),flex)
 
 # Default file for 'make defconfig'.
-export KBUILD_DEFCONFIG := $(ARCH)_defconfig
+# May be overruled on the command line or set in the environment.
+export KBUILD_DEFCONFIG ?= $(ARCH)_defconfig
 
 # Copy CFLAGS generated by "Config.mk" so they can be reused later without
 # reparsing Config.mk by e.g. arch/x86/boot/.