diff --git a/INSTALL b/INSTALL
index 58359da380..9aa9ebdddc 100644
--- a/INSTALL
+++ b/INSTALL
@@ -247,6 +247,10 @@ EXTRA_CFLAGS_XEN_TOOLS=
 EXTRA_CFLAGS_QEMU_TRADITIONAL=
 EXTRA_CFLAGS_QEMU_XEN=
 
+Additional CFLAGS may be supplied to the build of the hypervisor by
+using this variable.
+EXTRA_CFLAGS_XEN_CORE=
+
 This variable can be used to use DIR/include and DIR/lib during build.
 This is the same as PREPEND_LIB and PREPEND_INCLUDES. APPEND_LIB and
 APPEND_INCLUDES= will be appended to the CFLAGS/LDFLAGS variable.
diff --git a/xen/Rules.mk b/xen/Rules.mk
index 2918019b92..ef26b8d1bb 100644
--- a/xen/Rules.mk
+++ b/xen/Rules.mk
@@ -79,6 +79,8 @@ ALL_OBJS := $(ALL_OBJS-y)
 CFLAGS-y += -MMD -MF $(@D)/.$(@F).d
 
 CFLAGS += $(CFLAGS-y)
+# allow extra CFLAGS externally via EXTRA_CFLAGS_XEN_CORE
+CFLAGS += $(EXTRA_CFLAGS_XEN_CORE)
 
 # Most CFLAGS are safe for assembly files:
 #  -std=gnu{89,99} gets confused by #-prefixed end-of-line comments