diff --git a/xen/arch/x86/domain.c b/xen/arch/x86/domain.c
index 996b50af7a..6ebe6c5052 100644
--- a/xen/arch/x86/domain.c
+++ b/xen/arch/x86/domain.c
@@ -813,8 +813,8 @@ static bool emulation_flags_ok(const struct domain *d, uint32_t emflags)
 void __init arch_init_idle_domain(struct domain *d)
 {
     static const struct arch_csw idle_csw = {
-        .from = paravirt_ctxt_switch_from,
-        .to   = paravirt_ctxt_switch_to,
+        .from = FUNCTION_POISON,
+        .to   = FUNCTION_POISON,
         .tail = idle_loop,
     };
 
diff --git a/xen/arch/x86/include/asm/config.h b/xen/arch/x86/include/asm/config.h
index 1b28349a42..bf82cd082a 100644
--- a/xen/arch/x86/include/asm/config.h
+++ b/xen/arch/x86/include/asm/config.h
@@ -56,6 +56,9 @@
 #define LIST_POISON1  ((void *)0x0100100100100100UL)
 #define LIST_POISON2  ((void *)0x0200200200200200UL)
 
+/* Poison unreachable function hooks with a non-canonical value. */
+#define FUNCTION_POISON ((void *)0xDEAD0000DEAD0000UL)
+
 #include <xen/const.h>
 
 #define PML4_ENTRY_BITS  39