diff --git a/xen/include/asm-x86/desc.h b/xen/include/asm-x86/desc.h
index 4093c65faa..58efc1d896 100644
--- a/xen/include/asm-x86/desc.h
+++ b/xen/include/asm-x86/desc.h
@@ -106,8 +106,19 @@ struct desc_struct {
     u32 a, b;
 };
 
-typedef struct {
-    u64 a, b;
+typedef union {
+    struct {
+        uint64_t a, b;
+    };
+    struct {
+        uint16_t addr0;
+        uint16_t cs;
+        uint8_t  ist; /* :3, 5 bits rsvd, but this yields far better code. */
+        uint8_t  type:4, s:1, dpl:2, p:1;
+        uint16_t addr1;
+        uint32_t addr2;
+        /* 32 bits rsvd. */
+    };
 } idt_entry_t;
 
 /* Write the lower 64 bits of an IDT Entry. This relies on the upper 32
diff --git a/xen/include/asm-x86/processor.h b/xen/include/asm-x86/processor.h
index db9988ab33..1bc2f90d04 100644
--- a/xen/include/asm-x86/processor.h
+++ b/xen/include/asm-x86/processor.h
@@ -406,16 +406,14 @@ struct __packed __cacheline_aligned tss_struct {
 #define IST_MCE  3UL
 #define IST_MAX  3UL
 
-/* Set the interrupt stack table used by a particular interrupt
- * descriptor table entry. */
-static always_inline void set_ist(idt_entry_t *idt, unsigned long ist)
+/* Set the Interrupt Stack Table used by a particular IDT entry. */
+static inline void set_ist(idt_entry_t *idt, unsigned int ist)
 {
-    idt_entry_t new = *idt;
-
     /* IST is a 3 bit field, 32 bits into the IDT entry. */
     ASSERT(ist <= IST_MAX);
-    new.a = (idt->a & ~(7UL << 32)) | (ist << 32);
-    _write_gate_lower(idt, &new);
+
+    /* Typically used on a live idt.  Disuade any clever optimisations. */
+    ACCESS_ONCE(idt->ist) = ist;
 }
 
 static inline void enable_each_ist(idt_entry_t *idt)