There's no reason to keep the unused pages (of which there are actually
two; respective commentary also gets adjusted) mapped.

Signed-off-by: Jan Beulich <jbeul...@suse.com>

--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -5563,10 +5563,11 @@ void memguard_unguard_range(void *p, uns
 
 void memguard_guard_stack(void *p)
 {
-    BUILD_BUG_ON((PRIMARY_STACK_SIZE + PAGE_SIZE) > STACK_SIZE);
-    p = (void *)((unsigned long)p + STACK_SIZE -
-                 PRIMARY_STACK_SIZE - PAGE_SIZE);
-    memguard_guard_range(p, PAGE_SIZE);
+    /* IST_MAX IST pages + at least 1 guard page + primary stack. */
+    BUILD_BUG_ON((IST_MAX + 1) * PAGE_SIZE + PRIMARY_STACK_SIZE > STACK_SIZE);
+
+    memguard_guard_range(p + IST_MAX * PAGE_SIZE,
+                         STACK_SIZE - PRIMARY_STACK_SIZE - IST_MAX * 
PAGE_SIZE);
 }
 
 void memguard_unguard_stack(void *p)
--- a/xen/arch/x86/x86_64/traps.c
+++ b/xen/arch/x86/x86_64/traps.c
@@ -301,8 +301,8 @@ void subarch_percpu_traps_init(void)
     unsigned char *stub_page;
     unsigned int offset;
 
-    /* IST_MAX IST pages + 1 syscall page + 1 guard page + primary stack. */
-    BUILD_BUG_ON((IST_MAX + 2) * PAGE_SIZE + PRIMARY_STACK_SIZE > STACK_SIZE);
+    /* IST_MAX IST pages + at least 1 guard page + primary stack. */
+    BUILD_BUG_ON((IST_MAX + 1) * PAGE_SIZE + PRIMARY_STACK_SIZE > STACK_SIZE);
 
     stub_page = map_domain_page(_mfn(this_cpu(stubs.mfn)));
 
--- a/xen/include/asm-x86/current.h
+++ b/xen/include/asm-x86/current.h
@@ -16,9 +16,9 @@
  *
  * 7 - Primary stack (with a struct cpu_info at the top)
  * 6 - Primary stack
- * 5 - Optionally not preset (MEMORY_GUARD)
+ * 5 - Optionally not present (MEMORY_GUARD)
  * 4 - unused
- * 3 - Syscall trampolines
+ * 3 - unused
  * 2 - MCE IST stack
  * 1 - NMI IST stack
  * 0 - Double Fault IST stack




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