[RFC][PATCH 13/24] x86/task: Grow thread_union for shadow stacks

[Dave Hansen]

Define the size of an x86 shadow stack: one page. This will inflate the
virtual size and physical backing of thread_union, wasting one page
per task on non-shadow-stack configurations.

This waste can be cleaned up with the "double vmap()" approach discussed
earlier. For now, please ignore the gluttonous use of DRAM.

---

 b/arch/x86/include/asm/page_64_types.h |    9 +++++++++
 1 file changed, 9 insertions(+)

diff -puN arch/x86/include/asm/page_64_types.h~x86-define-shadow-stack-size arch/x86/include/asm/page_64_types.h
--- a/arch/x86/include/asm/page_64_types.h~x86-define-shadow-stack-size	2026-10-21 12:36:38.870882865 -0700
+++ b/arch/x86/include/asm/page_64_types.h	2026-10-21 12:36:38.872882869 -0700
@@ -15,6 +15,15 @@
 #define THREAD_SIZE_ORDER	(2 + KASAN_STACK_ORDER)
 #define THREAD_SIZE  (PAGE_SIZE << THREAD_SIZE_ORDER)
 
+/*
+ * Grow 'thread_union' to make space for the shadow stack
+ * page. For now, this wastes a page of space per task on
+ * systems that do not support shadow stacks.
+ */
+#ifdef CONFIG_X86_KERN_SHADOW_STACK
+#define TASK_SHADOW_STACK_SIZE PAGE_SIZE
+#endif
+
 #define EXCEPTION_STACK_ORDER (1 + KASAN_STACK_ORDER)
 #define EXCEPTION_STKSZ (PAGE_SIZE << EXCEPTION_STACK_ORDER)
 
_