x86/fred: Reserve space for the FRED stack frame

When using FRED, reserve space at the top of the stack frame, just
like i386 does.

Signed-off-by: H. Peter Anvin (Intel) <hpa@zytor.com>
Signed-off-by: Xin Li <xin3.li@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Tested-by: Shan Kang <shan.kang@intel.com>
Link: https://lore.kernel.org/r/20231205105030.8698-17-xin3.li@intel.com

arch/x86/include/asm/thread_info.h

@@ -31,16 +31,22 @@
  * In vm86 mode, the hardware frame is much longer still, so add 16
  * bytes to make room for the real-mode segments.
  *
- * x86_64 has a fixed-length stack frame.
+ * x86-64 has a fixed-length stack frame, but it depends on whether
+ * or not FRED is enabled. Future versions of FRED might make this
+ * dynamic, but for now it is always 2 words longer.
  */
 #ifdef CONFIG_X86_32
 # ifdef CONFIG_VM86
 #  define TOP_OF_KERNEL_STACK_PADDING 16
 # else
 #  define TOP_OF_KERNEL_STACK_PADDING 8
 # endif
-#else
-# define TOP_OF_KERNEL_STACK_PADDING 0
+#else /* x86-64 */
+# ifdef CONFIG_X86_FRED
+#  define TOP_OF_KERNEL_STACK_PADDING (2 * 8)
+# else
+#  define TOP_OF_KERNEL_STACK_PADDING 0
+# endif
 #endif
 
 /*