xtensa: check TLB sanity on return to userspace

- check that user TLB mappings correspond to the current page table;
- check that TLB mapping VPN is in the kernel/user address range
  in accordance with its ASID.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Chris Zankel <chris@zankel.net>

arch/xtensa/Kconfig.debug

@@ -2,6 +2,16 @@ menu "Kernel hacking"
 
 source "lib/Kconfig.debug"
 
+config DEBUG_TLB_SANITY
+	bool "Debug TLB sanity"
+	depends on DEBUG_KERNEL
+	help
+	  Enable this to turn on TLB sanity check on each entry to userspace.
+	  This check can spot missing TLB invalidation/wrong PTE permissions/
+	  premature page freeing.
+
+	  If unsure, say N.
+
 config LD_NO_RELAX
 	bool "Disable linker relaxation"
 	default n