xtensa: userspace: Warning about impl security

Add a Kconfig option (and build warning) alerting about the problem
of the kernel spilling register in behave of the userspace.

Signed-off-by: Flavio Ceolin <flavio.ceolin@intel.com>

arch/xtensa/CMakeLists.txt

@@ -3,3 +3,9 @@
 zephyr_syscall_header(${ZEPHYR_BASE}/include/zephyr/arch/xtensa/arch.h)
 set_property(GLOBAL PROPERTY PROPERTY_OUTPUT_FORMAT elf32-xtensa-le)
 add_subdirectory(core)
+
+if (CONFIG_XTENSA_INSECURE_USERSPACE)
+  message(WARNING "
+  This userspace implementation uses the window ABI this means that the kernel
+  will spill registers in behave of the userpsace. Use it carefully.")
+endif()

arch/xtensa/Kconfig

@@ -180,6 +180,11 @@ config XTENSA_SYSCALL_USE_HELPER
 	  This is a workaround for toolchains where they have
 	  issue modeling register usage.
 
+config XTENSA_INSECURE_USERSPACE
+	bool
+	default y if USERSPACE
+	depends on XTENSA_MMU
+
 endif # CPU_HAS_MMU
 
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