powerpc: Add framework for Kernel Userspace Protection

This patch adds a skeleton for Kernel Userspace Protection
functionnalities like Kernel Userspace Access Protection and Kernel
Userspace Execution Prevention

The subsequent implementation of KUAP for radix makes use of a MMU
feature in order to patch out assembly when KUAP is disabled or
unsupported. This won't work unless there's an entry point for KUP
support before the feature magic happens, so for PPC64 setup_kup() is
called early in setup.

On PPC32, feature_fixup() is done too early to allow the same.

Suggested-by: Russell Currey <ruscur@russell.cc>
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kernel/setup_64.c

@@ -68,6 +68,7 @@
 #include <asm/cputhreads.h>
 #include <asm/hw_irq.h>
 #include <asm/feature-fixups.h>
+#include <asm/kup.h>
 
 #include "setup.h"
 
@@ -331,6 +332,12 @@ void __init early_setup(unsigned long dt_ptr)
 	 */
 	configure_exceptions();
 
+	/*
+	 * Configure Kernel Userspace Protection. This needs to happen before
+	 * feature fixups for platforms that implement this using features.
+	 */
+	setup_kup();
+
 	/* Apply all the dynamic patching */
 	apply_feature_fixups();
 	setup_feature_keys();