powerpc: align stack to 2 * THREAD_SIZE with VMAP_STACK

In order to ease stack overflow detection, align
stack to 2 * THREAD_SIZE when using VMAP_STACK.
This allows overflow detection using a single bit check.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/60e9ae86b7d2cdcf21468787076d345663648f46.1576916812.git.christophe.leroy@c-s.fr

arch/powerpc/include/asm/thread_info.h

@@ -22,6 +22,19 @@
 
 #define THREAD_SIZE		(1 << THREAD_SHIFT)
 
+/*
+ * By aligning VMAP'd stacks to 2 * THREAD_SIZE, we can detect overflow by
+ * checking sp & (1 << THREAD_SHIFT), which we can do cheaply in the entry
+ * assembly.
+ */
+#ifdef CONFIG_VMAP_STACK
+#define THREAD_ALIGN_SHIFT	(THREAD_SHIFT + 1)
+#else
+#define THREAD_ALIGN_SHIFT	THREAD_SHIFT
+#endif
+
+#define THREAD_ALIGN		(1 << THREAD_ALIGN_SHIFT)
+
 #ifndef __ASSEMBLY__
 #include <linux/cache.h>
 #include <asm/processor.h>