powerpc/exec: Set thread.regs early during exec

In later patches during exec, we would like to access default regs.amr to
control access to the user mapping. Having thread.regs set early makes the
code changes simpler.
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20201127044424.40686-10-aneesh.kumar@linux.ibm.com

arch/powerpc/include/asm/thread_info.h

@@ -77,10 +77,8 @@ struct thread_info {
 /* how to get the thread information struct from C */
 extern int arch_dup_task_struct(struct task_struct *dst, struct task_struct *src);
 
-#ifdef CONFIG_PPC_BOOK3S_64
 void arch_setup_new_exec(void);
 #define arch_setup_new_exec arch_setup_new_exec
-#endif
 
 #endif /* __ASSEMBLY__ */
 

arch/powerpc/kernel/process.c

@@ -1531,14 +1531,22 @@ void flush_thread(void)
 #endif /* CONFIG_HAVE_HW_BREAKPOINT */
 }
 
-#ifdef CONFIG_PPC_BOOK3S_64
 void arch_setup_new_exec(void)
 {
-	if (radix_enabled())
-		return;
+
+#ifdef CONFIG_PPC_BOOK3S_64
+	if (!radix_enabled())
 		hash__setup_new_exec();
-}
 #endif
+	/*
+	 * If we exec out of a kernel thread then thread.regs will not be
+	 * set.  Do it now.
+	 */
+	if (!current->thread.regs) {
+		struct pt_regs *regs = task_stack_page(current) + THREAD_SIZE;
+		current->thread.regs = regs - 1;
+	}
+}
 
 #ifdef CONFIG_PPC64
 /**
@@ -1771,15 +1779,6 @@ void start_thread(struct pt_regs *regs, unsigned long start, unsigned long sp)
 		preload_new_slb_context(start, sp);
 #endif
 
-	/*
-	 * If we exec out of a kernel thread then thread.regs will not be
-	 * set.  Do it now.
-	 */
-	if (!current->thread.regs) {
-		struct pt_regs *regs = task_stack_page(current) + THREAD_SIZE;
-		current->thread.regs = regs - 1;
-	}
-
 #ifdef CONFIG_PPC_TRANSACTIONAL_MEM
 	/*
 	 * Clear any transactional state, we're exec()ing. The cause is