powerpc: copy_thread don't set _TIF_RESTOREALL

In the kernel user thread path, don't set _TIF_RESTOREALL because
the thread is required to call kernel_execve() before it returns,
which will set _TIF_RESTOREALL if necessary via start_thread().
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230325122904.2375060-8-npiggin@gmail.com

arch/powerpc/kernel/interrupt_64.S

@@ -748,6 +748,11 @@ _GLOBAL(ret_from_kernel_user_thread)
 #endif
 	bctrl
 	li	r3,0
+	/*
+	 * It does not matter whether this returns via the scv or sc path
+	 * because it returns as execve() and therefore has no calling ABI
+	 * (i.e., it sets registers according to the exec()ed entry point).
+	 */
 	b	.Lsyscall_exit
 
 _GLOBAL(start_kernel_thread)

arch/powerpc/kernel/process.c

@@ -1745,7 +1745,6 @@ int copy_thread(struct task_struct *p, const struct kernel_clone_args *args)
 	extern void start_kernel_thread(void);
 	void (*f)(void);
 	unsigned long sp = (unsigned long)task_stack_page(p) + THREAD_SIZE;
-	struct thread_info *ti = task_thread_info(p);
 #ifdef CONFIG_HAVE_HW_BREAKPOINT
 	int i;
 #endif
@@ -1784,7 +1783,6 @@ int copy_thread(struct task_struct *p, const struct kernel_clone_args *args)
 #ifdef CONFIG_PPC64
 			childregs->softe = IRQS_ENABLED;
 #endif
-			ti->flags |= _TIF_RESTOREALL;
 			f = ret_from_kernel_user_thread;
 		} else {
 			struct pt_regs *regs = current_pt_regs();