arm64: mark idle code as noinstr

Core code disables RCU when calling arch_cpu_idle(), so it's not safe
for arch_cpu_idle() or its calees to be instrumented, as the
instrumentation callbacks may attempt to use RCU or other features which
are unsafe to use in this context.

Mark them noinstr to prevent issues.

The use of local_irq_enable() in arch_cpu_idle() is similarly
problematic, and the "sched/idle: Fix arch_cpu_idle() vs tracing" patch
queued in the tip tree addresses that case.
Reported-by: Marco Elver <elver@google.com>
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: James Morse <james.morse@arm.com>
Cc: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20201130115950.22492-3-mark.rutland@arm.comSigned-off-by: Will Deacon <will@kernel.org>

arch/arm64/kernel/process.c

@@ -72,13 +72,13 @@ EXPORT_SYMBOL_GPL(pm_power_off);
 
 void (*arm_pm_restart)(enum reboot_mode reboot_mode, const char *cmd);
 
-static void __cpu_do_idle(void)
+static void noinstr __cpu_do_idle(void)
 {
 	dsb(sy);
 	wfi();
 }
 
-static void __cpu_do_idle_irqprio(void)
+static void noinstr __cpu_do_idle_irqprio(void)
 {
 	unsigned long pmr;
 	unsigned long daif_bits;
@@ -108,7 +108,7 @@ static void __cpu_do_idle_irqprio(void)
  *	ensure that interrupts are not masked at the PMR (because the core will
  *	not wake up if we block the wake up signal in the interrupt controller).
  */
-void cpu_do_idle(void)
+void noinstr cpu_do_idle(void)
 {
 	if (system_uses_irq_prio_masking())
 		__cpu_do_idle_irqprio();
@@ -119,7 +119,7 @@ void cpu_do_idle(void)
 /*
  * This is our default idle handler.
  */
-void arch_cpu_idle(void)
+void noinstr arch_cpu_idle(void)
 {
 	/*
 	 * This should do all the clock switching and wait for interrupt