arm64: kernel: restore HW breakpoint registers in cpu_suspend

When a CPU resumes from low-power, it restores HW breakpoint and
watchpoint slots through a CPU PM notifier. Since we want to enable
debugging as early as possible in the resume path, the mdscr content
is restored along the general purpose registers in the cpu_suspend API
and debug exceptions are reenabled when cpu_suspend returns. Since the
CPU PM notifier is run after a CPU has been resumed, we cannot expect
HW breakpoint registers to contain sane values till the notifier is run,
since the HW breakpoints registers content is unknown at reset; this means
that the CPU might run with debug exceptions enabled, mdscr restored but HW
breakpoint registers containing junk values that can trigger spurious
debug exceptions.

This patch fixes current HW breakpoints restore by moving the HW breakpoints
registers restoration to the cpu_suspend API, before the debug exceptions are
enabled. This way, as soon as the cpu_suspend function returns the
kernel can resume debugging with sane values in HW breakpoint registers.

Signed-off-by: Lorenzo Pieralisi <[e-mail]>
Acked-by: Will Deacon <[e-mail]>
Signed-off-by: Catalin Marinas <[e-mail]>

arch/arm64/kernel/hw_breakpoint.c

···
 	.notifier_call = hw_breakpoint_reset_notify,
 };
 
-#ifdef CONFIG_CPU_PM
-static int hw_breakpoint_cpu_pm_notify(struct notifier_block *self,
-				       unsigned long action,
-				       void *v)
-{
-	if (action == CPU_PM_EXIT) {
-		hw_breakpoint_reset(NULL);
-		return NOTIFY_OK;
-	}
-
-	return NOTIFY_DONE;
-}
-
-static struct notifier_block hw_breakpoint_cpu_pm_nb = {
-	.notifier_call = hw_breakpoint_cpu_pm_notify,
-};
-
-static void __init hw_breakpoint_pm_init(void)
-{
-	cpu_pm_register_notifier(&hw_breakpoint_cpu_pm_nb);
-}
+#ifdef CONFIG_ARM64_CPU_SUSPEND
+extern void cpu_suspend_set_dbg_restorer(void (*hw_bp_restore)(void *));
 #else
-static inline void hw_breakpoint_pm_init(void)
+static inline void cpu_suspend_set_dbg_restorer(void (*hw_bp_restore)(void *))
 {
 }
 #endif
···
 
 	/* Register hotplug notifier. */
 	register_cpu_notifier(&hw_breakpoint_reset_nb);
-	hw_breakpoint_pm_init();
+	/* Register cpu_suspend hw breakpoint restore hook */
+	cpu_suspend_set_dbg_restorer(hw_breakpoint_reset);
 
 	return 0;
 }

arch/arm64/kernel/suspend.c

···
 	return cpu_ops[cpu]->cpu_suspend(arg);
 }
 
+/*
+ * This hook is provided so that cpu_suspend code can restore HW
+ * breakpoints as early as possible in the resume path, before reenabling
+ * debug exceptions. Code cannot be run from a CPU PM notifier since by the
+ * time the notifier runs debug exceptions might have been enabled already,
+ * with HW breakpoints registers content still in an unknown state.
+ */
+void (*hw_breakpoint_restore)(void *);
+void __init cpu_suspend_set_dbg_restorer(void (*hw_bp_restore)(void *))
+{
+	/* Prevent multiple restore hook initializations */
+	if (WARN_ON(hw_breakpoint_restore))
+		return;
+	hw_breakpoint_restore = hw_bp_restore;
+}
+
 /**
  * cpu_suspend
  *
···
 	if (ret == 0) {
 		cpu_switch_mm(mm->pgd, mm);
 		flush_tlb_all();
+		/*
+		 * Restore HW breakpoint registers to sane values
+		 * before debug exceptions are possibly reenabled
+		 * through local_dbg_restore.
+		 */
+		if (hw_breakpoint_restore)
+			hw_breakpoint_restore(NULL);
 	}
 
 	/*