[PATCH] ACPI / processor_idle: use dead loop instead of io port access for wait

[Yin Fengwei]

In function acpi_idle_do_entry(), we do an io port access to guarantee
hardware behavior. But it could trigger unnecessary trap to HV for
virtualization environemnt.

>From the comments of this part of code, we could use busy wait instead
of io port access to guarantee hardware behavior and avoid unnecessary
trap to HV.

Signed-off-by: Yin Fengwei <fengwei.yin@xxxxxxxxx>
---
 drivers/acpi/processor_idle.c | 33 ++++++++++++++++++++++++++++++---
 1 file changed, 30 insertions(+), 3 deletions(-)

diff --git a/drivers/acpi/processor_idle.c b/drivers/acpi/processor_idle.c
index abb559cd28d7..dcc447e59885 100644
--- a/drivers/acpi/processor_idle.c
+++ b/drivers/acpi/processor_idle.c
@@ -68,6 +68,8 @@ struct cpuidle_driver acpi_idle_driver = {
 };
 
 #ifdef CONFIG_ACPI_PROCESSOR_CSTATE
+static struct timespec64 dummy_delta = {0L, 0L};
+
 static
 DEFINE_PER_CPU(struct acpi_processor_cx * [CPUIDLE_STATE_MAX], acpi_cstate);
 
@@ -77,6 +79,18 @@ static int disabled_by_idle_boot_param(void)
 		boot_option_idle_override == IDLE_HALT;
 }
 
+static void dummy_wait(void)
+{
+	struct timespec64 now, target;
+
+	ktime_get_real_ts64(&now);
+	target = timespec64_add(now, dummy_delta);
+
+	do {
+		ktime_get_real_ts64(&now);
+	} while (timespec64_compare(&now, &target) < 0);
+}
+
 /*
  * IBM ThinkPad R40e crashes mysteriously when going into C2 or C3.
  * For now disable this. Probably a bug somewhere else.
@@ -664,8 +678,12 @@ static void __cpuidle acpi_idle_do_entry(struct acpi_processor_cx *cx)
 		inb(cx->address);
 		/* Dummy wait op - must do something useless after P_LVL2 read
 		   because chipsets cannot guarantee that STPCLK# signal
-		   gets asserted in time to freeze execution properly. */
-		inl(acpi_gbl_FADT.xpm_timer_block.address);
+		   gets asserted in time to freeze execution properly.
+
+		   Previously, we do io port access here which could trigger
+		   unnecessary trap to HV for virtualization env. We use dead loop
+		   here to avoid the impact to virtualization env. */
+		dummy_wait();
 	}
 }
 
@@ -687,7 +705,7 @@ static int acpi_idle_play_dead(struct cpuidle_device *dev, int index)
 		else if (cx->entry_method == ACPI_CSTATE_SYSTEMIO) {
 			inb(cx->address);
 			/* See comment in acpi_idle_do_entry() */
-			inl(acpi_gbl_FADT.xpm_timer_block.address);
+			dummy_wait();
 		} else
 			return -ENODEV;
 	}
@@ -906,6 +924,7 @@ static inline void acpi_processor_cstate_first_run_checks(void)
 {
 	acpi_status status;
 	static int first_run;
+	struct timespec64 ts0, ts1;
 
 	if (first_run)
 		return;
@@ -916,6 +935,13 @@ static inline void acpi_processor_cstate_first_run_checks(void)
 			  max_cstate);
 	first_run++;
 
+	/* profiling the time used for dummy wait op */
+	ktime_get_real_ts64(&ts0);
+	inl(acpi_gbl_FADT.xpm_timer_block.address);
+	ktime_get_real_ts64(&ts1);
+
+	dummy_delta = timespec64_sub(ts1, ts0);
+
 	if (acpi_gbl_FADT.cst_control && !nocst) {
 		status = acpi_os_write_port(acpi_gbl_FADT.smi_command,
 					    acpi_gbl_FADT.cst_control, 8);
@@ -924,6 +950,7 @@ static inline void acpi_processor_cstate_first_run_checks(void)
 					"Notifying BIOS of _CST ability failed"));
 	}
 }
+
 #else
 
 static inline int disabled_by_idle_boot_param(void) { return 0; }
-- 
2.17.1