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SubjectRe: [RESEND] ACPI / processor_idle: use dead loop instead of io port access for wait
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On 10/11/2019 5:05 PM, Rafael J. Wysocki wrote:
> Sorry for the delay.
No problem.

>
> On Monday, September 9, 2019 9:39:37 AM CEST Yin Fengwei wrote:
>> In function acpi_idle_do_entry(), we do an io port access to guarantee
>> hardware behavior. But it could trigger unnecessary vmexit for
>> virtualization environemnt.
>
> Is this a theoretical problem, or do you actually see it?
>
> If you see it, I'd like to have a pointer to a bug report regarding it
> or similar.
We did see this issue when we run linux as guest with ACRN hypervisor
instead of kvm or xen. In our case, we export all native C states to
guest and let guest choose which C state it will enter.

And we observed many pm timer port access when guest tried to enter
deeper C state (Yes, we emulate pm timer so pm timer access will trigger
vmexit).


>
>> 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
>> vmexit.
>>
>> Signed-off-by: Yin Fengwei <fengwei.yin@intel.com>
>> ---
>> 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 ed56c6d20b08..676553228e8f 100644
>> --- a/drivers/acpi/processor_idle.c
>> +++ b/drivers/acpi/processor_idle.c
>> @@ -55,6 +55,8 @@ 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);
>>
>> @@ -64,6 +66,18 @@ 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);
>> +}
>
> Why not to use ndelay() instead of this? ->
Yes. ndelay should work also.

>
>> +
>> /*
>> * IBM ThinkPad R40e crashes mysteriously when going into C2 or C3.
>> * For now disable this. Probably a bug somewhere else.
>> @@ -660,8 +674,12 @@ 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();
>> }
>> }
>>
>> @@ -683,7 +701,7 @@ 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;
>> }
>> @@ -902,6 +920,7 @@ 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;
>> @@ -912,6 +931,13 @@ 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);
>
> -> And simply measure the number of nsecs this takes?
Yes. nsecs is fine here. and use udelay in dummy_wait.

Regards
Yin, Fengwei

>
>> +
>> + 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);
>> @@ -920,6 +946,7 @@ 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; }
>>
>
>
>
>

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