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

[Yin, Fengwei]

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@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 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; }