Re: [PATCH v3] ACPI/processor_idle: Remove dummy wait if kernel is in guest mode

[Yin Fengwei]

On 2019/10/23 äå4:45, David Laight wrote:
> From: Yin Fengwei
> > Sent: 23 October 2019 08:50
>
>
> > In function acpi_idle_do_entry(), an ioport access is used for dummy
> > wait to guarantee hardware behavior. But it could trigger unnecessary
> > vmexit if kernel is running as guest in virtualization environtment.
> >
> > If it's in virtualization environment, the deeper C state enter
> > operation (inb()) will trap to hyervisor. It's not needed to do
> > dummy wait after the inb() call. So we remove the dummy io port
> > access to avoid unnecessary VMexit.
> >
> > We keep dummy io port access to maintain timing for native environment.
> >
> > Signed-off-by: Yin Fengwei <fengwei.yin@xxxxxxxxx>
> > ---
> > ChangeLog:
> > v2 -> v3:
> >   - Remove dummy io port access totally for virtualization env.
> >
> > v1 -> v2:
> >   - Use ndelay instead of dead loop for dummy delay.
> >
> >   drivers/acpi/processor_idle.c | 36 ++++++++++++++++++++++++++++++++---
> >   1 file changed, 33 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/acpi/processor_idle.c b/drivers/acpi/processor_idle.c
> > index ed56c6d20b08..0c4a97dd6917 100644
> > --- a/drivers/acpi/processor_idle.c
> > +++ b/drivers/acpi/processor_idle.c
> > @@ -58,6 +58,17 @@ struct cpuidle_driver acpi_idle_driver = {
> >   static
> >   DEFINE_PER_CPU(struct acpi_processor_cx * [CPUIDLE_STATE_MAX], acpi_cstate);
> >
> > +static void (*dummy_wait)(u64 address);
> > +
> > +static void default_dummy_wait(u64 address)
> > +{
> > +	inl(address);
> > +}
> > +
> > +static void default_noop_wait(u64 address)
> > +{
> > +}
> > +
>
> Overengineered...
> Just add:
>
> static void wait_for_freeze(void)
> {
> #ifdef	CONFIG_X86
> 	/* No delay is needed if we are a guest */
> 	if (boot_cpu_has(X86_FEATURE_HYPERVISOR))
> 		return;
> #endif
> 	/* 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);
> }
>
> and use it to replace the inl().
OK. I was trying to avoid any impact to native case.

> ...
> > +#ifdef	CONFIG_X86
> > +	/* For x86, if we are running in guest, we don't need extra
> > +	 * access ioport as dummy wait.
> > +	 */
> > +	if (boot_cpu_has(X86_FEATURE_HYPERVISOR)) {
> > +		pr_err("We are in virtual env");
> > +		dummy_wait = default_noop_wait;
> > +	} else {
> > +		pr_err("We are not in virtual env");
> > +	}
> > +#endif
>
> WTF are the pr_err() for???
Sorry. Didn't remove my debug code...

Regards
Yin, Fengwei

> 	David
>
> -
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