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

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Date:   Wed, 23 Oct 2019 11:03:19 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     David Laight <David.Laight@...lab.com>
Cc:     Yin Fengwei <fengwei.yin@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-acpi@...r.kernel.org" <linux-acpi@...r.kernel.org>,
        "rjw@...ysocki.net" <rjw@...ysocki.net>,
        "lenb@...nel.org" <lenb@...nel.org>
Subject: Re: [PATCH v3] ACPI/processor_idle: Remove dummy wait if kernel is in
 guest mode

On Wed, Oct 23, 2019 at 10:45 AM David Laight <David.Laight@...lab.com> 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@...el.com>
> > ---
> > 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().

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