linux-kernel - Re: [PATCH v4] ACPI/processor_idle: Remove dummy wait if kernel is in guest

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Date:   Fri, 25 Oct 2019 11:06:25 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Yin Fengwei <fengwei.yin@...el.com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Len Brown <lenb@...nel.org>
Subject: Re: [PATCH v4] ACPI/processor_idle: Remove dummy wait if kernel is in guest

On Thu, Oct 24, 2019 at 9:04 AM Yin Fengwei <fengwei.yin@...el.com> wrote:
>
> 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
> environment.
>
> If it's in virtualization environment, the deeper C state enter
> operation (inb()) will trap to hypervisor. It's not needed to do
> dummy wait after the inb() call. So we could just remove the
> dummy io port access to avoid unnecessary VMexit.
>
> And keep dummy io port access to maintain timing for native
> environment.
>
> Signed-off-by: Yin Fengwei <fengwei.yin@...el.com>
> ---
> ChangeLog:
> v3 -> v4:
>  - Drop overengineered function pointer and do check whether
>    we are in guest before dummy inl call.
>
> 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 | 21 +++++++++++++++------
>  1 file changed, 15 insertions(+), 6 deletions(-)
>
> diff --git a/drivers/acpi/processor_idle.c b/drivers/acpi/processor_idle.c
> index ed56c6d20b08..2ae95df2e74f 100644
> --- a/drivers/acpi/processor_idle.c
> +++ b/drivers/acpi/processor_idle.c
> @@ -642,6 +642,19 @@ static int acpi_idle_bm_check(void)
>         return bm_status;
>  }
>
> +static void wait_for_freeze(void)
> +{
> +#ifdef CONFIG_X86
> +       /* No delay is needed if we are in 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);
> +}
> +
>  /**
>   * acpi_idle_do_entry - enter idle state using the appropriate method
>   * @cx: cstate data
> @@ -658,10 +671,7 @@ static void __cpuidle acpi_idle_do_entry(struct acpi_processor_cx *cx)
>         } else {
>                 /* IO port based C-state */
>                 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);
> +               wait_for_freeze();
>         }
>  }
>
> @@ -682,8 +692,7 @@ static int acpi_idle_play_dead(struct cpuidle_device *dev, int index)
>                         safe_halt();
>                 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);
> +                       wait_for_freeze();
>                 } else
>                         return -ENODEV;
>         }
> --

Applying as 5.5 material, thanks!