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

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Message-ID: <30ee0a348f624698801691f65eeecd87@AcuMS.aculab.com>
Date:   Wed, 23 Oct 2019 08:45:36 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     '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

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().

...
> +#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???

	David

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