Re: [PATCH] ACPI: processor idle: Practically limit "Dummy wait" workaround to old Intel systems

["Rafael J. Wysocki" <rafael.j.wysocki@intel.com>]

On 9/22/2022 8:47 PM, Dave Hansen wrote:
> Old, circa 2002 chipsets have a bug: they don't go idle when they are
> supposed to.  So, a workaround was added to slow the CPU down and
> ensure that the CPU waits a bit for the chipset to actually go idle.
> This workaround is ancient and has been in place in some form since
> the original kernel ACPI implementation.
>
> But, this workaround is very painful on modern systems.  The "inl()"
> can take thousands of cycles (see Link: for some more detailed
> numbers and some fun kernel archaeology).
>
> First and foremost, modern systems should not be using this code.
> Typical Intel systems have not used it in over a decade because it is
> horribly inferior to MWAIT-based idle.
>
> Despite this, people do seem to be tripping over this workaround on
> AMD system today.
>
> Limit the "dummy wait" workaround to Intel systems.  Keep Modern AMD
> systems from tripping over the workaround.  Remotely modern Intel
> systems use intel_idle instead of this code and will, in practice,
> remain unaffected by the dummy wait.
>
> Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
> Cc: Len Brown <lenb@kernel.org>
> Cc: Mario Limonciello <Mario.Limonciello@amd.com>
> Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
> Cc: Borislav Petkov <bp@alien8.de>
> Suggested-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> Reported-by: K Prateek Nayak <kprateek.nayak@amd.com>
> Link: https://lore.kernel.org/all/20220921063638.2489-1-kprateek.nayak@amd.com/

Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

or do you want me to pick this up?


> ---
>   drivers/acpi/processor_idle.c | 23 ++++++++++++++++++++---
>   1 file changed, 20 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/acpi/processor_idle.c b/drivers/acpi/processor_idle.c
> index 16a1663d02d4..9f40917c49ef 100644
> --- a/drivers/acpi/processor_idle.c
> +++ b/drivers/acpi/processor_idle.c
> @@ -531,10 +531,27 @@ static void wait_for_freeze(void)
>   	/* No delay is needed if we are in guest */
>   	if (boot_cpu_has(X86_FEATURE_HYPERVISOR))
>   		return;
> +	/*
> +	 * Modern (>=Nehalem) Intel systems use ACPI via intel_idle,
> +	 * not this code.  Assume that any Intel systems using this
> +	 * are ancient and may need the dummy wait.  This also assumes
> +	 * that the motivating chipset issue was Intel-only.
> +	 */
> +	if (boot_cpu_data.x86_vendor != X86_VENDOR_INTEL)
> +		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. */
> +	/*
> +	 * 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
> +	 *
> +	 * This workaround has been in place since the original ACPI
> +	 * implementation was merged, circa 2002.
> +	 *
> +	 * If a profile is pointing to this instruction, please first
> +	 * consider moving your system to a more modern idle
> +	 * mechanism.
> +	 */
>   	inl(acpi_gbl_FADT.xpm_timer_block.address);
>   }
>