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

["Limonciello, Mario" <mario.limonciello@amd.com>]

On 9/22/2022 13:53, Rafael J. Wysocki wrote:
> 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://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Flore.kernel.org%2Fall%2F20220921063638.2489-1-kprateek.nayak%40amd.com%2F&amp;data=05%7C01%7CMario.Limonciello%40amd.com%7C8460d9ef3add45bf571408da9ccbc58a%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637994696248641733%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000%7C%7C%7C&amp;sdata=23k2wKPZaBrgOTtcHw8ByNzfsus1RSsdXMlCACjl%2Bmc%3D&amp;reserved=0 

If agreeable, I think this should be @stable too.

Either way:

Reviewed-by: Mario Limonciello <mario.limonciello@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);
>>   }
> 
>