linux-kernel - Re: [PATCH] x86/ACPI/State: Optimize C3 entry on AMD CPUs

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Date:   Tue, 31 Aug 2021 19:10:55 -0700
From:   Deepak Sharma <deesharm@....com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Deepak Sharma <deepak.sharma@....com>
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Len Brown <len.brown@...el.com>, Pavel Machek <pavel@....cz>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        "open list:SUSPEND TO RAM" <linux-pm@...r.kernel.org>,
        "open list:X86 ARCHITECTURE (32-BIT AND 64-BIT)" 
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86/ACPI/State: Optimize C3 entry on AMD CPUs


On 8/26/21 4:04 PM, Thomas Gleixner wrote:
> On Wed, Aug 18 2021 at 17:43, Deepak Sharma wrote:
>
>> AMD CPU which support C3 shares cache. Its not necessary to flush the
>> caches in software before entering C3. This will cause performance drop
>> for the cores which share some caches. ARB_DIS is not used with current
>> AMD C state implementation. So set related flags correctly.
>>
>> Signed-off-by: Deepak Sharma <deepak.sharma@....com>
>> ---
>>   arch/x86/kernel/acpi/cstate.c | 15 +++++++++++++++
>>   1 file changed, 15 insertions(+)
>>
>> diff --git a/arch/x86/kernel/acpi/cstate.c b/arch/x86/kernel/acpi/cstate.c
>> index 7de599eba7f0..62a5986d625a 100644
>> --- a/arch/x86/kernel/acpi/cstate.c
>> +++ b/arch/x86/kernel/acpi/cstate.c
>> @@ -79,6 +79,21 @@ void acpi_processor_power_init_bm_check(struct acpi_processor_flags *flags,
>>   		 */
>>   		flags->bm_control = 0;
>>   	}
>> +	if (c->x86_vendor == X86_VENDOR_AMD) {
>> +		/*
>> +		 * For all AMD CPUs that support C3, caches should not be
>> +		 * flushed by software while entering C3 type state. Set
>> +		 * bm->check to 1 so that kernel doesn't need to execute
>> +		 * cache flush operation.
>> +		 */
>> +		flags->bm_check = 1;
>> +		/*
>> +		 * In current AMD C state implementation ARB_DIS is no longer
> Fine for current implementations, but what about older implementations?
We are internally discussing about its validity on much older 
implementations. Will send subsequent patch based on the conclusion.


Thanks,

Deepak