linux-kernel - Re: [PATCH RFC] ACPI: disable extra P

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Date:   Mon, 23 Apr 2018 10:36:09 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        "Michael S. Tsirkin" <mst@...hat.com>
Cc:     linux-kernel@...r.kernel.org,
        Radim Krčmář <rkrcmar@...hat.com>,
        kvm@...r.kernel.org, Len Brown <lenb@...nel.org>,
        linux-acpi@...r.kernel.org
Subject: Re: [PATCH RFC] ACPI: disable extra P_LVLx access on KVM

On 23/04/2018 10:07, Rafael J. Wysocki wrote:
> On Friday, March 30, 2018 7:43:58 PM CEST Michael S. Tsirkin wrote:
>> As documented by commit b488f021 "ACPI: restore comment justifying
>> 'extra' P_LVLx access", Linux does an extra IO read after entering idle
>> because on (some) chipsets STPCLK# doesn't get asserted in time
>> to prevent further instruction processing.
>>
>> This can never be the case on KVM, and a timer read causes an expensive
>> VM exit in turn causing useless load on host system. Detect KVM and skip
>> the read.  TODO: whitelist more hypervisors?
>>
>> Note: very lightly tested.  Pls don't apply this yet, I am working on a
>> _CST implementation for KVM and will repost this without the RFC tag
>> when it's been tested properly.
>>
>> Posting now for early flames/feedback.
>>
>> Cc: Paolo Bonzini <pbonzini@...hat.com>
>> Cc: Radim Krčmář <rkrcmar@...hat.com>
>> Cc: kvm@...r.kernel.org
>> Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
>> ---
>>  drivers/acpi/processor_idle.c | 7 +++++--
>>  1 file changed, 5 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/acpi/processor_idle.c b/drivers/acpi/processor_idle.c
>> index abb559c..8ae28dc 100644
>> --- a/drivers/acpi/processor_idle.c
>> +++ b/drivers/acpi/processor_idle.c
>> @@ -33,6 +33,7 @@
>>  #include <linux/cpuidle.h>
>>  #include <linux/cpu.h>
>>  #include <acpi/processor.h>
>> +#include <linux/kvm_para.h>
>>  
>>  /*
>>   * Include the apic definitions for x86 to have the APIC timer related defines
>> @@ -665,7 +666,8 @@ static void __cpuidle acpi_idle_do_entry(struct acpi_processor_cx *cx)
>>  		/* 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);
>> +		if (!kvm_para_available())
>> +			inl(acpi_gbl_FADT.xpm_timer_block.address);
>>  	}
>>  }
>>  
>> @@ -687,7 +689,8 @@ static int acpi_idle_play_dead(struct cpuidle_device *dev, int index)
>>  		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);
>> +			if (!kvm_para_available())
>> +				inl(acpi_gbl_FADT.xpm_timer_block.address);
>>  		} else
>>  			return -ENODEV;
>>  	}
>>
> 
> While I have no objections to this change from the ACPI side, I'd like someone
> from the KVM land to comment on this.

It would be nicer to blacklist the problematic chipsets, or as Michael
said have a way to whitelist more hypervisors, but this is certainly
okay from KVM land too.

Thanks,

paolo