[RFC] ACPI: disable extra P_LVLx access on KVM
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Message ID 1522431741-4678-1-git-send-email-mst@redhat.com
State New
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Commit Message

Michael S. Tsirkin March 30, 2018, 5:43 p.m. UTC
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@redhat.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Cc: kvm@vger.kernel.org
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---
 drivers/acpi/processor_idle.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

Comments

Rafael J. Wysocki April 23, 2018, 8:07 a.m. UTC | #1
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@redhat.com>
> Cc: Radim Krčmář <rkrcmar@redhat.com>
> Cc: kvm@vger.kernel.org
> Signed-off-by: Michael S. Tsirkin <mst@redhat.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.

Thanks,
Rafael
Paolo Bonzini April 23, 2018, 8:36 a.m. UTC | #2
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@redhat.com>
>> Cc: Radim Krčmář <rkrcmar@redhat.com>
>> Cc: kvm@vger.kernel.org
>> Signed-off-by: Michael S. Tsirkin <mst@redhat.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

Patch
diff mbox

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;
 	}