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@xxxxxxxxxx>
>> Cc: Radim Krčmář <rkrcmar@xxxxxxxxxx>
>> Cc: kvm@xxxxxxxxxxxxxxx
>> Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
>> ---
>> 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
