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 <[email protected]> >> Cc: Radim Krčmář <[email protected]> >> Cc: [email protected] >> Signed-off-by: Michael S. Tsirkin <[email protected]> >> --- >> 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
