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; } -- MST
