ACPI: disable extra P_LVLx access on KVM

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(-)

drivers/acpi/processor_idle.c +3 -1

index abb559c..8ae28dc 100644
@@ -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);
 	}
 }
 

Quoted 45 lines · CEST Michael S. Tsirkin

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(-)

drivers/acpi/processor_idle.c +3 -1

index abb559c..8ae28dc 100644
@@ -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);
 	}
 }
 

drivers/acpi/processor_idle.c static int acpi_idle_play_dead(struct cpuidle_device *dev, int index) · line 696

Quoted 11 lines

drivers/acpi/processor_idle.c +2 -1

@@ -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.