unimore/kernel-hacking-2024-linux-steffo
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x86/i8253: Disable PIT timer 0 when not in use

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Leaving the PIT interrupt running can cause noticeable steal time for
virtual guests. The VMM generally has a timer which toggles the IRQ input
to the PIC and I/O APIC, which takes CPU time away from the guest. Even
on real hardware, running the counter may use power needlessly (albeit
not much).

Make sure it's turned off if it isn't going to be used.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Michael Kelley <mhkelley@outlook.com>
Link: https://lore.kernel.org/all/20240802135555.564941-1-dwmw2@infradead.org
This commit is contained in:
David Woodhouse 2024-08-02 14:55:54 +01:00 committed by Thomas Gleixner
parent b4bac27931
commit 70e6b7d9ae
3 changed files with 19 additions and 6 deletions
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11
arch/x86/kernel/i8253.c
View file

@ -8,6 +8,7 @@
#include <linux/timex.h>
#include <linux/i8253.h>
#include <asm/hypervisor.h>
#include <asm/apic.h>
#include <asm/hpet.h>
#include <asm/time.h>
@ -39,9 +40,15 @@ static bool __init use_pit(void)
bool __init pit_timer_init(void)
{
if (!use_pit())
if (!use_pit()) {
/*
* Don't just ignore the PIT. Ensure it's stopped, because
* VMMs otherwise steal CPU time just to pointlessly waggle
* the (masked) IRQ.
*/
clockevent_i8253_disable();
return false;
}
clockevent_i8253_init(true);
global_clock_event = &i8253_clockevent;
return true;

13
drivers/clocksource/i8253.c
View file

@ -108,11 +108,8 @@ int __init clocksource_i8253_init(void)
#endif
#ifdef CONFIG_CLKEVT_I8253
static int pit_shutdown(struct clock_event_device *evt)
void clockevent_i8253_disable(void)
{
if (!clockevent_state_oneshot(evt) && !clockevent_state_periodic(evt))
return 0;
raw_spin_lock(&i8253_lock);
outb_p(0x30, PIT_MODE);
@ -123,6 +120,14 @@ static int pit_shutdown(struct clock_event_device *evt)
}
raw_spin_unlock(&i8253_lock);
}
static int pit_shutdown(struct clock_event_device *evt)
{
if (!clockevent_state_oneshot(evt) && !clockevent_state_periodic(evt))
return 0;
clockevent_i8253_disable();
return 0;
}

1
include/linux/i8253.h
View file

@ -24,6 +24,7 @@ extern raw_spinlock_t i8253_lock;
extern bool i8253_clear_counter_on_shutdown;
extern struct clock_event_device i8253_clockevent;
extern void clockevent_i8253_init(bool oneshot);
extern void clockevent_i8253_disable(void);
extern void setup_pit_timer(void);