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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
75 lines
1.8 KiB
C
75 lines
1.8 KiB
C
// SPDX-License-Identifier: GPL-2.0
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/*
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* 8253/PIT functions
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*
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*/
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#include <linux/clockchips.h>
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#include <linux/init.h>
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#include <linux/timex.h>
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#include <linux/i8253.h>
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#include <asm/hypervisor.h>
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#include <asm/apic.h>
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#include <asm/hpet.h>
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#include <asm/time.h>
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#include <asm/smp.h>
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/*
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* HPET replaces the PIT, when enabled. So we need to know, which of
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* the two timers is used
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*/
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struct clock_event_device *global_clock_event;
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/*
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* Modern chipsets can disable the PIT clock which makes it unusable. It
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* would be possible to enable the clock but the registers are chipset
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* specific and not discoverable. Avoid the whack a mole game.
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*
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* These platforms have discoverable TSC/CPU frequencies but this also
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* requires to know the local APIC timer frequency as it normally is
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* calibrated against the PIT interrupt.
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*/
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static bool __init use_pit(void)
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{
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if (!IS_ENABLED(CONFIG_X86_TSC) || !boot_cpu_has(X86_FEATURE_TSC))
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return true;
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/* This also returns true when APIC is disabled */
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return apic_needs_pit();
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}
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bool __init pit_timer_init(void)
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{
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if (!use_pit()) {
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/*
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* Don't just ignore the PIT. Ensure it's stopped, because
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* VMMs otherwise steal CPU time just to pointlessly waggle
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* the (masked) IRQ.
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*/
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clockevent_i8253_disable();
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return false;
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}
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clockevent_i8253_init(true);
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global_clock_event = &i8253_clockevent;
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return true;
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}
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#ifndef CONFIG_X86_64
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static int __init init_pit_clocksource(void)
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{
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/*
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* Several reasons not to register PIT as a clocksource:
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*
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* - On SMP PIT does not scale due to i8253_lock
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* - when HPET is enabled
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* - when local APIC timer is active (PIT is switched off)
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*/
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if (num_possible_cpus() > 1 || is_hpet_enabled() ||
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!clockevent_state_periodic(&i8253_clockevent))
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return 0;
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return clocksource_i8253_init();
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}
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arch_initcall(init_pit_clocksource);
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#endif /* !CONFIG_X86_64 */
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