5.15-stable review patch. If anyone has any objections, please let me know.
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From: David Woodhouse dwmw@amazon.co.uk
commit 70e6b7d9ae3c63df90a7bba7700e8d5c300c3c60 upstream.
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 Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- arch/x86/kernel/i8253.c | 11 +++++++++-- drivers/clocksource/i8253.c | 13 +++++++++---- include/linux/i8253.h | 1 + 3 files changed, 19 insertions(+), 6 deletions(-)
--- a/arch/x86/kernel/i8253.c +++ b/arch/x86/kernel/i8253.c @@ -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; --- a/drivers/clocksource/i8253.c +++ b/drivers/clocksource/i8253.c @@ -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_eve }
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; }
--- a/include/linux/i8253.h +++ b/include/linux/i8253.h @@ -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);