Commit afa8b475 authored Sep 08, 2019 by Jan Stancek Committed by Thomas Gleixner Sep 08, 2019

x86/timer: Force PIT initialization when !X86_FEATURE_ARAT

KVM guests with commit c8c40767 ("x86/timer: Skip PIT initialization on
modern chipsets") applied to guest kernel have been observed to have
unusually higher CPU usage with symptoms of increase in vm exits for HLT
and MSW_WRITE (MSR_IA32_TSCDEADLINE).

This is caused by older QEMUs lacking support for X86_FEATURE_ARAT. lapic
clock retains CLOCK_EVT_FEAT_C3STOP and nohz stays inactive. There's no
usable broadcast device either.

Do the PIT initialization if guest CPU lacks X86_FEATURE_ARAT. On real
hardware it shouldn't matter as ARAT and DEADLINE come together.

Fixes: c8c40767 ("x86/timer: Skip PIT initialization on modern chipsets")
Signed-off-by: Jan Stancek <jstancek@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

parent e16c2983

arch/x86/kernel/apic/apic.c

+4 −0

Original line number	Diff line number	Diff line
		@@ -834,6 +834,10 @@ bool __init apic_needs_pit(void)
		if (!boot_cpu_has(X86_FEATURE_APIC))
		return true;

		/* Virt guests may lack ARAT, but still have DEADLINE */
		if (!boot_cpu_has(X86_FEATURE_ARAT))
		return true;

		/* Deadline timer is based on TSC so no further PIT action required */
		if (boot_cpu_has(X86_FEATURE_TSC_DEADLINE_TIMER))
		return false;

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