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x86/timer: Force PIT initialization when !X86_FEATURE_ARAT

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KVM guests with commit c8c4076723 ("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: c8c4076723 ("x86/timer: Skip PIT initialization on modern chipsets")
Signed-off-by: Jan Stancek <jstancek@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
This commit is contained in:
Jan Stancek 2019-09-08 00:50:40 +02:00 committed by Thomas Gleixner
parent e16c2983fb
commit afa8b475c1
1 changed files with 4 additions and 0 deletions
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4
arch/x86/kernel/apic/apic.c
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@ -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;