From afa8b475c1aec185a8e106c48b3832e0b88bc2de Mon Sep 17 00:00:00 2001 From: Jan Stancek Date: Sun, 8 Sep 2019 00:50:40 +0200 Subject: [PATCH] x86/timer: Force PIT initialization when !X86_FEATURE_ARAT KVM guests with commit c8c4076723da ("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: c8c4076723da ("x86/timer: Skip PIT initialization on modern chipsets") Signed-off-by: Jan Stancek Signed-off-by: Thomas Gleixner --- arch/x86/kernel/apic/apic.c | 4 ++++ 1 file changed, 4 insertions(+) diff --git a/arch/x86/kernel/apic/apic.c b/arch/x86/kernel/apic/apic.c index dba2828b779a..f91b3ff9dc03 100644 --- a/arch/x86/kernel/apic/apic.c +++ b/arch/x86/kernel/apic/apic.c @@ -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; -- 2.20.1