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Subject[tip: x86/urgent] x86/timer: Force PIT initialization when !X86_FEATURE_ARAT
The following commit has been merged into the x86/urgent branch of tip:

Commit-ID: afa8b475c1aec185a8e106c48b3832e0b88bc2de
Gitweb: https://git.kernel.org/tip/afa8b475c1aec185a8e106c48b3832e0b88bc2de
Author: Jan Stancek <jstancek@redhat.com>
AuthorDate: Sun, 08 Sep 2019 00:50:40 +02:00
Committer: Thomas Gleixner <tglx@linutronix.de>
CommitterDate: Sun, 08 Sep 2019 09:01:15 +02:00

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 <jstancek@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

---
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 dba2828..f91b3ff 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;
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