linux-kernel - [tip: x86/urgent] x86/timer: Force PIT initialization when !X86_FEATURE

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Date:   Sun, 08 Sep 2019 07:07:09 -0000
From:   "tip-bot2 for Jan Stancek" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     Jan Stancek <jstancek@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>,
        linux-kernel@...r.kernel.org
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@...hat.com>
AuthorDate:    Sun, 08 Sep 2019 00:50:40 +02:00
Committer:     Thomas Gleixner <tglx@...utronix.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@...hat.com>
Signed-off-by: Thomas Gleixner <tglx@...utronix.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;