[4.2.y-ckt stable] Patch "KVM: i8254: change PIT discard tick policy" has been added to the 4.2.y-ckt tree

[Kamal Mostafa]

This is a note to let you know that I have just added a patch titled

    KVM: i8254: change PIT discard tick policy

to the linux-4.2.y-queue branch of the 4.2.y-ckt extended stable tree 
which can be found at:

    http://kernel.ubuntu.com/git/ubuntu/linux.git/log/?h=linux-4.2.y-queue

This patch is scheduled to be released in version 4.2.8-ckt7.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 4.2.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Kamal

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>From 1fac294e83233896f6b10bec2485515610612538 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Radim=20Kr=C4=8Dm=C3=A1=C5=99?= <rkrcmar at redhat.com>
Date: Wed, 2 Mar 2016 22:56:38 +0100
Subject: KVM: i8254: change PIT discard tick policy
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

commit 7dd0fdff145c5be7146d0ac06732ae3613412ac1 upstream.

Discard policy uses ack_notifiers to prevent injection of PIT interrupts
before EOI from the last one.

This patch changes the policy to always try to deliver the interrupt,
which makes a difference when its vector is in ISR.
Old implementation would drop the interrupt, but proposed one injects to
IRR, like real hardware would.

The old policy breaks legacy NMI watchdogs, where PIT is used through
virtual wire (LVT0): PIT never sends an interrupt before receiving EOI,
thus a guest deadlock with disabled interrupts will stop NMIs.

Note that NMI doesn't do EOI, so PIT also had to send a normal interrupt
through IOAPIC.  (KVM's PIT is deeply rotten and luckily not used much
in modern systems.)

Even though there is a chance of regressions, I think we can fix the
LVT0 NMI bug without introducing a new tick policy.

Reported-by: Yuki Shibuya <shibuya.yk at ncos.nec.co.jp>
Reviewed-by: Paolo Bonzini <pbonzini at redhat.com>
Signed-off-by: Radim Krčmář <rkrcmar at redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini at redhat.com>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 arch/x86/kvm/i8254.c | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kvm/i8254.c b/arch/x86/kvm/i8254.c
index a5b5997..3ec922b 100644
--- a/arch/x86/kvm/i8254.c
+++ b/arch/x86/kvm/i8254.c
@@ -244,7 +244,7 @@ static void kvm_pit_ack_irq(struct kvm_irq_ack_notifier *kian)
 		 * PIC is being reset.  Handle it gracefully here
 		 */
 		atomic_inc(&ps->pending);
-	else if (value > 0)
+	else if (value > 0 && ps->reinject)
 		/* in this case, we had multiple outstanding pit interrupts
 		 * that we needed to inject.  Reinject
 		 */
@@ -287,7 +287,9 @@ static void pit_do_work(struct kthread_work *work)
 	 * last one has been acked.
 	 */
 	spin_lock(&ps->inject_lock);
-	if (ps->irq_ack) {
+	if (!ps->reinject)
+		inject = 1;
+	else if (ps->irq_ack) {
 		ps->irq_ack = 0;
 		inject = 1;
 	}
@@ -316,10 +318,10 @@ static enum hrtimer_restart pit_timer_fn(struct hrtimer *data)
 	struct kvm_kpit_state *ps = container_of(data, struct kvm_kpit_state, timer);
 	struct kvm_pit *pt = ps->kvm->arch.vpit;

-	if (ps->reinject || !atomic_read(&ps->pending)) {
+	if (ps->reinject)
 		atomic_inc(&ps->pending);
-		queue_kthread_work(&pt->worker, &pt->expired);
-	}
+
+	queue_kthread_work(&pt->worker, &pt->expired);

 	if (ps->is_periodic) {
 		hrtimer_add_expires_ns(&ps->timer, ps->period);
--
2.7.4