Patch "KVM: i8254: change PIT discard tick policy" has been added to the 3.14-stable tree

This is a note to let you know that I've just added the patch titled

KVM: i8254: change PIT discard tick policy

to the 3.14-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
kvm-i8254-change-pit-discard-tick-policy.patch
and it can be found in the queue-3.14 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: =?UTF-8?q?Radim=20Kr=C4=8Dm=C3=A1=C5=99?= <[email protected]>
Date: Wed, 2 Mar 2016 22:56:38 +0100
Subject: KVM: i8254: change PIT discard tick policy
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From: Radim Krčmář <[email protected]>

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.

---
 arch/x86/kvm/i8254.c |   12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

@@ -244,7 +244,7 @@ static void kvm_pit_ack_irq(struct kvm_i
 		 * 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_w
 	 * 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 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);

Patches currently in stable-queue which might be from [email protected] are

queue-3.14/kvm-i8254-change-pit-discard-tick-policy.patch