linux-kernel - [PATCH 3.2 020/115] KVM: i8254: change PIT discard tick policy

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Message-ID: <lsq.1461711744.370830636@decadent.org.uk>
Date:	Wed, 27 Apr 2016 01:02:24 +0200
From:	Ben Hutchings <ben@...adent.org.uk>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:	akpm@...ux-foundation.org, "Paolo Bonzini" <pbonzini@...hat.com>,
	"Yuki Shibuya" <shibuya.yk@...s.nec.co.jp>,
	"Radim Krčmář" <rkrcmar@...hat.com>
Subject: [PATCH 3.2 020/115] KVM: i8254: change PIT discard tick policy

3.2.80-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Radim Krčmář <rkrcmar@...hat.com>

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@...s.nec.co.jp>
Reviewed-by: Paolo Bonzini <pbonzini@...hat.com>
Signed-off-by: Radim Krčmář <rkrcmar@...hat.com>
Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
[bwh: Backported to 3.2:
 - s/ps->reinject/ps->pit_timer.reinject/
 - Adjust context]
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 arch/x86/kvm/i8254.c | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

--- a/arch/x86/kvm/i8254.c
+++ b/arch/x86/kvm/i8254.c
@@ -246,7 +246,7 @@ static void kvm_pit_ack_irq(struct kvm_i
 		 * PIC is being reset.  Handle it gracefully here
 		 */
 		atomic_inc(&ps->pit_timer.pending);
-	else if (value > 0)
+	else if (value > 0 && ps->pit_timer.reinject)
 		/* in this case, we had multiple outstanding pit interrupts
 		 * that we needed to inject.  Reinject
 		 */
@@ -300,7 +300,9 @@ static void pit_do_work(struct work_stru
 	 * last one has been acked.
 	 */
 	spin_lock(&ps->inject_lock);
-	if (ps->irq_ack) {
+	if (!ps->pit_timer.reinject)
+		inject = 1;
+	else if (ps->irq_ack) {
 		ps->irq_ack = 0;
 		inject = 1;
 	}
@@ -329,10 +331,10 @@ static enum hrtimer_restart pit_timer_fn
 	struct kvm_timer *ktimer = container_of(data, struct kvm_timer, timer);
 	struct kvm_pit *pt = ktimer->kvm->arch.vpit;
 
-	if (ktimer->reinject || !atomic_read(&ktimer->pending)) {
+	if (ktimer->reinject)
 		atomic_inc(&ktimer->pending);
-		queue_work(pt->wq, &pt->expired);
-	}
+
+	queue_work(pt->wq, &pt->expired);
 
 	if (ktimer->t_ops->is_periodic(ktimer)) {
 		hrtimer_add_expires_ns(&ktimer->timer, ktimer->period);