Patch "KVM: x86: Inject pending interrupt even if pending nmi exist" has been added to the 4.4-stable tree

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

KVM: x86: Inject pending interrupt even if pending nmi exist

to the 4.4-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-x86-inject-pending-interrupt-even-if-pending-nmi-exist.patch
and it can be found in the queue-4.4 subdirectory.

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

From: Yuki Shibuya <[email protected]>
Date: Thu, 24 Mar 2016 05:17:03 +0000
Subject: KVM: x86: Inject pending interrupt even if pending nmi exist

From: Yuki Shibuya <[email protected]>

commit 321c5658c5e9192dea0d58ab67cf1791e45b2b26 upstream.

Non maskable interrupts (NMI) are preferred to interrupts in current implementation. If a NMI is pending and NMI is blocked by the result of nmi_allowed(), pending interrupt is not injected and enable_irq_window() is not executed, even if interrupts injection is allowed.

In old kernel (e.g. 2.6.32), schedule() is often called in NMI context. In this case, interrupts are needed to execute iret that intends end of NMI. The flag of blocking new NMI is not cleared until the guest execute the iret, and interrupts are blocked by pending NMI. Due to this, iret can't be invoked in the guest, and the guest is starved until block is cleared by some events (e.g. canceling injection).

This patch injects pending interrupts, when it's allowed, even if NMI is blocked. And, If an interrupts is pending after executing inject_pending_event(), enable_irq_window() is executed regardless of NMI pending counter.

---
 arch/x86/kvm/x86.c |   20 ++++++++++----------
 1 file changed, 10 insertions(+), 10 deletions(-)

@@ -6024,12 +6024,10 @@ static int inject_pending_event(struct k
 	}
 
 	/* try to inject new event if pending */
-	if (vcpu->arch.nmi_pending) {
-		if (kvm_x86_ops->nmi_allowed(vcpu)) {
-			--vcpu->arch.nmi_pending;
-			vcpu->arch.nmi_injected = true;
-			kvm_x86_ops->set_nmi(vcpu);
-		}
+	if (vcpu->arch.nmi_pending && kvm_x86_ops->nmi_allowed(vcpu)) {
+		--vcpu->arch.nmi_pending;
+		vcpu->arch.nmi_injected = true;
+		kvm_x86_ops->set_nmi(vcpu);
 	} else if (kvm_cpu_has_injectable_intr(vcpu)) {
 		/*
 		 * Because interrupts can be injected asynchronously, we are

@@ -6474,10 +6472,12 @@ static int vcpu_enter_guest(struct kvm_v
 		if (inject_pending_event(vcpu, req_int_win) != 0)
 			req_immediate_exit = true;
 		/* enable NMI/IRQ window open exits if needed */
-		else if (vcpu->arch.nmi_pending)
-			kvm_x86_ops->enable_nmi_window(vcpu);
-		else if (kvm_cpu_has_injectable_intr(vcpu) || req_int_win)
-			kvm_x86_ops->enable_irq_window(vcpu);
+		else {
+			if (vcpu->arch.nmi_pending)
+				kvm_x86_ops->enable_nmi_window(vcpu);
+			if (kvm_cpu_has_injectable_intr(vcpu) || req_int_win)
+				kvm_x86_ops->enable_irq_window(vcpu);
+		}
 
 		if (kvm_lapic_enabled(vcpu)) {
 			update_cr8_intercept(vcpu);

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

queue-4.4/kvm-x86-inject-pending-interrupt-even-if-pending-nmi-exist.patch