[PATCH 1/2] KVM: x86: ioapic: Update state only after successful delivery

From: Hao Zhang <[email protected]>

The I/O APIC tracks delivered interrupts in state that is later used to
decide whether an interrupt is still pending or blocked waiting for an
EOI.

For level-triggered interrupts, remote_irr means that a local APIC
accepted the interrupt and that the I/O APIC must wait for the
corresponding EOI before delivering the interrupt again.  For
edge-triggered interrupts, irr_delivered is used to hide delivered
interrupts from KVM_GET_IRQCHIP so that userspace does not reinject an
interrupt that has already left the I/O APIC.

But ioapic_service() currently updates that state before or without
checking that interrupt delivery actually succeeded.
kvm_irq_delivery_to_apic() can return -1 when no destination is found.
Treating failed delivery as success can either leave a level-triggered
pin blocked forever waiting for an EOI that will never be generated, or
cause KVM_GET_IRQCHIP to drop an undelivered edge-triggered interrupt
from the saved IRR state.

Update I/O APIC delivery state only when the delivery result is positive,
i.e. when at least one local APIC accepted the interrupt.

Fixes: 4925663a079c ("KVM: Report IRQ injection status to userspace.")
Fixes: 5bda6eed2e36 ("KVM: ioapic: Record edge-triggered interrupts delivery status")
Signed-off-by: Hao Zhang <[email protected]>
---
Changes in v2:
 - Address Kai Huang's review by deferring both remote_irr and
   irr_delivered updates until interrupt delivery succeeds.
 - Extend the selftest to cover failed edge-triggered delivery and verify
   that the interrupt remains pending in IRR.

Link to v1: https://lore.kernel.org/all/[email protected]/

 arch/x86/kvm/ioapic.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kvm/ioapic.c b/arch/x86/kvm/ioapic.c
index 757667fb2bfa..540e5665fbe4 100644
--- a/arch/x86/kvm/ioapic.c
+++ b/arch/x86/kvm/ioapic.c
@@ -474,9 +474,6 @@ static int ioapic_service(struct kvm_ioapic *ioapic, int irq, bool line_status)
 	irqe.shorthand = APIC_DEST_NOSHORT;
 	irqe.msi_redir_hint = false;
 
-	if (irqe.trig_mode == IOAPIC_EDGE_TRIG)
-		ioapic->irr_delivered |= 1 << irq;
-
 	if (irq == RTC_GSI && line_status) {
 		/*
 		 * pending_eoi cannot ever become negative (see
@@ -491,8 +488,12 @@ static int ioapic_service(struct kvm_ioapic *ioapic, int irq, bool line_status)
 	} else
 		ret = kvm_irq_delivery_to_apic(ioapic->kvm, NULL, &irqe);
 
-	if (ret && irqe.trig_mode == IOAPIC_LEVEL_TRIG)
-		entry->fields.remote_irr = 1;
+	if (ret > 0) {
+		if (irqe.trig_mode == IOAPIC_EDGE_TRIG)
+			ioapic->irr_delivered |= 1 << irq;
+		else if (irqe.trig_mode == IOAPIC_LEVEL_TRIG)
+			entry->fields.remote_irr = 1;
+	}
 
 	return ret;
 }

base-commit: c21bb4193868a8de71fc4693fa741e195fdf5d86
-- 
2.15.0