KVM: Fix race between APIC TMR and IRR

commit a5d36f82c4f3e852b61fdf1fee13463c8aa91b90 upstream.

When we queue an interrupt to the local apic, we set the IRR before the TMR.
The vcpu can pick up the IRR and inject the interrupt before setting the TMR,
and perhaps even EOI it, causing incorrect behaviour.

The race is really insignificant since it can only occur on the first
interrupt (usually following interrupts will not change TMR), but it's better
closed than open.

Fixed by reordering setting the TMR vs IRR.

Signed-off-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index 41659fb080ba7d213bab5c05271398ac25246af0..8dfeaaa7079569badd5165989347154d9180c0d2 100644 (file)
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -374,6 +374,12 @@ static int __apic_accept_irq(struct kvm_lapic *apic, int delivery_mode,
                if (unlikely(!apic_enabled(apic)))
                        break;
 
+               if (trig_mode) {
+                       apic_debug("level trig mode for vector %d", vector);
+                       apic_set_vector(vector, apic->regs + APIC_TMR);
+               } else
+                       apic_clear_vector(vector, apic->regs + APIC_TMR);
+
                result = !apic_test_and_set_irr(vector, apic);
                trace_kvm_apic_accept_irq(vcpu->vcpu_id, delivery_mode,
                                          trig_mode, vector, !result);
@@ -384,11 +390,6 @@ static int __apic_accept_irq(struct kvm_lapic *apic, int delivery_mode,
                        break;
                }
 
-               if (trig_mode) {
-                       apic_debug("level trig mode for vector %d", vector);
-                       apic_set_vector(vector, apic->regs + APIC_TMR);
-               } else
-                       apic_clear_vector(vector, apic->regs + APIC_TMR);
                kvm_vcpu_kick(vcpu);
                break;