The code was blindly assuming that kvm_cpu_get_interrupt never returns -1
when there is a pending interrupt.

While this should be true, a bug in KVM can still cause this.

If -1 is returned, the code before this patch was converting it to 0xFF,
and 0xFF interrupt was injected to the guest, which results in an issue
which was hard to debug.

Add WARN_ON_ONCE to catch this case and	skip the injection
if this happens again.

Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
---
 arch/x86/kvm/x86.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index a6b9bea62fb8ac..00b87fcf6da4af 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -10203,9 +10203,13 @@ static int kvm_check_and_inject_events(struct kvm_vcpu *vcpu,
 		if (r < 0)
 			goto out;
 		if (r) {
-			kvm_queue_interrupt(vcpu, kvm_cpu_get_interrupt(vcpu), false);
-			static_call(kvm_x86_inject_irq)(vcpu, false);
-			WARN_ON(static_call(kvm_x86_interrupt_allowed)(vcpu, true) < 0);
+			int irq = kvm_cpu_get_interrupt(vcpu);
+
+			if (!WARN_ON_ONCE(irq == -1)) {
+				kvm_queue_interrupt(vcpu, irq, false);
+				static_call(kvm_x86_inject_irq)(vcpu, false);
+				WARN_ON(static_call(kvm_x86_interrupt_allowed)(vcpu, true) < 0);
+			}
 		}
 		if (kvm_cpu_has_injectable_intr(vcpu))
 			static_call(kvm_x86_enable_irq_window)(vcpu);
-- 
2.26.3