Re: [PATCH v3 2/4] KVM: X86: Fix loss of exception which has not yet injected

[Paolo Bonzini <pbonzini@redhat.com>]

On 24/08/2017 08:52, Wanpeng Li wrote:
>> @@ -6862,6 +6876,7 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
>>                                 kvm_x86_ops->enable_nmi_window(vcpu);
>>                         if (kvm_cpu_has_injectable_intr(vcpu) || req_int_win)
>>                                 kvm_x86_ops->enable_irq_window(vcpu);
>> +                       WARN_ON(vcpu->arch.exception.pending);
>
> This WARN_ON() is suggested during the review of last version,
> however, there are many cases in inject_pending_event() can result in
> return directly w/ vcpu->arch.exception.pending is true. Actually I
> have already catched the warning several times during the testing. I
> think we should remove it when committing.

No, it's a good thing that it's failing, because it's finding a bug.  
There's no such thing as an "exception window", so at the very least it 
should set req_immediate_exit to true.

Does this help?

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index b698b2f135a2..76d5a192be6c 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -6365,14 +6365,20 @@ static int inject_pending_event(struct kvm_vcpu *vcpu, bool req_int_win)
 		return 0;
 	}
 
-	if (vcpu->arch.nmi_injected) {
-		kvm_x86_ops->set_nmi(vcpu);
-		return 0;
-	}
+	/*
+	 * Exceptions must be injected immediately, or the exception
+	 * frame will have the address of the NMI or interrupt handler.
+	 */
+	if (!vcpu->arch.exception.pending) {
+		if (vcpu->arch.nmi_injected) {
+			kvm_x86_ops->set_nmi(vcpu);
+			return 0;
+		}
 
-	if (vcpu->arch.interrupt.pending) {
-		kvm_x86_ops->set_irq(vcpu);
-		return 0;
+		if (vcpu->arch.interrupt.pending) {
+			kvm_x86_ops->set_irq(vcpu);
+			return 0;
+		}
 	}
 
 	if (is_guest_mode(vcpu) && kvm_x86_ops->check_nested_events) {

Paolo