KVM: detect if VCPU triple faults

In the current inject_page_fault path KVM only checks if there is another PF
pending and injects a DF then. But it has to check for a pending DF too to
detect a shutdown condition in the VCPU.  If this is not detected the VCPU goes
to a PF -> DF -> PF loop when it should triple fault. This patch detects this
condition and handles it with an KVM_SHUTDOWN exit to userspace.
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>
Signed-off-by: Avi Kivity <avi@qumranet.com>

arch/x86/kvm/x86.c

@@ -155,11 +155,16 @@ void kvm_inject_page_fault(struct kvm_vcpu *vcpu, unsigned long addr,
 			   u32 error_code)
 {
 	++vcpu->stat.pf_guest;
-	if (vcpu->arch.exception.pending && vcpu->arch.exception.nr == PF_VECTOR) {
+	if (vcpu->arch.exception.pending) {
+		if (vcpu->arch.exception.nr == PF_VECTOR) {
 			printk(KERN_DEBUG "kvm: inject_page_fault:"
 					" double fault 0x%lx\n", addr);
 			vcpu->arch.exception.nr = DF_VECTOR;
 			vcpu->arch.exception.error_code = 0;
+		} else if (vcpu->arch.exception.nr == DF_VECTOR) {
+			/* triple fault -> shutdown */
+			set_bit(KVM_REQ_TRIPLE_FAULT, &vcpu->requests);
+		}
 		return;
 	}
 	vcpu->arch.cr2 = addr;
@@ -2676,6 +2681,11 @@ again:
 			r = 0;
 			goto out;
 		}
+		if (test_and_clear_bit(KVM_REQ_TRIPLE_FAULT, &vcpu->requests)) {
+			kvm_run->exit_reason = KVM_EXIT_SHUTDOWN;
+			r = 0;
+			goto out;
+		}
 	}
 
 	kvm_inject_pending_timer_irqs(vcpu);

include/linux/kvm_host.h

@@ -38,6 +38,7 @@
 #define KVM_REQ_MIGRATE_TIMER      1
 #define KVM_REQ_REPORT_TPR_ACCESS  2
 #define KVM_REQ_MMU_RELOAD         3
+#define KVM_REQ_TRIPLE_FAULT       4
 
 struct kvm_vcpu;
 extern struct kmem_cache *kvm_vcpu_cache;