KVM: LAPIC: cap __delay at lapic_timer_advance_ns

The host timer which emulates the guest LAPIC TSC deadline
timer has its expiration diminished by lapic_timer_advance_ns
nanoseconds. Therefore if, at wait_lapic_expire, a difference
larger than lapic_timer_advance_ns is encountered, delay at most
lapic_timer_advance_ns.

This fixes a problem where the guest can cause the host
to delay for large amounts of time.
Reported-by: Alan Jenkins <alan.christopher.jenkins@gmail.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/lapic.c

@@ -1310,7 +1310,8 @@ void wait_lapic_expire(struct kvm_vcpu *vcpu)
 
 	/* __delay is delay_tsc whenever the hardware has TSC, thus always.  */
 	if (guest_tsc < tsc_deadline)
-		__delay(tsc_deadline - guest_tsc);
+		__delay(min(tsc_deadline - guest_tsc,
+			nsec_to_cycles(vcpu, lapic_timer_advance_ns)));
 }
 
 static void start_apic_timer(struct kvm_lapic *apic)