KVM: x86: Do not update EFLAGS on faulting emulation

If the emulation ends in fault, eflags should not be updated.  However, several
instruction emulations (actually all the fastops) currently update eflags, if
the fault was detected afterwards (e.g., #PF during writeback).
Signed-off-by: Nadav Amit <namit@cs.technion.ac.il>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/x86.c

@@ -5360,7 +5360,9 @@ int x86_emulate_instruction(struct kvm_vcpu *vcpu,
 		kvm_rip_write(vcpu, ctxt->eip);
 		if (r == EMULATE_DONE)
 			kvm_vcpu_check_singlestep(vcpu, rflags, &r);
-		__kvm_set_rflags(vcpu, ctxt->eflags);
+		if (!ctxt->have_exception ||
+		    exception_type(ctxt->exception.vector) == EXCPT_TRAP)
+			__kvm_set_rflags(vcpu, ctxt->eflags);
 
 		/*
 		 * For STI, interrupts are shadowed; so KVM_REQ_EVENT will