KVM: x86: Avoid issuing wbinvd twice

Micro optimization to avoid calling wbinvd twice on the CPU that has to
emulate it. As we might be preempted between smp_call_function_many and
the local wbinvd, the cache might be filled again so that real work
could be done uselessly.
Signed-off-by: NJan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: NMarcelo Tosatti <mtosatti@redhat.com>

arch/x86/kvm/x86.c

@@ -4007,13 +4007,15 @@ int kvm_emulate_wbinvd(struct kvm_vcpu *vcpu)
 		return X86EMUL_CONTINUE;
 
 	if (kvm_x86_ops->has_wbinvd_exit()) {
-		preempt_disable();
+		int cpu = get_cpu();
+
+		cpumask_set_cpu(cpu, vcpu->arch.wbinvd_dirty_mask);
 		smp_call_function_many(vcpu->arch.wbinvd_dirty_mask,
 				wbinvd_ipi, NULL, 1);
-		preempt_enable();
+		put_cpu();
 		cpumask_clear(vcpu->arch.wbinvd_dirty_mask);
-	}
-	wbinvd();
+	} else
+		wbinvd();
 	return X86EMUL_CONTINUE;
 }
 EXPORT_SYMBOL_GPL(kvm_emulate_wbinvd);