KVM: x86: add comments where MMIO does not return to the emulator

Support for single-step in the emulator (new in 3.12) does not work for
MMIO or PIO writes, because they are completed without returning to
the emulator.  This is not worse than what we had in 3.11; still, add
comments so that the issue is not forgotten.
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Gleb Natapov <gleb@redhat.com>

arch/x86/kvm/x86.c

@@ -5122,9 +5122,10 @@ int x86_emulate_instruction(struct kvm_vcpu *vcpu,
 		inject_emulated_exception(vcpu);
 		r = EMULATE_DONE;
 	} else if (vcpu->arch.pio.count) {
-		if (!vcpu->arch.pio.in)
+		if (!vcpu->arch.pio.in) {
+			/* FIXME: return into emulator if single-stepping.  */
 			vcpu->arch.pio.count = 0;
-		else {
+		} else {
 			writeback = false;
 			vcpu->arch.complete_userspace_io = complete_emulated_pio;
 		}
@@ -6176,6 +6177,8 @@ static int complete_emulated_mmio(struct kvm_vcpu *vcpu)
 
 	if (vcpu->mmio_cur_fragment == vcpu->mmio_nr_fragments) {
 		vcpu->mmio_needed = 0;
+
+		/* FIXME: return into emulator if single-stepping.  */
 		if (vcpu->mmio_is_write)
 			return 1;
 		vcpu->mmio_read_completed = 1;