Re: [Qemu-devel] [PATCH/s390-next 3/3] s390x/flic: migrate ais states

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Christian Borntraeger (borntraeger@de.ibm.com) wrote:
> On 07/13/2017 04:49 PM, Dr. David Alan Gilbert wrote:
> > * Halil Pasic (pasic@linux.vnet.ibm.com) wrote:
> >>
> >>
> >> On 07/13/2017 02:27 PM, Cornelia Huck wrote:
> >>>> +static void kvm_flic_ais_pre_save(void *opaque)
> >>>> +{
> >>>> +    KVMS390FLICStateMigTmp *tmp = opaque;
> >>>> +    KVMS390FLICState *flic = tmp->parent;
> >>>> +    struct kvm_s390_ais_all ais;
> >>>> +    struct kvm_device_attr attr = {
> >>>> +        .group = KVM_DEV_FLIC_AISM_ALL,
> >>>> +        .addr = (uint64_t)&ais,
> >>>> +        .attr = sizeof(ais),
> >>>> +    };
> >>>> +
> >>>> +    if (ioctl(flic->fd, KVM_GET_DEVICE_ATTR, &attr)) {
> >>>> +        error_report("Failed to retrieve kvm flic ais states");
> >>> There's not much else we can do in that case, is there?
> >>>
> >>
> >> I think this is a very good question! The ioctl should not fail
> >> under any circumstances, but if it does we have a problem.
> >>
> >> Carrying on happily (what we do now) means effectively discarding
> >> ais state. In general just discarding state ain't a good idea.
> >>
> >> In particular it might be OK, but the patch should explain that!
> >>
> >> Regarding what could/should we do in such a case (instead
> >> of discarding state and carrying on happily) I don't know, so
> >> I tend to agree with you regarding 'not much else we can do'.
> >>
> >> Adding Dave and Juan. Maybe they can tell.
> > 
> > I keep meaning to make the pre_save give a return value for failure,
> > but it hasn't currently got one.
> 
> Would you accept patches for that?

Sure.

> > 
> > You could try something like:
> > 
> >   qemu_file_set_error(migrate_get_current()->to_dst_file, -EINVAL);
> > 
> >   I *think* the migration code should spot that before it finishes
> > but it might carry on for a little while before it does.
> 
> I will keep this patch as is, since this is one of the "should not happen"
> cases.

And you've got a log message, so when the impossible happens at least
we should be able to debug it.

Dave
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK