Re: [Qemu-devel] [PULL 02/15] migration: extend VMStateInfo

[Dr. David Alan Gilbert]

* Cornelia Huck (address@hidden) wrote:
> On Wed, 25 Jan 2017 14:44:20 +0000
> "Dr. David Alan Gilbert" <address@hidden> wrote:
> 
> > * Cornelia Huck (address@hidden) wrote:
> > > On Wed, 25 Jan 2017 13:22:55 +0000
> > > "Dr. David Alan Gilbert" <address@hidden> wrote:
> > > 
> > > > * Cornelia Huck (address@hidden) wrote:
> > > > > On Wed, 25 Jan 2017 12:00:53 +0000
> > > > > "Dr. David Alan Gilbert" <address@hidden> wrote:
> 
> > > > > > OK, so it looks like that's a failure path, adding a return -ENOMEM 
> > > > > > would seem to make
> > > > > > sense there.
> > > > > 
> > > > > Just saw this. I don't think we want -ENOMEM, as that would change the
> > > > > actual state being saved, no?
> > > > 
> > > > But isn't that the intention of this function?
> > > > 
> > > >     buf = g_try_malloc0(len);
> > > >     if (!buf) {
> > > >         /* Storing FLIC_FAILED into the count field here will cause the
> > > >          * target system to fail when attempting to load irqs from the
> > > >          * migration state */
> > > >         error_report("flic: couldn't allocate memory");
> > > >         qemu_put_be64(f, FLIC_FAILED);
> > > >         return;
> > > >     }
> > > > 
> > > > What should happen on the destination - should the migration fail?
> > > > If we want the migration to fail then we should now return an error
> > > > status rather than 0, and then we see a failed migration on the source
> > > > as well.
> > > 
> > > Yes. There's also another error case below where we should return an
> > > error instead of putting FLIC_FAILED, then.
> > > 
> > > The problem is that this is rather hard to test: So I'd prefer to fix
> > > the compile for now and introduce error return codes in a separate
> > > patch.
> > 
> > OK, that's fair.
> 
> I've coded something up and tried to test it with error injection to
> trigger the failed case, but I can't really see an improvement :(
> 
> Before: source logs error, target fails to load the flic with 'invalid
> argument'
> 
> After: source logs error, target fails to load the flic with 'could not
> allocate memory'
> 
> The migration code does not seem to do anything with the return code of
> put methods for now, so that's not too surprising. Is anything in the
> works?

OK, I hadn't remembered that wasn't wired up.

> For now, I'd prefer to keep the old behaviour as 'invalid argument'
> seems like a more obvious error on the target.

Yes, agreed.

If you feel like, then I'd just change the return -ENOMEM but still
send the FLIC_FAILED.  That way the destination still fails
as before, and the destination will fail nicely when we wire up the
error checks.

Dave

> diff --git a/hw/intc/s390_flic_kvm.c b/hw/intc/s390_flic_kvm.c
> index e86a84e49a..3c62ef8258 100644
> --- a/hw/intc/s390_flic_kvm.c
> +++ b/hw/intc/s390_flic_kvm.c
> @@ -293,27 +293,21 @@ static int kvm_flic_save(QEMUFile *f, void *opaque, 
> size_t size,
>      int len = FLIC_SAVE_INITIAL_SIZE;
>      void *buf;
>      int count;
> +    int r = 0;
>  
>      flic_disable_wait_pfault((struct KVMS390FLICState *) opaque);
>  
>      buf = g_try_malloc0(len);
>      if (!buf) {
> -        /* Storing FLIC_FAILED into the count field here will cause the
> -         * target system to fail when attempting to load irqs from the
> -         * migration state */
>          error_report("flic: couldn't allocate memory");
> -        qemu_put_be64(f, FLIC_FAILED);
> -        return 0;
> +        return -ENOMEM;
>      }
>  
>      count = __get_all_irqs(flic, &buf, len);
>      if (count < 0) {
>          error_report("flic: couldn't retrieve irqs from kernel, rc %d",
>                       count);
> -        /* Storing FLIC_FAILED into the count field here will cause the
> -         * target system to fail when attempting to load irqs from the
> -         * migration state */
> -        qemu_put_be64(f, FLIC_FAILED);
> +        r = count;
>      } else {
>          qemu_put_be64(f, count);
>          qemu_put_buffer(f, (uint8_t *) buf,
> @@ -321,7 +315,7 @@ static int kvm_flic_save(QEMUFile *f, void *opaque, 
> size_t size,
>      }
>      g_free(buf);
>  
> -    return 0;
> +    return r;
>  }
>  
>  /**
> 
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK