Re: [PATCH v16 QEMU 09/16] vfio: Add save state functions to SaveVMHandlers

[Yan Zhao]

On Mon, May 11, 2020 at 05:53:37PM +0800, Kirti Wankhede wrote:
> 
> 
> On 5/5/2020 10:07 AM, Alex Williamson wrote:
> > On Tue, 5 May 2020 04:48:14 +0530
> > Kirti Wankhede <address@hidden> wrote:
> > 
> >> On 3/26/2020 3:33 AM, Alex Williamson wrote:
> >>> On Wed, 25 Mar 2020 02:39:07 +0530
> >>> Kirti Wankhede <address@hidden> wrote:
> >>>    

<...>

> >>>> +static int vfio_save_iterate(QEMUFile *f, void *opaque)
> >>>> +{
> >>>> +    VFIODevice *vbasedev = opaque;
> >>>> +    int ret, data_size;
> >>>> +
> >>>> +    qemu_put_be64(f, VFIO_MIG_FLAG_DEV_DATA_STATE);
> >>>> +
> >>>> +    data_size = vfio_save_buffer(f, vbasedev);
> >>>> +
> >>>> +    if (data_size < 0) {
> >>>> +        error_report("%s: vfio_save_buffer failed %s", vbasedev->name,
> >>>> +                     strerror(errno));
> >>>> +        return data_size;
> >>>> +    }
> >>>> +
> >>>> +    qemu_put_be64(f, VFIO_MIG_FLAG_END_OF_STATE);
> >>>> +
> >>>> +    ret = qemu_file_get_error(f);
> >>>> +    if (ret) {
> >>>> +        return ret;
> >>>> +    }
> >>>> +
> >>>> +    trace_vfio_save_iterate(vbasedev->name, data_size);
> >>>> +    if (data_size == 0) {
> >>>> +        /* indicates data finished, goto complete phase */
> >>>> +        return 1;
> >>>
> >>> But it's pending_bytes not data_size that indicates we're done.  How do
> >>> we get away with ignoring pending_bytes for the save_live_iterate phase?
> >>>    
> >>
> >> This is requirement mentioned above qemu_savevm_state_iterate() which
> >> calls .save_live_iterate.
> >>
> >> /* 
> >>    * this function has three return values:
> >>    *   negative: there was one error, and we have -errno.
> >>    *   0 : We haven't finished, caller have to go again
> >>    *   1 : We have finished, we can go to complete phase
> >>    */
> >> int qemu_savevm_state_iterate(QEMUFile *f, bool postcopy)
> >>
> >> This is to serialize savevm_state.handlers (or in other words devices).
> > 
> > I've lost all context on this question in the interim, but I think this
> > highlights my question.  We use pending_bytes to know how close we are
> > to the end of the stream and data_size to iterate each transaction
> > within that stream.  So how does data_size == 0 indicate we've
> > completed the current phase?  It seems like pending_bytes should
> > indicate that.  Thanks,
> > 
> 
> Fixing this by adding a read on pending_bytes if its 0 and return 
> accordingly.
>      if (migration->pending_bytes == 0) {
>          ret = vfio_update_pending(vbasedev);
>          if (ret) {
>              return ret;
>          }
> 
>          if (migration->pending_bytes == 0) {
>              /* indicates data finished, goto complete phase */
>              return 1;
>          }
>      }
> 

just a question. if 1 is only returned when migration->pending_bytes is 0,
does that mean .save_live_iterate of vmstates after "vfio-pci"
would never be called until migration->pending_bytes is 0 ?

as in qemu_savevm_state_iterate(),

qemu_savevm_state_iterate {
...
  QTAILQ_FOREACH(se, &savevm_state.handlers, entry) {
        ...
        ret = se->ops->save_live_iterate(f, se->opaque);
        ...
        if (ret <= 0) {
            /* Do not proceed to the next vmstate before this one reported
               completion of the current stage. This serializes the migration
               and reduces the probability that a faster changing state is
               synchronized over and over again. */
            break;
        }
  }
  return ret;
}

in ram's migration code, its pending_bytes(remaining_size) is only updated in
ram_save_pending() when it's below threshold, which means in
ram_save_iterate() the pending_bytes is possible to be 0, so other
vmstates have their chance to be called.

Thanks
Yan