Re: [PATCH 07/14] migration: Make switchover-ack re-usable

[Peter Xu <peterx@redhat.com>]

On Tue, May 26, 2026 at 12:08:34PM +0300, Avihai Horon wrote:
> 
> On 5/25/2026 6:01 PM, Peter Xu wrote:
> > External email: Use caution opening links or attachments
> > 
> > 
> > On Sun, May 24, 2026 at 09:34:48AM +0300, Avihai Horon wrote:
> > > Yes I think so.
> > > 
> > > We just need to indicate modules that it’s the last query during switchover
> > > so they can handle it properly.
> > > Do you think it would be reasonable to add a "bool final" param to
> > > save_query_pending handler?
> > > 
> > > For RAM it will be used to indicate we are running under the BQL (since
> > > currently save_query_pending runs only outside BQL) and to pass the proper
> > > last_stage param into migration_bitmap_sync_precopy().
> > > For VFIO it will indicate we should not do a query precopy info ioctl (which
> > > is only valid in VFIO precopy states, not while VM is stopped).
> > Yes, a final boolean sounds reasonable, implying both (1) last sync before
> > switchover, VM stopped, (2) BQL held.
> > 
> > For VFIO, I double checked the complete() that does not depend on the
> > precopy_bytes fetched, then it should be fine indeed,
> > 
> > vfio_save_complete_precopy():
> >      do {
> >          data_size = vfio_save_block(f, vbasedev->migration);
> >          if (data_size < 0) {
> >              return data_size;
> >          }
> >      } while (data_size);
> > 
> > It's just werid to see that it doesn't depend on either precopy_bytes or
> > initial_bytes, even if logically it should.. this will be confusing to
> > whoever start reading this code.. but I understand not much we can do with
> > the current kernel API.
> > 
> > Side note: should we still better update these fields to make sure they'll
> > be zero after migration? That means vfio_update_estimated_pending_data() in
> > vfio_save_complete_precopy() too, with/without further sanity checks.  That
> > seems to be missing right now.  I'm not sure if it's intentional.
> 
> Yes, it's intentional, since we don't use these values after calling
> vfio_save_complete_precopy() -- they are only used for downtime estimation
> prior switchover.
> Precopy_init/dirty sizes are zeroed in vfio_save_cleanup() though, but not
> stopcopy_size (however, that's benign, since upon new migration it will be
> reset before used).
> 
> So calling vfio_update_estimated_pending_data() here seems redundant to me.

Logically migration can still fail during complete():

qemu_savevm_state_complete_precopy():
    ret = qemu_savevm_state_complete_precopy_iterable(f, false);
    if (ret) {
        return ret;
    }

If vfio_update_estimated_pending_data() has the safe guard for any form of
overflow, then IMHO we should try to maintain those counters if possible.

Thanks,

-- 
Peter Xu