Re: [Qemu-devel] [PATCH v4 30/32] migration: delay the postcopy-active state switch

[Peter Xu <peterx@redhat.com>]

On Fri, Dec 01, 2017 at 12:34:32PM +0000, Dr. David Alan Gilbert wrote:
> * Peter Xu (peterx@redhat.com) wrote:
> > Switch the state until we try to start the VM on destination side.  The
> > problem is that without doing this we may have a very small window that
> > we'll be in such a state:
> > 
> > - dst VM is in postcopy-active state,
> > - main thread is handling MIG_CMD_PACKAGED message, which loads all the
> >   device states,
> > - ram load thread is reading memory data from source.
> > 
> > Then if we failed at this point when reading the migration stream we'll
> > also switch to postcopy-paused state, but that is not what we want.  If
> > device states failed to load, we should fail the migration directly
> > instead of pause.
> > 
> > Postponing the state switch to the point when we have already loaded the
> > devices' states and been ready to start running destination VM.
> > 
> > Signed-off-by: Peter Xu <peterx@redhat.com>
> 
> If it's the only way, then this is OK; but I'd prefer you use a separate
> flag somewhere to let you know this, because this means that
> POSTCOPY_ACTIVE on the destination happens at a different point that it
> does on the source (and changing it on the source I think will break
> lots of things).

Yes, I thought it was fine to postpone it a bit to the point even
after receiving the packed data, but I fully understand your point.

> Can't you use the PostcopyState value and check if it's in
> POSTCOPY_INCOMING_RUNNING?

I think, yes. :)

Let me drop this patch, instead I'll check explicitly on
POSTCOPY_INCOMING_RUNNING state in patch 6.  Thanks,

-- 
Peter Xu