Re: [PATCH v1] migration: fail the cap check if it requires the use of deferred incoming

[Peter Xu]

On Fri, May 19, 2023 at 09:26:23AM +0100, Daniel P. Berrangé wrote:
> On Thu, May 18, 2023 at 03:20:02PM -0400, Peter Xu wrote:
> > On Fri, May 19, 2023 at 12:00:26AM +0800, Wei Wang wrote:
> > > qemu_start_incoming_migration needs to check the number of multifd
> > > channels or postcopy ram channels to configure the backlog parameter (i.e.
> > > the maximum length to which the queue of pending connections for sockfd
> > > may grow) of listen(). So multifd and postcopy-preempt caps require the
> > > use of deferred incoming, that is, calling qemu_start_incoming_migration
> > > should be deferred via qmp or hmp commands after the cap of multifd and
> > > postcopy-preempt are configured.
> > > 
> > > Check if deferred incoming is used when enabling multifd or
> > > postcopy-preempt, and fail the check with error messages if not.
> > > 
> > > Signed-off-by: Wei Wang <wei.w.wang@intel.com>
> > 
> > IIUC this will unfortunately break things like:
> > 
> >   -global migration.x-postcopy-preempt=on
> > 
> > where the cap is actually applied before incoming starts even with !defer
> > so it should still work.
> > 
> > Can we just make socket_start_incoming_migration_internal() listen on a
> > static but larger value?
> 
> Why do we need todo that ? I thought we just determined the problem was
> a configuration error, not a code error.

Yes, sorry I just read the other thread for more context.  So it seems my
concern wasn't really a concern anyway, now I'm fine with the current
approach.  Thanks,

-- 
Peter Xu