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

[Daniel P . Berrangé]

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.

With regards,
Daniel
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