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

[Wang, Wei W]

On Friday, May 19, 2023 3:20 AM, 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.

Actually the patch doesn’t check "!defer". It just checks if incoming has been 
started
or not. It allows the 2 caps to be set only before incoming starts. So I think 
the above
should work.

> 
> Can we just make socket_start_incoming_migration_internal() listen on a
> static but larger value?

Yes, agree for this and that's out initial change.
This needs listen() to create a longer queue for pending connections (seems OK 
to me).
Need to see Daniel and Juan's opinion about this.