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

[Peter Xu]

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?

-- 
Peter Xu