[Xenomai] New domain migration process with ipipe-core and forge

[Jan Kiszka <jan.kiszka@siemens.com>]

Hi Philippe,

I'm trying to understand how the new Linux-to-RT migration process works
under forge and specifically I-pipe core. It currently looks to me like
we are delaying this needlessly:

- __ipipe_migrate_head() suspends the calling Linux thread and triggers
  a context switch
- after the switch, __ipipe_switch_tail and then
  complete_domain_migration is invoked
- complete_domain_migration stalls head and calls ipipe_migration_hook.
  We are still running over the root domain.
- ipipe_migration_hook in forge wakes up the RT thread and calls
  xnpod_schedule
- as we are in root, xnarch_escalate triggers and calls ipipe_raise_irq
- as the head domain is stalled, dispatch_irq_head just marks the
  escalation VIRQ pending, not handler is called, no actual RT
  rescheduling can be performed on its return
- we eventually return to complete_domain_migration and simply clear
  the stall flag. No pending IRQs are process, again no VIRQ is
  delivered
- only when we receive the next IRQ or have some other reason for
  checking the head domain's IRQ log, the pending VIRQ will finally be
  processed

Did I miss something or should we actually perform some
__ipipe_sync_stage in complete_domain_migration to accelerate the switch?

Jan

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