[PATCHv7 0/2] Xen: FIFO-based event channel fixes

[David Vrabel <david.vrabel@citrix.com>]

This series fixes two bugs in the FIFO-based event channel ABI.

With the first bug, the priority of a newly bound event may not be the
default (it might have been an old priority for that port or 0).

The second bug is triggered by moving events between queues (either
moving VCPUs or changing their priority).  This would cause events to
be lost.

Testing with a process continually moving all event channels between
VCPUs has been done.  This would previously fail in under an hour but
with this fix the system stayed up for over 10 days.

It has also been through a complete set of XenServer's automated
regression tests and no issues were found.

Changes in v7:

- Add patch to initialize priority of all newly bound events.

Changes in v6:

- Limit loop to acquire old_q->lock to 3 iterations.

Changes in v5:

- Only set READY bits for new heads.
- Rework old tail bug fix to cover all cases.

Changes in v4:

- const struct domain *
- Clear BUSY with existing cmpxchg() where possible.
- Fix BUSY bit debug output.

Changes in v3:

- Use a new BUSY bit to block guests from clearing UNMASKED, this is
  lower overhead than the previous solution (which required a
  hypercall).
- Fix another problem with moving events between queues.
- Add evtchn->last_vpcu_id and evtchn->last_priority instead of
  evtchn->q.  This keeps the structure at 32 bytes long.

Changes in v2:

- Add MAINTAINERS patch
- Remove some unnecessary temporary pending state clears
- Add fix for DoS

David