[PATCHv4 0/2] Xen: FIFO-based event channel ABI fixes

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

This series address two design flaws in the FIFO-based event channel ABI.

1. Fix a potential DoS caused by an unbounded loop when setting LINK.

2. Fix queue corruption that may occurs when events are moved between
queues.

An updated design document is available from:

  http://xenbits.xen.org/people/dvrabel/event-channels-H.pdf

- Add the BUSY bit to indicate that the guest must not clear MASKED.

v9 of the Linux patches have been posted already.

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