[PATCHv5 0/2] Xen: FIFO-based event channel ABI fixes

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

Two further fixes for bugs in the FIFO-based event channel
implementation.

1. Correctly set READY bits to avoid races with guests emptying queues.

2. Handle problems with relinking events that were the tails on now
   empty queues.  This fixes more cases than a previously posted
   patch.

I have now backported the FIFO-based event channel patches (including
the above fixes) to Xen 4.3 and Linux 3.10 and run then through some
of XenServer's system tests.  Approximately 40-50 machine hours of
testing of dom0 have been done with a variety of guests and workloads.
No event channel failures were found.

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