[Xen-devel] [PATCHv6 0/1] Xen: FIFO-based event channel fix

[David Vrabel <david.vrabel@xxxxxxxxxx>]

This fixes a bug triggered by moving events between queues (either
moving VCPUs or changing their priority).

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 reduced set of XenServer's automated tests
and no issues were found.  I would have preferred to run though the
full set of tests but it didn't look like I'd get a slot before the
first 4.4 release candidate.

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



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