[PATCHv5 0/2] x86/ept: reduce translation invalidation impact

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

This series improves the performance of EPT by further reducing the
impact of the translation invalidations (ept_sync_domain()). By:

a) Deferring invalidations until the p2m write lock is released.

Prior to this change a 16 VCPU guest could not be successfully
migrated on an (admittedly slow) 160 PCPU box because the p2m write
lock was held for such extended periods of time.  This starved the
read lock needed (by the toolstack) to map the domain's memory,
triggering the watchdog.

After this change a 64 VCPU guest could be successfully migrated.

ept_sync_domain() is very expensive because:

a) it uses on_selected_cpus() and the IPI cost can be particularly
   high for a multi-socket machine.

b) on_selected_cpus() is serialized by its own spin lock.

On this particular box, ept_sync_domain() could take ~3-5 ms.

Simply using a fair rw lock was not sufficient to resolve this (but it
was an improvement) as the cost of the ept_sync_domain calls() was
still delaying the read locks enough for the watchdog to trigger (the
toolstack maps a batch of 1024 GFNs at a time, which means trying to
acquire the p2m read lock 1024 times).

Changes in v5:

- Fix PoD by explicitly doing an invalidation before reclaiming zero
  pages.
- Use the same mechanism for dealing with freeing page table pages.
  This isn't a common path and its simpler than the deferred list.

Changes in v4:

- __ept_sync_domain() is a no-op -- invalidates are done before VMENTER.
- initialize ept->invalidate to all ones so the initial invalidate is
  always done.

Changes in v3:

- Drop already applied "x86/ept: remove unnecessary sync after
  resolving misconfigured entries".
- Replaced "mm: don't free pages until mm locks are released" with
  "x86/ept: invalidate guest physical mappings on VMENTER".

Changes in v2:

- Use a per-p2m (not per-CPU) list for page table pages to be freed.
- Hold the write lock while updating the synced_mask.

David