[PATCH v9 for-xen-4.5] Fix interrupt latency of HVM PCIpassthrough devices.

[Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>]

Changelog:
since v8 (http://lists.xen.org/archives/html/xen-devel/2014-10/msg02564.html)
 - Went back-n-forth on remote softirq, fixed styleguide violations.
 - Moved call to pt_pirq_softirq_reset in pt_irq_create_bind
 - Removed 'case default'
 - Streamlined 'pci_clean_dpci_irqs' return usage.

since v7 (http://lists.xen.org/archives/html/xen-devel/2014-09/msg04385.html)
 - Put ref-counting back, added two bits to allow ref-counting from other places.
since v6 (http://lists.xen.org/archives/html/xen-devel/2014-09/msg03208.html)
 - Squashed #1 + #2.
 - Added more comments, redid it based on Jan's feedback.
since v5 (http://lists.xen.org/archives/html/xen-devel/2014-09/msg02868.html)
 - Redid the series based on Jan's feedback
since v4
(http://lists.xen.org/archives/html/xen-devel/2014-09/msg01676.html):
 - Ditch the domain centric mechansim.
 - Fix issues raised by Jan.


The patches are an performance bug-fixes for PCI passthrough for machines
with many sockets. On those machines we have observed awful latency issues
with interrupts and with high steal time on idle guests. The root cause
was that the tasklet lock which was shared across all sockets. Each interrupt
that was to be delivered to a guest took the tasket lock - and with many
guests and many PCI passthrough devices - the performance and latency were
atrocious. These two patches fix the outstanding issues.

 xen/arch/x86/domain.c         |   4 +-
 xen/drivers/passthrough/io.c  | 282 +++++++++++++++++++++++++++++++++++++-----
 xen/drivers/passthrough/pci.c |  29 +++--
 xen/include/asm-x86/softirq.h |   3 +-
 xen/include/xen/hvm/irq.h     |   5 +-
 xen/include/xen/pci.h         |   2 +-
 6 files changed, 283 insertions(+), 42 deletions(-)

Konrad Rzeszutek Wilk (2):
      dpci: Move from an hvm_irq_dpci (and struct domain) to an hvm_dirq_dpci model.
      dpci: Replace tasklet with an softirq (v12)