3.19-rc4: Xen pci-passthrough regression, bisected to commit cffe0a2b5a34c95a4dadc9ec7132690a5b0f6687 "x86, irq: Keep balance of IOAPIC pin reference count"

[Sander Eikelenboom <linux@eikelenboom.it>]

Hi Gerry / David / Konrad,

Some more testing uncovered another issue under Xen, this time with PCI-passthrough.

I have bisected it to the following commit: 
cffe0a2b5a34c95a4dadc9ec7132690a5b0f6687 "x86, irq: Keep balance of IOAPIC pin reference count"

It causes these symptoms:

- On Intel
  - Running on Xen with pci devices seized on host boot with xen-pciback.hide= parameter
  - Running a HVM guest with PCI passthrough of two devices (NIC + wireless NIC)
  - While the driver loads fine, the device isn't working properly, looking in /proc/interrupts in the guest
    shows that it doesn't receive any interrupts.
  - Reverting this particular commit (in the dom0 kernel only) makes the device receive interrupts and work properly again.

- On AMD (more subtle symptom) 
  - Running on Xen with pci devices seized on host boot with xen-pciback.hide= parameter
  - Running a HVM guest with PCI passthrough of one devices (videograbber)
  - While the driver loads fine and the device looks like it's working, the videostream isn't stable and it skips or repeats frames.
  - Reverting this particular commit (in the dom0 kernel only) makes the device work properly again with a stable videostream.

--
Sander