pv-ops domU not working with MSI interrupts on Nehalem

[Bruce Edge <bruce.edge@gmail.com>]

One of our developers who is working on a tachyon driver is
complaining that the pvops domU kernel is not working for these MSI
interrupts.
This is using the current head of xen/2.6.32.x on both a single
Nahelam 920 and a dual E5540. This behavior is consistent with Xen
4.0.1, 4.0.2.rc1-pre and 4.1.

Here are his comments:

- the driver has no problem to enable msi interrupt and request the
interrupt through kernel functions pci_enable_msi & request_irq
- the interrupt does happen. But the interrupt service routine of
tachyon driver doesn't detect any interrupt status related to this
interrupt, which inhibits the tachyon chip from coming on-line. And
there are high count of tachyon interrupt in /proc/interrupts

kaan-18-dpm:~# cat /proc/interrupts | grep TACH
124:     760415          0          0          0          0          0
         0          0          0          0          0          0
    0          0  xen-pirq-pcifront-msi  HW_TACHYON
125:     762234          0          0          0          0          0
         0          0          0          0          0          0
    0          0  xen-pirq-pcifront-msi  HW_TACHYON
126:     764180          0          0          0          0          0
         0          0          0          0          0          0
    0          0  xen-pirq-pcifront-msi  HW_TACHYON
127:     764164          0          0          0          0          0
         0          0          0          0          0          0
    0          0  xen-pirq-pcifront-msi  HW_TACHYON


-Bruce