pciback bug "secondary bus reset failed"

[Alex Zeffertt <alex.zeffertt@eu.citrix.com>]

Hi,

I'm trying to passthrough a PCI network card to a domU guest, but I'm hitting a 
bug that prevents the IRQ being unbound from dom0.

Here are the steps I am taking:

  * configure Xen to pass $pci (0000:06:00.0) through to guest VM
  * unbind from current driver in dom0 and rebind to pciback:

      echo -n $pci > /sys/bus/pci/devices/$pci/driver/unbind
      modprobe pciback
      echo -n $pci > /sys/bus/pci/drivers/pciback/new_slot
      echo -n $pci > /sys/bus/pci/drivers/pciback/bind

  * start guest VM

I then get the following on the dom0 console

   pciback 0000:06:00.0: secondary bus reset failed for device - all functions \
   need to be co-assigned - err: 6
   pciback 0000:06:00.0: FLR functionality not supported; attempts to use \
   secondary bus reset unsuccessful;
   pciback 0000:06:00.0: FLR not performed for device

I don't understand the above, but the upshot is that pirq_guest_unbind() does 
not get called.  Later when the guest does an "ifconfig up" the IRQ gets sent to 
dom0 instead of the domU, and this causes the following to appear on the dom0 
console:

   irq 21: nobody cared (try booting with the "irqpoll" option)
   Pid: 0, comm: swapper Tainted: G          2.6.27.42-0.1.1.xs5.6.0.44.111158xen
    #1
    [<c0154517>] __report_bad_irq+0x27/0x90
    [<c015487c>] note_interrupt+0x2fc/0x330
    [<f009e92d>] ? usb_hcd_irq+0x4d/0xe0 [usbcore]
    [<c0153951>] ? handle_IRQ_event+0x31/0x90
    [<c0155204>] handle_level_irq+0xe4/0x110
    [<c0107733>] do_IRQ+0x43/0x90

I have ioapic_ack=old on the Xen command line.  This causes Xen to EOI the 
interrupt straight away instead of waiting for a guest to EOI and forwarding 
that on to the IO APIC.  If I remove this then since no guest handles the 
interrupt, all lower priority IRQs get blocked forever and I lose access to my 
hard disk!

Any help would be much appreciated.

Regards,

Alex Zeffertt