[libvirt] question about PCI new_id sysfs interface

[Jim Fehlig]

After updating the dom0 kernel on one of my Xen test hosts, I noticed problems 
with PCI hostdev management. E.g

# virsh nodedev-detach pci_0000_07_10_1
error: Failed to detach device pci_0000_07_10_1
error: Failed to add PCI device ID '8086 1520' to pciback: File exists

It turns out there was a small interface change to new_id with the following 
commit to 3.16 kernel

https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/drivers/pci/pci-driver.c?h=v3.16&id=8895d3bcb8ba960b1b83f95d772b641352ea8e51

which now causes xen_pciback to fail writes of "vendorid productid" to new_id. e.g.

# echo "8086 1520" > /sys/bus/pci/drivers/pciback/new_id
-bash: echo: write error: File exists

Interestingly, vfio doesn't encounter the same error

# echo "8086 1520" > /sys/bus/pci/drivers/vfio-pci/new_id
# echo $?
0

vfio-pci has:
static struct pci_driver vfio_pci_driver = {
         .name           = "vfio-pci",
         .id_table       = NULL, /* only dynamic ids */

while xen-pciback has:
static const struct pci_device_id pcistub_ids[] = {
         {
          .vendor = PCI_ANY_ID,
          .device = PCI_ANY_ID,
          .subvendor = PCI_ANY_ID,
          .subdevice = PCI_ANY_ID,
          },
         {0,},
};
static struct pci_driver xen_pcibk_pci_driver = {
         .name = "pciback",
         .id_table = pcistub_ids,

So any vendor/device pair will match for xen-pciback, while none will match for 
vfio-pci.

But after reading that commit and the associated thread, it is not clear to me 
how to best fix this. Options are

1. set .id_table to NULL for xen-pciback
2. drop using the new_id interface from libvirt
3. pass more values (subvendor, subdevice, class, etc) to the new_id interface

I'm not sure what problems, if any, options 1 and 2 might cause. Option 2 seems 
the best approach since new_id seems to be a rather unsafe interface.

Thanks for your opinions!

Regards,
Jim