On 06/28/2016 01:39 PM, Jim Fehlig wrote:
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.
Regardless of your current problem (as Dan says in his reply, this is
kernel breakage and should be fixed)...
"Unsafe" was *one* of the words that came to my mind when I first saw
the new_id interface. These days there is a sysfs interface called
driver_override that seems much more thoughtfully designed - you just
write the name of the desired driver to /sys/devices/[rest of path to
device]/driver_override. I didn't check if this is the version of the
patch that was pushed upstream, but the commit log message does give a
nice synopsis of its use:
https://www.redhat.com/archives/libvir-list/2014-April/msg00382.html
It would be nice to completely get rid of new_id in libvirt, but
driver_override doesn't exist in 2.6 kernels, so we have to keep it
around for compatibility with RHEL6/CentOS6. In the meantime, I wouldn't
complain at all if someone added support for driver_override that would
fallback to new_id if the driver_override node wasn't found. (A nice
side effect would be that your problem would be solved even when the
kernel wasn't fixed - driver_override is present at least as far back as
kernel 3.10, and you say your problem doesn't occur until 3.16).
Thanks for your opinions!
Regards,
Jim
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