What:           /sys/bus/pci/drivers/pciback/quirks
Date:           Oct 2011
KernelVersion:  3.1
Contact:        xen-devel@lists.xenproject.org
Description:
                If the permissive attribute is set, then writing a string in
                the format of DDDD:BB:DD.F-REG:SIZE:MASK will allow the guest
                to write and read from the PCI device. That is Domain:Bus:
                Device.Function-Register:Size:Mask (Domain is optional).
                For example:
                #echo 00:19.0-E0:2:FF > /sys/bus/pci/drivers/pciback/quirks
                will allow the guest to read and write to the configuration
                register 0x0E.