PCI/IOV: Add pci-pf-stub driver for PFs that only enable VFs

Some SR-IOV PF devices provide no functionality other than acting as a
means of enabling VFs.  For these devices, we want to enable the VFs and
assign them to guest virtual machines, but there's no need to have a driver
for the PF itself.

Add a new pci-pf-stub driver to claim those PF devices and provide the
generic VF enable functionality.  An administrator can use the sysfs
"sriov_numvfs" file to enable VFs, then assign them to guests.

For now I only have one example ID provided by Amazon in terms of devices
that require this functionality.  The general idea is that in the future we
will see other devices added as vendors come up with devices where the PF
is more or less just a lightweight shim used to allocate VFs.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
[bhelgaas: changelog]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Greg Rose <gvrose8192@gmail.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>

drivers/pci/Kconfig

@@ -71,6 +71,18 @@ config PCI_STUB
 
 	  When in doubt, say N.
 
+config PCI_PF_STUB
+	tristate "PCI PF Stub driver"
+	depends on PCI
+	depends on PCI_IOV
+	help
+	  Say Y or M here if you want to enable support for devices that
+	  require SR-IOV support, while at the same time the PF itself is
+	  not providing any actual services on the host itself such as
+	  storage or networking.
+
+	  When in doubt, say N.
+
 config XEN_PCIDEV_FRONTEND
         tristate "Xen PCI Frontend"
         depends on PCI && X86 && XEN