[libvirt] [PATCH] PCI: Introduce new device binding path using pci_dev.driver_override

[Stuart Yoder]

> -----Original Message-----
> From: Alex Williamson [mailto:alex.williamson at redhat.com]
> Sent: Friday, April 04, 2014 3:19 PM
> To: bhelgaas at google.com; linux-pci at vger.kernel.org
> Cc: agraf at suse.de; kvm at vger.kernel.org; konrad.wilk at oracle.com;
> kim.phillips at linaro.org; gregkh at linuxfoundation.org; Yoder Stuart-B08248;
> linux-kernel at vger.kernel.org; libvir-list at redhat.com; iommu at lists.linux-
> foundation.org; tech at virtualopensystems.com;
> kvmarm at lists.cs.columbia.edu; christoffer.dall at linaro.org
> Subject: [PATCH] PCI: Introduce new device binding path using
> pci_dev.driver_override
> 
> The driver_override field allows us to specify the driver for a device
> rather than relying on the driver to provide a positive match of the
> device.  This shortcuts the existing process of looking up the vendor
> and device ID, adding them to the driver new_id, binding the device,
> then removing the ID, but it also provides a couple advantages.
> 
> First, the above existing process allows the driver to bind to any
> device matching the new_id for the window where it's enabled.  This is
> often not desired, such as the case of trying to bind a single device
> to a meta driver like pci-stub or vfio-pci.  Using driver_override we
> can do this deterministically using:
> 
> echo pci-stub > /sys/bus/pci/devices/0000:03:00.0/driver_override
> echo 0000:03:00.0 > /sys/bus/pci/devices/0000:03:00.0/driver/unbind
> echo 0000:03:00.0 > /sys/bus/pci/drivers_probe
> 
> Previously we could not invoke drivers_probe after adding a device
> to new_id for a driver as we get non-deterministic behavior whether
> the driver we intend or the standard driver will claim the device.
> Now it becomes a deterministic process, only the driver matching
> driver_override will probe the device.
> 
> To return the device to the standard driver, we simply clear the
> driver_override and reprobe the device:
> 
> echo > /sys/bus/pci/devices/0000:03:00.0/preferred_driver
> echo 0000:03:00.0 > /sys/bus/pci/devices/0000:03:00.0/driver/unbind
> echo 0000:03:00.0 > /sys/bus/pci/drivers_probe
> 
> Another advantage to this approach is that we can specify a driver
> override to force a specific binding or prevent any binding.  For
> instance when an IOMMU group is exposed to userspace through VFIO
> we require that all devices within that group are owned by VFIO.
> However, devices can be hot-added into an IOMMU group, in which case
> we want to prevent the device from binding to any driver (preferred
> driver = "none") or perhaps have it automatically bind to vfio-pci.
> With driver_override it's a simple matter for this field to be set
> internally when the device is first discovered to prevent driver
> matches.
> 
> Signed-off-by: Alex Williamson <alex.williamson at redhat.com>
> ---

Reviewed-by: Stuart Yoder <stuart.yoder at freescale.com>