vfio/pci: Require vfio_device_ops.name

vfio-pci-core code makes use of the vfio_device_ops.name field in order
to set a default driver_override for VFs created on a user-owned PF.
This avoids default driver matching, which might otherwise bind those
VFs to native drivers.

The mechanism for this currently uses kasprintf(), which will set
driver_override to the literal "(null)" if name is NULL.  This is
effective in sequestering the device, but presents a challenging debug
situation to differentiate driver_override being set to "(null)" versus
being NULL and interpreted as "(null)" via the sysfs show attribute.

There's also a tree-wide effort to convert to generic driver_override
support, where passing NULL will generate an error, resulting in a
WARN_ON without setting any driver_override.

All drivers making use of vfio-pci-core already set a driver name,
therefore by requiring this behavior, all of these corner cases are
rendered moot.  This is expected to have no impact on current
in-kernel drivers.

Suggested-by: Danilo Krummrich <dakr@kernel.org>
Signed-off-by: Alex Williamson <alex.williamson@nvidia.com>
Reviewed-by: Danilo Krummrich <dakr@kernel.org>
Link: https://lore.kernel.org/r/20260331202443.2598404-1-alex.williamson@nvidia.com
Signed-off-by: Alex Williamson <alex@shazbot.org>

Author: Alex Williamson

drivers/vfio/pci/vfio_pci_core.c

@@ -2139,6 +2139,10 @@ int vfio_pci_core_register_device(struct vfio_pci_core_device *vdev)
 	if (WARN_ON(vdev != dev_get_drvdata(dev)))
 		return -EINVAL;
 
+	/* Drivers must set a name.  Required for sequestering SR-IOV VFs */
+	if (WARN_ON(!vdev->vdev.ops->name))
+		return -EINVAL;
+
 	if (pdev->hdr_type != PCI_HEADER_TYPE_NORMAL)
 		return -EINVAL;