[PATCH v2] util: basic support for VFIO variant drivers

[Alex Williamson]

On Wed, 31 May 2023 17:46:50 -0300
Jason Gunthorpe <jgg at nvidia.com> wrote:

> On Wed, May 31, 2023 at 02:40:01PM -0600, Alex Williamson wrote:
> 
> > Also note that we're saying "vfio" not "vfio-pci".  Only the mdev
> > interface has the device_api attribute to indicate the exported vfio
> > device interface.  The "vfio_pci:" match in modalias indicates a vfio
> > PCI driver, not necessarily a driver that provides a vfio-pci API.    
> 
> modalias was designed so you take the /sys/.../modalias file, prepend
> vfio_ then do a standard modalias search on that string. The matching
> module should be loaded and the module name bound to the device as the
> driver name.
> 
> There should be no bus type dependencies in any of this in management
> code.

For example, modalias of a random wifi adapter:

pci:v00008086d00002723sv00008086sd00000084bc02sc80i00

The bus name is prepended because the encoding is bus specific.  Yes,
code doesn't really need to interpret that, it simply adds "vfio_" to
the beginning of the string and finds the matching driver with the
fewest number of wildcards in modules.alias.

We are not code, we have a vfio_pci driver, a vfio-pci device API, and
a vfio_pci: modalias prefix, it's easy to get them confused and infer
information that isn't intended.  All I'm trying (poorly) to clarify is
that a vfio_pci: modalias prefix only indicates a vfio driver for a PCI
device.  It does not guarantee the vfio device API exposed to userspace
is vfio-pci.  Therefore management tools should be cautious to make
assumptions about the type of device the VM will see even though we've
got vfio-pci written all over the place.  Thanks,

Alex