Re: [PATCH 0/2] rust: pci: expose is_virtfn() and reject VFs in nova-core

["Danilo Krummrich" <dakr@kernel.org>]

On Wed Oct 1, 2025 at 3:22 AM CEST, John Hubbard wrote:
> On 9/30/25 5:29 PM, Alistair Popple wrote:
>> On 2025-10-01 at 08:07 +1000, John Hubbard <jhubbard@nvidia.com> wrote...
>>> Post-Kangrejos, the approach for NovaCore + VFIO has changed a bit: the
>>> idea now is that VFIO drivers, for NVIDIA GPUs that are supported by
>>> NovaCore, should bind directly to the GPU's VFs. (An earlier idea was to
>>> let NovaCore bind to the VFs, and then have NovaCore call into the upper
>>> (VFIO) module via Aux Bus, but this turns out to be awkward and is no
>>> longer in favor.) So, in order to support that:
>>>
>>> Nova-core must only bind to Physical Functions (PFs) and regular PCI
>>> devices, not to Virtual Functions (VFs) created through SR-IOV.
>>>
>>> Add a method to check if a PCI device is a Virtual Function (VF). This
>>> allows Rust drivers to determine whether a device is a VF created
>>> through SR-IOV. This is required in order to implement VFIO, because
>>> drivers such as NovaCore must only bind to Physical Functions (PFs) or
>>> regular PCI devices. The VFs must be left unclaimed, so that a VFIO
>>> kernel module can claim them.
>> 
>> Curiously based on a quick glance I didn't see any other drivers doing this
>> which makes me wonder why we're different here. But it seems likely their
>> virtual functions are supported by the same driver rather than requiring a
>> different VF specific driver (or I glanced too quickly!).
>
> I haven't checked into that, but it sounds reasonable.

There are multiple cases:

Some devices have different PCI device IDs for their physical and virtual
functions and different drivers handling then. One example for that is Intel
IXGBE.

But there are also some drivers, which do a similar check and just stop probing
if they detect a virtual function.

So, this patch series does not do anything uncommon.

>> I'm guessing the proposal is to fail the probe() function in nova-core for
>> the VFs - I'm not sure but does the driver core continue to try probing other
>> drivers if one fails probe()? It seems like this would be something best
>> filtered on in the device id table, although I understand that's not possible
>> today.

Yes, the driver core keeps going until it finds a driver that succeeds probing
or no driver is left to probe. (This behavior is also the reason for the name
probe() in the first place.)

However, nowadays we ideally know whether a driver fits a device before probe()
is called, but there are still exceptions; with PCI virtual functions we've just
hit one of those.

Theoretically, we could also indicate whether a driver handles virtual functions
through a boolean in struct pci_driver, which would be a bit more elegant.

If you want I can also pick this up with my SR-IOV RFC which will probably touch
the driver structure as well; I plan to send something in a few days.