Nouveau - Oct 2025 - [PATCH v2 1/2] rust: pci: skip probing VFs if driver doesn't support VFs

On Thu Oct 2, 2025 at 2:11 PM CEST, Jason Gunthorpe
wrote:
> On Wed, Oct 01, 2025 at 07:00:09PM -0700, John Hubbard wrote:
>> Add a "supports_vf" flag to struct pci_driver to let drivers
declare
>> Virtual Function (VF) support. If a driver does not support VFs, then
>> the PCI driver core will not probe() any VFs for that driver's
devices.
>> 
>> On the Rust side, add a const "SUPPORTS_VF" Driver trait,
defaulting to
>> false: drivers must explicitly opt into VF support.
>
> As I said in the other thread - please no.
>
> Linux drivers are expected to run on their VFs.
The consequence would be that drivers for HW that can export VFs would need to
be rejected upstream if they only support the PF, but no VFs. IMHO, that's
an
unreasonable requirement.

Don't get me wrong, I agree that it'd be great if all drivers would (be
able to)
support the corresponding VFs.

But in practice that's not always the case, so I'd rather have a common
way to
let drivers assert what they support over going back to the "old
model" of
probing drivers, where we just rely on probe() to fail.
> This temporary
> weirdness of novacore should not be elevated to a core behavior that
> people will misuse.
It's not just nova-core, please see [1].

[1] https://lore.kernel.org/lkml/DD7TP31FEE92.2E0AKAHUOHVVF at kernel.org/

On Thu, Oct 02, 2025 at 02:18:36PM +0200, Danilo Krummrich
wrote:
> On Thu Oct 2, 2025 at 2:11 PM CEST, Jason Gunthorpe wrote:
> > On Wed, Oct 01, 2025 at 07:00:09PM -0700, John Hubbard wrote:
> >> Add a "supports_vf" flag to struct pci_driver to let
drivers declare
> >> Virtual Function (VF) support. If a driver does not support VFs,
then
> >> the PCI driver core will not probe() any VFs for that driver's
devices.
> >> 
> >> On the Rust side, add a const "SUPPORTS_VF" Driver
trait, defaulting to
> >> false: drivers must explicitly opt into VF support.
> >
> > As I said in the other thread - please no.
> >
> > Linux drivers are expected to run on their VFs.
> 
> The consequence would be that drivers for HW that can export VFs would need
to
> be rejected upstream if they only support the PF, but no VFs. IMHO,
that's an
> unreasonable requirement.
Not rejected, they just need to open code a simple isvf check and fail
during probe if they really have a (hopefully temporary) problem.

This not really a realistic case. Linux running in the VM *should*
have drivers that operate the VF, and those existing drivers *should*
work in the PF context.

Drivers that work in VM but not in a host should not be encouraged!!

AFAICT this is even true for novacore, the driver should "work" but
the VF won't be provisioned today so it should fail startup in some
way. eg "no vram" or something like that.
> > This temporary
> > weirdness of novacore should not be elevated to a core behavior that
> > people will misuse.
> 
> It's not just nova-core, please see [1].
> 
> [1] https://lore.kernel.org/lkml/DD7TP31FEE92.2E0AKAHUOHVVF at kernel.org/
I responded there, I don't think the reasons those were added to ICE
and then cargo-culted are very good, not good enough to justify adding
it to the core code.

Jason