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

On Thu, Oct 02, 2025 at 03:03:38PM +0200, Danilo Krummrich wrote:
> I think it's not unreasonable to have a driver for the PF and a
separate driver
> for the VFs if they are different enough; the drivers can still share
common
> code of course.
This isn't feasible without different PCI IDs. ICE does this for
example where they have two totally different drivers, and of course
two different PCI IDs.
> Surely, you can argue that if they have different enough requirements they
> should have different device IDs, but "different enough
requirements" is pretty
> vague and it's not under our control either.
If you want two drivers in Linux you need two PCI IDs.

We can't reliably select different drivers based on VFness because
VFness is wiped out during virtualization.
> But, if there is another solution for VFs already, e.g. in the case of
nova-core
> vGPU, why restrict drivers from opt-out of VFs. (In a previous reply I
mentioned
> I prefer opt-in, but you convinced me that it should rather be
> opt-out.)
I think nova-core has a temporary (OOT even!) issue that should be
resolved - that doesn't justify adding core kernel infrastructure that
will encourage more drivers to go away from our kernel design goals of
drivers working equally in host and VM.

Nova core should work in that it probes, detects an unprovisioned VF
and then fails probe. This edge case could perhaps arise in a VM
anyhow and needs to be handled anyhow.

Also, this is all pointless until nova-core gets an sriov_configure
callback - you can't even turn on SRIOV without that!

Jason

On Thu Oct 2, 2025 at 3:56 PM CEST, Jason Gunthorpe
wrote:
> On Thu, Oct 02, 2025 at 03:03:38PM +0200, Danilo Krummrich wrote:
>
>> I think it's not unreasonable to have a driver for the PF and a
separate driver
>> for the VFs if they are different enough; the drivers can still share
common
>> code of course.
>
> This isn't feasible without different PCI IDs.
At least on the host you can obviously differentiate them.
>> Surely, you can argue that if they have different enough requirements
they
>> should have different device IDs, but "different enough
requirements" is pretty
>> vague and it's not under our control either.
>
> If you want two drivers in Linux you need two PCI IDs.
>
> We can't reliably select different drivers based on VFness because
> VFness is wiped out during virtualization.
Sure, but I thought the whole point is that some VFs are not given directly to
the VM, but have some kind of intermediate layer, such as vGPU. I.e. some kind
of hybrid approach between full pass-through and mediated devices?
>> But, if there is another solution for VFs already, e.g. in the case of
nova-core
>> vGPU, why restrict drivers from opt-out of VFs. (In a previous reply I
mentioned
>> I prefer opt-in, but you convinced me that it should rather be
>> opt-out.)
>
> I think nova-core has a temporary (OOT even!) issue that should be
> resolved - that doesn't justify adding core kernel infrastructure that
> will encourage more drivers to go away from our kernel design goals of
> drivers working equally in host and VM.
My understanding is that vGPU will ensure that the device exposed to the VM will
be set up to be (at least mostly) compatible with nova-core's PF code paths?

So, there is a semantical difference between vGPU and nova-core that makes a
differentiation between VF and PF meaningful and justified.

But maybe this understanding is not correct. If so, please educate me.