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

On 10/2/25 8:31 PM, Jason Gunthorpe wrote:
> This exactly how this function is used.
> 
> The core PF driver provides an API:
> 
> struct mlx5_core_dev *mlx5_vf_get_core_dev(struct pci_dev *pdev)
> 
> Which takes in the VF as pdev and internally it invokes:
> 
> 	mdev = pci_iov_get_pf_drvdata(pdev, &mlx5_core_driver);
Oh, I see, that makes sense then. Thanks for clarifying. I think I already had
in mind how this would look like in the Rust abstraction, and there we don't
need pci_iov_get_pf_drvdata() to achieve the same thing.
> /**
>  * pci_iov_get_pf_drvdata - Return the drvdata of a PF
>  * @dev: VF pci_dev
>  * @pf_driver: Device driver required to own the PF
>  *
>  * This must be called from a context that ensures that a VF driver is
attached.
>  * The value returned is invalid once the VF driver completes its remove()
>  * callback.
>  *
>  * Locking is achieved by the driver core. A VF driver cannot be probed
until
>  * pci_enable_sriov() is called and pci_disable_sriov() does not return
until
>  * all VF drivers have completed their remove().
>  *
>  * The PF driver must call pci_disable_sriov() before it begins to destroy
the
>  * drvdata.
>  */
> 
> Meaning nova-core has to guarentee to call pci_disable_sriov() before
> remove completes or before a failing probe returns as part of
> implementing SRIOV support.
Yes, I already thought about this. In the context of adding support for SR-IOV
in the Rust abstractions I'm planning on sending an RFC to let the subsystem
provide this guarantee instead (at least under certain conditions).

This will allow us to assert the device to be bound by the type system in the
Rust PCI abstraction, rather than having the driver to provide a guarantee to
call pci_disable_sriov() manually. :)

On Thu, Oct 02, 2025 at 08:42:58PM +0200, Danilo Krummrich
wrote:
> On 10/2/25 8:31 PM, Jason Gunthorpe wrote:
> > This exactly how this function is used.
> > 
> > The core PF driver provides an API:
> > 
> > struct mlx5_core_dev *mlx5_vf_get_core_dev(struct pci_dev *pdev)
> > 
> > Which takes in the VF as pdev and internally it invokes:
> > 
> > 	mdev = pci_iov_get_pf_drvdata(pdev, &mlx5_core_driver);
> 
> Oh, I see, that makes sense then. Thanks for clarifying. I think I already
had
> in mind how this would look like in the Rust abstraction, and there we
don't
> need pci_iov_get_pf_drvdata() to achieve the same thing.
I'm skeptical, there is nothing about rust that should avoid having to
us pci_iov_get_pf_drvdata().. It does a number of safety checks
related to the linux driver model that are not optional.

Don't forget in linux you actually can bind VFIO to the PF, start
SRIOV and then bind VFs to other drivers which then fail
pci_iov_get_pf_drvdata(). Blindly converting a struct device to an
instance memory without this check will be buggy.
> Yes, I already thought about this. In the context of adding support for
SR-IOV
> in the Rust abstractions I'm planning on sending an RFC to let the
subsystem
> provide this guarantee instead (at least under certain conditions).
Certain conditions may be workable, some drivers seem to have
preferences not to call disable, though I think that is wrong :\

Jason