[PATCH v4 2/5] PCI/ATS: Validate STU for VFs in pci_prepare_ats()

[Nicolin Chen]

On Mon, May 25, 2026 at 06:43:44PM +0000, Pranjal Shrivastava wrote:
> While every PCI Function that implements ATS has an independent ATS
> Extended Capability structure with a Read/Write Smallest Translation
> Unit (STU) field, the kernel manages SR-IOV ATS by requiring the IOMMU
> driver to configure the STU on the Physical Function (PF) before any
> any Virtual Functions (VFs) are created.
> 
> Currently, pci_prepare_ats() bails out early for VFs, assuming that the
> PF has already been correctly prepared. However, this creates a potential
> mismatch if a VF is subsequently prepared with a different page shift.
> 
> Update pci_prepare_ats() to validate that the requested page shift (ps)
> matches the STU already configured in the associated PF. This ensures
> early detection of incompatible configurations and maintains the kernel's
> policy of consistent STU sizing across all functions associated with a
> given SMMU.
> 
> Reviewed-by: Jason Gunthorpe <jgg at nvidia.com>
> Reviewed-by: Samiullah Khawaja <skhawaja at google.com>
> Signed-off-by: Pranjal Shrivastava <praan at google.com>

Reviewed-by: Nicolin Chen <nicolinc at nvidia.com>

Nit:

> @@ -73,8 +73,13 @@ int pci_prepare_ats(struct pci_dev *dev, int ps)
>  	if (ps < PCI_ATS_MIN_STU)
>  		return -EINVAL;
>  
> -	if (dev->is_virtfn)
> +	if (dev->is_virtfn) {
> +		struct pci_dev *pdev = pci_physfn(dev);
> +
> +		if (pdev->ats_stu != ps)
> +			return -EINVAL;
>  		return 0;
> +	}

pdev doesn't seem useful. Dropping it reads better to me:
		if (pci_physfn(dev)->ats_stu != ps)
			return -EINVAL;

Nicolin