Re: [PATCH v8 2/4] PCI/ATS: Validate STU for VFs in pci_prepare_ats()

[Bjorn Helgaas <helgaas@kernel.org>]

On Thu, Jun 04, 2026 at 06:21:14PM +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@nvidia.com>
> Reviewed-by: Samiullah Khawaja <skhawaja@google.com>
> Reviewed-by: Nicolin Chen <nicolinc@nvidia.com>
> Reviewed-by: Lu Baolu <baolu.lu@linux.intel.com>
> Reviewed-by: Kevin Tian <kevin.tian@intel.com>
> Signed-off-by: Pranjal Shrivastava <praan@google.com>

Acked-by: Bjorn Helgaas <bhelgaas@google.com>

> ---
>  drivers/pci/ats.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/pci/ats.c b/drivers/pci/ats.c
> index 679a3c3c1d54..9cb23780093d 100644
> --- a/drivers/pci/ats.c
> +++ b/drivers/pci/ats.c
> @@ -73,8 +73,12 @@ 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) {
> +		if (pci_physfn(dev)->ats_stu != ps)
> +			return -EINVAL;
> +
>  		return 0;
> +	}
>  
>  	dev->ats_stu = ps;
>  	ctrl = PCI_ATS_CTRL_STU(dev->ats_stu - PCI_ATS_MIN_STU);
> -- 
> 2.54.0.1032.g2f8565e1d1-goog
>