Re: [PATCH v3 2/3] PCI/ATS: Validate STU for VFs in pci_prepare_ats()

[Samiullah Khawaja <skhawaja@google.com>]

On Tue, May 19, 2026 at 01:53:21PM +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.
>
>Signed-off-by: Pranjal Shrivastava <praan@google.com>
>---
> drivers/pci/ats.c | 7 ++++++-
> 1 file changed, 6 insertions(+), 1 deletion(-)
>
>diff --git a/drivers/pci/ats.c b/drivers/pci/ats.c
>index a5fa7745bce8..54319854bfd8 100644
>--- a/drivers/pci/ats.c
>+++ b/drivers/pci/ats.c
>@@ -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;
>+	}
>
> 	dev->ats_stu = ps;
> 	ctrl = PCI_ATS_CTRL_STU(dev->ats_stu - PCI_ATS_MIN_STU);
>-- 
>2.54.0.563.g4f69b47b94-goog
>

Reviewed-by: Samiullah Khawaja <skhawaja@google.com>