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

[Pranjal Shrivastava <praan@google.com>]

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>
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);
-- 
2.54.0.746.g67dd491aae-goog