From: Jason Gunthorpe <jgg@nvidia.com>
Sent: Thursday, August 8, 2024 2:19 AM

PCI ATS has a global Smallest Translation Unit field that is located in
the PF but shared by all of the VFs.

The expectation is that the STU will be set to the root port's global STU
capability which is driven by the IO page table configuration of the iommu
HW. Today it becomes set when the iommu driver first enables ATS.

Thus, to enable ATS on the VF, the PF must have already had the correct
STU programmed, even if ATS is off on the PF.

Unfortunately the PF only programs the STU when the PF enables ATS. The
iommu drivers tend to leave ATS disabled when IDENTITY translation is
being used.

Is there more context on this?

Looking at intel-iommu driver ATS is disabled for IDENETITY when
the iommu is in legacy mode:

dmar_domain_attach_device()
{
	...
	if (sm_supported(info->iommu) || !domain_type_is_si(info->domain))
		iommu_enable_pci_caps(info);
	...
}

But this follows what VT-d spec says (section 9.3):

TT: Translate Type
10b: Untranslated requests are processed as pass-through. The SSPTPTR
field is ignored by hardware. Translated and Translation Requests are
blocked.

 

+/**
+ * pci_prepare_ats - Setup the PS for ATS
+ * @dev: the PCI device
+ * @ps: the IOMMU page shift
+ *
+ * This must be done by the IOMMU driver on the PF before any VFs are
created to
+ * ensure that the VF can have ATS enabled.
+ *
+ * Returns 0 on success, or negative on failure.
+ */
+int pci_prepare_ats(struct pci_dev *dev, int ps)
+{
+	u16 ctrl;
+
+	if (!pci_ats_supported(dev))
+		return -EINVAL;
+
+	if (WARN_ON(dev->ats_enabled))
+		return -EBUSY;
+
+	if (ps < PCI_ATS_MIN_STU)
+		return -EINVAL;
+
+	if (dev->is_virtfn)
+		return 0;

missed a check that 'ps' matches pf's ats_stu.

+
+	dev->ats_stu = ps;
+	ctrl = PCI_ATS_CTRL_STU(dev->ats_stu - PCI_ATS_MIN_STU);
+	pci_write_config_word(dev, dev->ats_cap + PCI_ATS_CTRL, ctrl);
+	return 0;
+}
+EXPORT_SYMBOL_GPL(pci_prepare_ats);
+

Then there is no need to keep the 'ps' parameter in pci_enable_ats().