RE: [PATCH v7 2/5] PCI/ATS: Validate STU for VFs in pci_prepare_ats()

[Tian, Kevin]

> From: Pranjal Shrivastava <praan@xxxxxxxxxx>
> Sent: Monday, June 1, 2026 10:37 PM
> 
> 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.

It's the PCIe spec defining STU must be hardwired to zero for VF, i.e.
the PF's value applies to all related VFs.

above reads like a software decision.

> 
> 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@xxxxxxxxxx>
> Reviewed-by: Samiullah Khawaja <skhawaja@xxxxxxxxxx>
> Reviewed-by: Nicolin Chen <nicolinc@xxxxxxxxxx>
> Reviewed-by: Lu Baolu <baolu.lu@xxxxxxxxxxxxxxx>
> Signed-off-by: Pranjal Shrivastava <praan@xxxxxxxxxx>

Reviewed-by: Kevin Tian <kevin.tian@xxxxxxxxx>