Re: [PATCH 2/2] iommu/amd: Force identity mode for selected GPUs only

[Bjorn Helgaas <helgaas@kernel.org>]

On Thu, Jul 23, 2026 at 11:32:49AM -0500, Mario Limonciello wrote:
> On 7/23/26 11:23, Vasant Hegde wrote:
> > On 7/23/2026 9:41 PM, Mario Limonciello wrote:
> > > On 7/23/26 10:59, Bjorn Helgaas wrote:
> > > > On Thu, Jul 23, 2026 at 06:15:48AM +0000, Vasant Hegde wrote:
> > > > > Certain AMD GPU's must always be in identity mode. Currently its enforced
> > > > > using PASID check. It worked fine as most GPU's has PASID feature. But
> > > > > this means, identity mode enforcement is done for all PASID capable devices.
> ...

> > +static bool quirks_force_identity_mapping(struct pci_dev *pdev)
> > +{
> > +       struct pci_dev *root_port;
> > +       int class = pdev->class >> 8;
> > +
> > +       /* AMD GPU vendor ID */
> > +       if (pdev->vendor != PCI_VENDOR_ID_ATI)
> > +               return false;
> > +
> > +       /* GPU class */
> > +       if (class != PCI_CLASS_DISPLAY_VGA &&
> > +           class != PCI_CLASS_DISPLAY_OTHER)
> > +               return false;
> > +
> > +       if (pci_upstream_bridge(pdev) &&
> 
> I don't think you need to check for pci_upstream_bridge() to be
> non-NULL.  You already checked that it's an endpoint by looking at
> the class.  So an endpoint will be connected to a bridge of some
> sort (either a switch internal to the dGPU or to a root port).

I think devices can be passed through to virtualized guests with no
upstream bridge visible to the guest, can't they?