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

On 7/23/26 14:53, Bjorn Helgaas wrote:
> 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?

None of the GPUs in APUs support SRIOV, so there isn't a VF to be passed 
in to a guest.  But I guess you could pass the whole endpoint in.

When passed into a guest doesn't it end up associated behind a root port 
in the guest though?