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

[Vasant Hegde <vasant.hegde@amd.com>]

Ankit,


On 7/23/2026 9:23 PM, Ankit Soni 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.
>>
>> Previously it made sense as domain allocation API
>> (iommu_ops->domain_alloc()) was just passing domain type. So it couldn't
>> check device capability and decide best suited page table type (v1 or
>> v2). With recent enhancement to driver code, it uses
>> domain_alloc_paging_flags() ops for all paging mode domain allocation.
>> This can check device/flags and allocate best suited page table (v1 or v2).
>> Hence fix amd_iommu_def_domain_type() to force identity mapping for selected
>> GPUs only.
>>
>> With this change system booted with DMA translation mode will select:
>>  * Guest (v2) page table for PASID capable device
>>  * Host (v1) page table for non-PASID capable device
>>
>> Cc: Alex Deucher <alexander.deucher@amd.com>
>> Cc: Mario Limonciello <mario.limonciello@amd.com>
>> Signed-off-by: Vasant Hegde <vasant.hegde@amd.com>
>> Tested-by: Amandeep Kaur Longia <AmandeepKaur.Longia@amd.com>
>> ---
>>  drivers/iommu/amd/iommu.c | 56 +++++++++++++++++++++++++++++++--------
>>  1 file changed, 45 insertions(+), 11 deletions(-)
>>
>> diff --git a/drivers/iommu/amd/iommu.c b/drivers/iommu/amd/iommu.c
>> index 563f9c2672d5..fe642e33c87d 100644
>> --- a/drivers/iommu/amd/iommu.c
>> +++ b/drivers/iommu/amd/iommu.c
>> @@ -3112,6 +3112,32 @@ static bool amd_iommu_is_attach_deferred(struct device *dev)
>>  	return dev_data->defer_attach;
>>  }
>>  
>> +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;
>> +
>> +	root_port = pcie_find_root_port(pdev);
>> +	if (!root_port)
>> +		return false;
>> +
>> +	/* If bridge vendor is not ATI then its APU and force IDENTITY mode */
>> +	if (root_port->vendor != PCI_VENDOR_ID_ATI)
>> +		return true;
> 
> Hi Vasant,
> 
> For a discrete GPU, pcie_find_root_port() may walk up to the system root port. 
> On an AMD platform that root port is vendored PCI_VENDOR_ID_AMD, which is not ATI. 

We want to filter based on vendor as ATI only. As Mario pointed out, logic got
inverted. Will fix it in v2.

-Vasant