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

On 7/28/26 14:10, Kuehling, Felix wrote:
> On 2026-07-28 01:31, Mario Limonciello wrote:
>> +Felix
>>
>> On 7/27/26 23:52, Vasant Hegde wrote:
>>> Mario,
>>>
>>>
>>> On 7/27/2026 8:15 PM, Mario Limonciello wrote:
>>>>
>>>>
>>>> On 7/26/26 23:19, Vasant Hegde wrote:
>>>>>
>>>>>
>>>
>>> .../...
>>>
>>>>>>
>>>>>> I think this can probably be dropped now? There is no intersection of
>>>>>> this old broken embedded GPU and a CPU that supports CC right?
>>>>>
>>>>>
>>>>> I did consider dropping these two checks, but I was not entirely 
>>>>> sure. So I kept
>>>>> as is for now. Once these series settles and I can go with next 
>>>>> step of dropping
>>>>> these two checks.
>>>>>
>>>>> @Alex, @Mario, Any suggestions ?
>>>>>
>>>>
>>>> The background comes from this series:
>>>>
>>>> https://lore.kernel.org/all/[email protected]/
>>>
>>>
>>> Thanks for old link. So it looks like we are good to drop SME check?
>>
>> Yeah I think so.
>>
>>>
>>> Also is it safe to assume SNP is not enabled on these platform and 
>>> drop that
>>> check? Anyway SNP requirement (which doesn't support identity 
>>> mapping) and APU
>>> requirement (needs identity mapping) is conflicting.
>>>
>>
>> The current platforms I think this is a safe assumption.
>>
>> Felix,
>>
>> Can we revisit the requirement of identity mapping for APU?  Does it 
>> still hold with the current hardware and software?
> 
> I'm not sure what this means. Is Identity mode something like pass- 
> through? I don't know why this is required on APUs. If it's related to 
> IOMMUv2 support, then it's probably no longer needed. We're not using 
> IOMMUv2 on our APUs any more. Support for that was removed from the 
> amdgpu driver a few years ago.
> 

Hmm, I just had a try with this on a few years old APU (PHX).  It's 
flicker central with this (7.2-rc5~ish tree):

diff --git a/drivers/iommu/amd/iommu.c b/drivers/iommu/amd/iommu.c
index 29dc18d3d22e5..1e792cee7f4cd 100644
--- a/drivers/iommu/amd/iommu.c
+++ b/drivers/iommu/amd/iommu.c
@@ -3134,19 +3134,6 @@ static int amd_iommu_def_domain_type(struct 
device *dev)
         if (dev_is_pci(dev) && to_pci_dev(dev)->untrusted)
                 return IOMMU_DOMAIN_DMA;

-       /*
-        * Do not identity map IOMMUv2 capable devices when:
-        *  - memory encryption is active, because some of those devices
-        *    (AMD GPUs) don't have the encryption bit in their DMA-mask
-        *    and require remapping.
-        *  - SNP is enabled, because it prohibits DTE[Mode]=0.
-        */
-       if (pdev_pasid_supported(dev_data) &&
-           !cc_platform_has(CC_ATTR_MEM_ENCRYPT) &&
-           !amd_iommu_snp_en) {
-               return IOMMU_DOMAIN_IDENTITY;
-       }
-
         return 0;
  }

The flicking is reminencent of issues moving BOs between VRAM and GTT 
from a few years ago.

So I don't think it can be dropped (just) yet.