Re: [PATCH] iommu/amd: Add support for hw_info for iommu capability query

["Suthikulpanit, Suravee" <suravee.suthikulpanit@amd.com>]

On 8/26/2025 12:58 PM, Jason Gunthorpe wrote:
> On Tue, Aug 26, 2025 at 12:36:23PM -0500, Suthikulpanit, Suravee wrote:
>>> I think you should probably just pass the raw HW value through and
>>> require the VMM to figure out what bits it needs based on feature
>>> flags elsewhere.
>>
>> The problem is some of the features are virtualized by hardware, which needs
>> enabling from the Linux AMD IOMMU driver. We cannot just provide all flags
>> since VMM would not know if the kernel has the support enabled.
> 
> The VMM is not supposed to forward these flags as-is! It is sort of
> some kind of maximum what the underlying HW can support.
> 
> If you forward as-is then the VMM will forward broken flags it doesn't
> support when the kernel gets updated, that isn't OK.

I got this part. That's why we mask out unsupported feature flags before 
returning the EFR/EFR2 to the VMM.

> Each and every feature the VMM wants to show in the EFR has to figured
> out on its own if it can be supported based on other kernel features.
> 
> The utility of the get_info return is for HW features that don't
> require any special kernel enablement.

Not sure if I got this part. Are you referring to the struct 
vfio_iommu_type1_info and vfio_iommu_type1_get_info()?

Suravee