Re: [PATCH v4 02/11] iommufd: Introduce IOMMUFD_OBJ_VIOMMU and its related struct

[Baolu Lu <baolu.lu@linux.intel.com>]

On 2024/10/22 21:15, Jason Gunthorpe wrote:
> On Tue, Oct 22, 2024 at 04:59:07PM +0800, Baolu Lu wrote:
> 
>> Is it feasible to make vIOMMU object more generic, rather than strictly
>> tying it to nested translation? For example, a normal paging domain that
>> translates gPAs to hPAs could also have a vIOMMU object associated with
>> it.
>>
>> While we can only support vIOMMU object allocation uAPI for S2 paging
>> domains in the context of this series, we could consider leaving the
>> option open to associate a vIOMMU object with other normal paging
>> domains that are not a nested parent?
> Why? The nested parent flavour of the domain is basically free to
> create, what reason would be to not do that?

Above addressed my question. The software using vIOMMU should allocate a
domain of nested parent type.

> 
> If the HW doesn't support it, then does the HW really need/support a
> VIOMMU?
> 
> I suppose it could be made up to the driver, but for now I think we
> should leave it as is in the core code requiring it until we have a
> reason to relax it.

Yes. In such cases, the iommu driver could always allow nested parent
domain allocation, but fails to allocate a nested domain if the hardware
is not capable of nesting translation.

Thanks,
baolu