Re: [RFC] iommu_ops->domain_alloc_paging() enhancement to support AMD IOMMU driver

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

On 2024/6/28 14:43, Vasant Hegde wrote:
> We are working on adding domain_alloc_paging() support in AMD driver and came
> across below issue.
> 
> BACKGROUND:
> ============
> - AMD IOMMU HW has two different page tables : V1 (host page table) and V2
> (guest page table). Only V2 page table supports PASID and PRI features.
> 
> - With V2 page table we have an aliasing issue. Hence we added
> per-device-domain-id when domain is configured with v2 page table. See upstream
> commit 87a6f1f22c97 ("iommu/amd: Introduce per-device domain ID to fix potential
> TLB aliasing issue")
> 
> 
> 
> PROBLEM :
> =========
> With iommu_ops->domain_alloc_paging(dev) API AMD driver will chose best page
> table based on device capabilities (V2 for PASID capable device and V1 page
> table for rest of the devices).
> 
> But we would like to continue enforcing V1 page table for UNMANAGED domain. As
> in terms of IOMMU caching, it performs better than V2 page table.
> 
> Also while adding SVA in AMD driver Jason mentioned that we should support PASID
> with UNMANAGED domain. As I understand currently we don't have this feature in
> upstream but we would like to support it in future. Keeping this use case also
> in mind, we came up with below two options :
> 
>    1 - Pass domain type : domain_alloc_paging(dev, type)
>     When we add PASID with UNMANAGED domain we need a way to differentiate the
> domain 'type' (See below attach sample code)

The domain type is not enough for the iommu driver to differentiate
between device or PASID. For example, a domain of type UNMANAGED could
be attached to either a device or a PASID. Furthermore, SVA domain is
not a paging domain.

>    2 - Introduce new flag for domain_alloc_paging()
>      Something like :
>      #define DOMAIN_FLAG_UNMANAGED	0x01
>      #define DOMAIN_FLAG_UNMANAGED_SVA	0x02
> 
>      domain_alloc_paging(dev, flag)
>      For now we will have 'DOMAIN_PAGING_UNMANAGED'.

Add an allocation flag seems to work. It doesn't indicate any type of
domain. Instead, it indicates the required iommu feature.

Something like

#define DOMAIN_ALLOC_FLAG_PASID		0x1

means the allocated domain requires PASID to support on both device and
IOMMU. If the hardware lacks this support, it should return failure.

Best regards,
baolu