Re: [RFC PATCH 0/6] Auxiliary IOMMU domains and Arm SMMUv3

[Lu Baolu <baolu.lu-VuQAYsv1563Yd54FQh9/CA@public.gmane.org>]

Hi Joerg,

On 11/8/18 12:43 AM, joro-zLv9SwRftAIdnm+yROfE0A@public.gmane.org wrote:
> Hi,
> 
> On Wed, Nov 07, 2018 at 11:40:30AM +0800, Lu Baolu wrote:
>> Hi Joerg,
>>
>> On 11/7/18 12:25 AM, joro-zLv9SwRftAIdnm+yROfE0A@public.gmane.org wrote:
>> Nowadays, we can find PASID granular DMA translation on both ARM and x86
>> platforms. With PASID granular DMA translation supported in system iommu, if
>> a device divides itself into multiple subsets and tag the DMA
>> transfers of each subset with a unique PASID, each subset become
>> assignable. We call the assignable subset as an ADI (Assignable Device
>> Interface). As the result, each ADI could be attached with a domain.
> 
> Yeah, I know the background. The point is, the IOMMU-API as of today
> implements a strict one-to-one relationship between domains and devices,
> every device can only have one domain assigned at a given time. If we
> assign a new domain to a device, the old gets unassigned.
> 
> If we allow to attach multiple domains to a single device we
> fundamentally break that semantic.

Yes. In the latest v4 submission, we use the aux-domain specific APIs to
attach/detach the domain to a device. Do you see other APIs that will
possibly break this semantic?

> 
> Therefore I think it is better is support the ADI devices with
> subdomains and a new set of functions in the API to handle only these
> sub-domains.

Can you please elaborate a bit more about the concept of subdomains?
 From my point of view, an aux-domain is a normal un-managed domain which
has a PASID and could be attached to any ADIs through the aux-domain
specific attach/detach APIs. It could also be attached to a device with
the existing domain_attach/detach_device() APIs at the same time, hence
mdev and pci devices in a vfio container could share a domain.

Best regards,
Lu Baolu

> 
>> Further more, a single domain might be attached to an ADI of device A,
>> while attached to another legacy device B which doesn't support PASID
>> features. In this case, we say "Domain 4" is attached to ADI(PASID#x) in
>> aux mode and attached to device B in primary mode.
> 
> This will of course not work with subdomains, but is that really
> necessary? VFIO already allocates a separate domain for each device
> anyway (iirc), so it is unlikely that is uses the same domain for a
> legacy and an ADI device.
> 
> 
> Regards,
> 
> 	Joerg
>