Hi,
On Wed, Nov 07, 2018 at 11:40:30AM +0800, Lu Baolu wrote:
> Hi Joerg,
>
> On 11/7/18 12:25 AM, j...@8bytes.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.
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.
> 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
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