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

[Alex Williamson <alex.williamson-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>]

On Wed, 7 Nov 2018 17:43:23 +0100
"joro-zLv9SwRftAIdnm+yROfE0A@public.gmane.org" <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.
> 
> 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.

VFIO will attempt to use the same domain for all groups within a
container, only falling back to separate domains if there's an
incompatibility.  Multiple domains means that we need to mirror mapping
and unmapping across each domain, so there's more work to do and
theoretically more overhead in the IOMMU implementation assuming those
domains land on the same IOMMU driver.  Thanks,

Alex