Re: [RFC PATCH 03/10] iommu/vt-d: Allocate groups for mediated devices

[Jean-Philippe Brucker <jean-philippe.brucker@arm.com>]

On 26/07/18 04:28, Tian, Kevin wrote:
>> hierarchical domain might be the right way to go, but let's do more
>> thinking on any corner cases.
>>
> 
> btw maybe we don't need make it 'hierarchical', as maintaining
> hierarchy usually brings more work. What we require is possibly
> just the capability of having one device bind to multiple 
> iommu_domains. One domain is reserved for parent driver's
> own DMA activities (e.g. serving DMA APIs), while other domains
> are auxiliary and can be tagged with a PASID (or any other identifier
> which IOMMU can use to support multiple domains). Such identifiers 
> may be automatically provisioned when auxiliary domain is attached, 
> i.e. not requiring an explicit request from caller. IMO it's safe to 
> assume that supporting multiple iommu domains anyway implies 
> some finer-grained capability than RID-based in underlying IOMMU.
> Then there is no need of parent/child concept.

Right, we probably don't need a hierarchy. I like this model (it's
actually the one I favor for supporting PASID in virtio-iommu), though
I'm hoping we can avoid changing the logic of iommu_attach/detach, and
instead introduce a new "get_child_domain", "attach_aux_domain" or
simply "clone" op.

Currently the attach_dev() op toggles the domain of a device. For
example a device automatically gets a default domain for kernel DMA,
then VFIO attaches a new domain for assigning to a VM. attach_dev()
disables the default domain and installs fresh page tables. detach_dev()
isn't called, and the SMMU drivers don't even implement it. If we wanted
to reuse attach_dev() for auxiliary domains, we'd need some flag to
differentiate the "add auxiliary domain" operation from the "detach
everything and attach one new domain" one

Thanks,
Jean