Re: [PATCH v2 5/8] iommu: Add PASID support for DMA mapping API users

[Jason Gunthorpe]

On Tue, Mar 15, 2022 at 09:31:35AM -0700, Jacob Pan wrote:

> > IMHO it is a device mis-design of IDXD to require all DMA be PASID
> > tagged. Devices should be able to do DMA on their RID when the PCI

> IDXD can do DMA w/ RID, the PASID requirement is only for shared WQ where
> ENQCMDS is used. ENQCMDS has the benefit of avoiding locking where work
> submission is done from multiple CPUs.
> Tony, Dave?

This is what I mean, it has an operating mode you want to use from the
kernel driver that cannot do RID DMA. It is a HW mis-design, IMHO.

Something like PASID0 in the ENQCMDS should have triggered RID DMA.

> > In any case I think we are better to wait for an actual user for multi
> > DMA API iommu_domains to come forward before we try to build an API
> > for it.
> 
> What would you recommend in the interim?

Oh, I mean this approach at a high level is fine - I was saying we
shouldn't try to broaden it like Robin was suggesting without a driver
that needs multiple iommu_domains for the DMA API.

Jason